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The human foot is a complex structure that plays an important role in our capacity for upright locomotion. Comparisons of our feet with those of our closest extinct and extant relatives have linked shape features (e.g. the longitudinal and transverse arches, heel size and toe length) to specific mechanical functions. However, foot shape varies widely across the human population, so it remains unclear if and how specific shape variants are related to locomotor mechanics. Here we constructed a statistical shape-function model (SFM) from 100 healthy participants to directly explore the relationship between the shape and function of our feet. We also examined if we could predict the joint motion and moments occurring within a person's foot during locomotion based purely on shape features. The SFM revealed that the longitudinal and transverse arches, relative foot proportions and toe shape along with their associated joint mechanics were most variable. However, each of these only accounted for small proportions of the overall variation in shape, deformation and joint mechanics, most likely owing to the high structural complexity of the foot. Nevertheless, a leave-one-out analysis showed that the SFM can accurately predict joint mechanics of a novel foot, based on its shape and deformation.

The cost of walking and running on uneven terrain is not directly explained by external mechanical work. Although metabolic cost of transport increases linearly with gradient at uphill and downhill gradients exceeding 15%, at shallower gradients, the relationship is nonlinear, with the minimum cost occurring at ∼10% downhill grade. Given these nonlinear relationships between grade and metabolic cost, we projected a significant difference in the total metabolic cost of two walking conditions that required the same total external mechanical work be performed over the same total period of time; in one condition, time was spent walking to gradients that were fixed at +10.5% and -10.5% and in the other condition time was spent walking to gradients that varied from 0 to +21% and from -21 to 0%. We compared these two conditions experimentally, using an approach to quantify nonsteady-state oxidative energy expenditure. In line with our projection, the "variable" grade condition resulted in an 8.3 ± 2.2% higher total cumulative oxidative energy expenditure (J·kg -1 ) compared with the "fixed" grade condition ( P < 0.001). Future work should aim to apply our approach across different gradients, speeds, and forms of locomotion; especially those that might provide insight into how humans optimize locomotion on variable grade routes. NEW & NOTEWORTHY We use a method for quantifying nonsteady-state energetics to show that regardless of whether the same total gain and loss in elevation (i.e., same total external mechanical work) is achieved over the same period of time, the total energy expenditure of different graded walking conditions can vary depending on the grades that are walked at and for how long they are walked at.

Computational musculoskeletal modelling has emerged as an alternative, less-constrained technique to indirect calorimetry for estimating energy expenditure. However, predictions from modelling tools depend on many assumptions around muscle architecture and function and motor control. Therefore, these tools need to continue to be validated if we are to eventually develop subject-specific simulations that can accurately and reliably model rates of energy consumption for any given task. In this study, we used OpenSim software and experimental motion capture data to simulate muscle activations, muscle fascicle dynamics and whole-body metabolic power across mechanically and energetically disparate hopping tasks, and then evaluated these outputs at a group- and individual-level against experimental electromyography, ultrasound and indirect calorimetry data. Comparing simulated and experimental outcomes, we found weak to strong correlations for peak muscle activations, moderate to strong correlations for absolute fascicle shortening and mean shortening velocity, and strong correlations for gross metabolic power. These correlations tended to be stronger on a group-level rather than individual-level. We encourage the community to use our publicly available dataset from SimTK.org to experiment with different musculoskeletal models, muscle models, metabolic cost models, optimal control policies, modelling tools and algorithms, data filtering etc. with subject-specific simulations being a focal goal., Competing Interests: We declare we have no competing interests., (© 2023 The Authors.)

Purpose: To investigate the effect of muscle force during active stretch on quantitative and qualitative indicators of exercise-induced muscle damage (EIMD) in the medial gastrocnemius (MG) muscle., Methods: Twelve recreationally active volunteers performed two trials of an eccentric heel drop exercise. Participants performed a single bout of low-load (body weight) and high-load (body weight + 30% body weight) exercises on separate legs. The total mechanical work output for each condition was matched between legs. Before, two hours and 48 h after each bout of eccentric exercise, electrically stimulated triceps surae twitch torque, muscle soreness, MG active fascicle length at maximum twitch torque and muscle passive stiffness were collected. Triceps surae electromyographic (EMG) activity, MG fascicle stretch and MG muscle-tendon unit (MTU) length were measured during the eccentric tasks., Results: The high-load condition increased triceps surae muscle activity by 6-9%, but reduced MG fascicle stretch (p < 0.001). MTU stretch was similar between conditions. The greater muscle force during stretch did not give rise to additional torque loss (5 vs 6%) or intensify muscle soreness., Conclusions: Adding 30% body weight during eccentric contractions has a modest impact on exercise-induced muscle damage in the medial gastrocnemius muscle. These results suggest that muscle load may not be an important determinant of stretch-induced muscle damage in the human MG muscle. The muscle investigated does exhibit large pennation angles and high series elastic compliance; architectural features that likely buffer muscle fibres against stretch and damage., (© 2023. The Author(s).)

Many models have been developed to predict metabolic energy expenditure based on biomechanical proxies of muscle function. However, current models may only perform well for select forms of locomotion, not only because the models are rarely rigorously tested across subtle and broad changes in locomotor task but also because previous research has not adequately characterised different forms of locomotion to account for the potential variability in muscle function and thus metabolic energy expenditure. To help to address the latter point, the present study imposed frequency and height constraints to hopping and quantified gross metabolic power as well as the activation requirements of medial gastrocnemius (MG), lateral gastrocnemius (GL), soleus (SOL), tibialis anterior (TA), vastus lateralis (VL), rectus femoris (RF) and biceps femoris (BF), and the work requirements of GL, SOL and VL. Gross metabolic power increased with a decrease in hop frequency and increase in hop height. There was no hop frequency or hop height effect on the mean electromyography (EMG) data of ankle musculature; however, the mean EMG of VL and RF increased with a decrease in hop frequency and that of BF increased with an increase in hop height. With a reduction in hop frequency, GL, SOL and VL fascicle shortening, fascicle shortening velocity and fascicle to MTU shortening ratio increased, whereas with an increase in hop height, only SOL fascicle shortening velocity increased. Therefore, within the constraints that we imposed, decreases in hop frequency and increases in hop height resulted in increases in metabolic power that could be explained by increases in the activation requirements of knee musculature and/or increases in the work requirements of both knee and ankle musculature., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

Background: Studies on motor unit behaviour with varying rates of force development have focussed predominantly on comparisons between slow and ballistic ( i.e. , very fast) contractions. It remains unclear how motor units respond to less extreme changes in rates of force development. Here, we studied a small intrinsic foot muscle, flexor hallucis brevis (FHB) where the aim was to compare motor unit discharge rates and recruitment thresholds at two rates of force development. We specifically chose to investigate relatively slow to moderate rates of force development, not ballistic, as the chosen rates are more akin to those that presumably occur during daily activity., Methods: We decomposed electromyographic signals to identify motor unit action potentials obtained from indwelling fine-wire electrodes in FHB, from ten male participants. Participants performed isometric ramp-and-hold contractions from relaxed to 50% of a maximal voluntary contraction. This was done for two rates of force development; one with the ramp performed over 5 s (slow condition) and one over 2.5 s (fast condition). Recruitment thresholds and discharge rates were calculated over the ascending limb of the ramp and compared between the two ramp conditions for matched motor units. A repeated measures nested linear mixed model was used to compare these parameters statistically. A linear repeated measures correlation was used to assess any relationship between changes in recruitment threshold and mean discharge rate between the two conditions., Results: A significant increase in the initial discharge rate ( i.e. , at recruitment) in the fast (mean: 8.6 ±  2.4 Hz) compared to the slow (mean: 7.8 ± 2.3 Hz) condition ( P = 0.027), with no changes in recruitment threshold ( P = 0.588), mean discharge rate ( P = 0.549) or final discharge rate ( P = 0.763) was observed. However, we found substantial variability in motor unit responses within and between conditions. A small but significant negative correlation (R 2 = 0.33, P = 0.003) was found between the difference in recruitment threshold and the difference in mean discharge rate between the two conditions., Conclusion: These findings suggest that as force increases for contractions with slower force development, increasing the initial discharge rate of recruited motor units produces the increase in rate of force development, without a change in their recruitment thresholds, mean or final discharge rate. However, an important finding was that for only moderate changes in rate of force development, as studied here, not all units respond similarly. This is different from what has been described in the literature for ballistic contractions in other muscle groups, where all motor units respond similarly to the increase in neural drive. Changing the discharge behaviour of a small group of motor units may be sufficient in developing force at the required rate rather than having the discharge behaviour of the entire motor unit pool change equally., Competing Interests: The authors declare there are no competing interests., (©2023 Aeles et al.)

Modern human feet are considered unique among primates in their capacity to transmit propulsive forces and re-use elastic energy. Considered central to both these capabilities are their arched configuration and the plantar aponeurosis (PA). However, recent evidence has shown that their interactions are not as simple as proposed by the theoretical and mechanical models that established their significance. Using three-dimensional foot scans and statistical shape and deformation modelling, we show that the shape of the longitudinal and transverse arches varies widely among the healthy adult population, and that the former is subject to load-induced arch flattening, whereas the latter is not. However, longitudinal arch shape and flattening are only one of the various foot shape-deformation relationships. PA stiffness was also found to vary widely. Yet only a small amount of this variability (approx. 10-18%) was explained by variations in foot shape, deformation and their combination. These findings add to the mounting evidence showing that foot mechanics are complex and cannot be accurately represented by simple models. Especially the interactions between longitudinal arch and PA appear to be far less constrained than originally proposed, most likely due to the many degrees of freedom provided by the structural complexity of our feet.

To preserve motion, humans must adopt actuator-like dynamics to replace energy that is dissipated during contact with damped surfaces. Our ankle plantar flexors are credited as the primary source of work generation. Our feet and their intrinsic foot muscles also appear to be an important source of generative work, but their contributions to restoring energy to the body remain unclear. Here, we test the hypothesis that our feet help to replace work dissipated by a damped surface through controlled activation of the intrinsic foot muscles. We used custom-built platforms to provide both elastic and damped surfaces and asked participants to perform a bilateral hopping protocol on each. We recorded foot motion and ground reaction forces, alongside muscle activation, using intramuscular electromyography from flexor digitorum brevis, abductor hallucis, soleus, and tibialis anterior. Hopping in the Damped condition resulted in significantly greater positive work and contact-phase muscle activation compared with the Elastic condition. The foot contributed 25% of the positive work performed about the ankle, highlighting the importance of the foot when humans adapt to different surfaces. NEW & NOTEWORTHY Adaptable foot mechanics play an important role in how we adjust to elastic surfaces. However, natural substrates are rarely perfectly elastic and dissipate energy. Here, we highlight the important role of the foot and intrinsic foot muscles in contributing to replacing dissipated work on damped surfaces and uncover an important energy-saving mechanism that may be exploited by the designers of footwear and other wearable devices.

The integration of electromyography (EMG) and ultrasound imaging has provided important information about the mechanisms of muscle activation and contraction. Unfortunately, conventional bipolar EMG does not allow an accurate assessment of the interplay between the neural drive received by muscles, changes in fascicle length and torque. We aimed to assess the relationship between modulations in tibialis anterior muscle (TA) motor unit (MU) discharge, fascicle length, and dorsiflexion torque using ultrasound-transparent high-density EMG electrodes. EMG and ultrasound images were recorded simultaneously from TA using a 32-electrode silicon matrix while performing isometric dorsiflexion contractions at two ankle joint positions (0° or 30° plantar flexion) and torques (20% or 40% of maximum). EMG signals were decomposed into MUs and changes in fascicle length were assessed with a fascicle-tracking algorithm. MU firings were converted into a cumulative spike train (CST) that was cross-correlated with torque (CST-torque) and fascicle length (CST-length). High cross-correlations were found for CST-length (0.60, range: 0.31-0.85) and CST-torque (0.71, range: 0.31-0.88). Cross-correlation delays revealed that the delay between CST-fascicle length (∼75 ms) was smaller than CST-torque (∼150 ms, P < 0.001). These delays affected MU recruitment and de-recruitment thresholds since the fascicle length at which MUs were recruited and de-recruited was similar but MU recruitment-de-recruitment torque varied. This study demonstrates that changes in TA fascicle length are related to modulations in MU firing and dorsiflexion torque. These relationships allow assessment of the interplay between neural drive, muscle contraction and torque, enabling the time required to convert neural activity into movement to be quantified. NEW & NOTEWORTHY By employing ultrasound-transparent high-density EMG electrodes, we show that modulations in tibialis anterior muscle motor unit discharge rate were related to both changes in fascicle length and resultant torque. These relationships permitted the quantification of the relative delays between fluctuations in neural drive, muscle contraction, and transfer of torque via the tendon during sustained isometric dorsiflexion contractions, providing information on the conversion of neural activity into muscle force during a contraction.

We developed and validated a multi-segment foot and ankle model for human walking and running. The model has 6-segments, and 7 degrees of freedom; motion between foot segments were constrained with a single oblique axis to enable triplanar motion [Joint Constrained (JC) model]. The accuracy of the JC model and that of a conventional model using a 6 degrees of freedom approach were assessed by comparison to segment motion determined with biplanar videoradiography. Compared to the 6-DoF model, our JC model demonstrated significantly smaller RMS differences [JC: 2.19° (1.43-2.73); 6-DoF: 3.25° (1.37-5.89)] across walking and running. The JC model is thus capable of more accurate musculoskeletal analyses and is also well suited for predictive simulations.

When rate of force development is increased, neural drive increases. There is presently no accepted explanation for this effect. We propose and experimentally test the theory that a small increase in rate of force development increases medial gastrocnemius fascicle shortening velocity, reducing the muscle's force-generating capacity, leading to active motor units being recruited at lower forces and with increased discharge frequencies. Participants produced plantar flexion torques at three different rates of force development (slow: 2% MVC/s, medium: 10% MVC/s, fast: 20% MVC/s). Ultrasound imaging showed that increased rate of force development was related to higher fascicle shortening velocity (0.4 ± 0.2 mm/s, 2.0 ± 0.9 mm/s, 4.1 ± 1.9 mm/s in slow, medium, fast, respectively). In separate experiments, medial gastrocnemius motor unit recruitment thresholds and discharge frequencies were measured using fine-wire electromyography (EMG), together with surface EMG. Recruitment thresholds were lower in the fast (12.8 ± 9.2% MVC) and medium (14.5 ± 9.9% MVC) conditions compared to the slow (18.2 ± 8.9% MVC) condition. The initial discharge frequency was lower in the slow (5.8 ± 3.1 Hz) than the fast (6.7 ± 1.4 Hz), but not than the medium (6.4 ± 2.4 Hz) condition. The surface EMG was greater in the fast (mean RMS: 0.029 ± 0.017 mV) compared to the slow condition (0.019 ± 0.013 mV). We propose that the increase in muscle fascicle shortening velocity reduces the force-generating capacity of the muscle, therefore requiring greater neural drive to generate the same forces., (© 2022. The Author(s).)

Background: Following stretch of an active muscle, muscle force is enhanced, which is known as residual force enhancement (rFE). As earlier studies found apparent corticospinal excitability modulations in the presence of rFE, this study aimed to test whether corticospinal excitability modulations contribute to rFE., Methods: Fourteen participants performed submaximal plantar flexion stretch-hold and fixed-end contractions at 30% of their maximal voluntary soleus muscle activity in a dynamometer. During the steady state of the contractions, participants either received subthreshold or suprathreshold transcranial magnetic stimulation (TMS) of their motor cortex, while triceps surae muscle responses to stimulation were obtained via electromyography (EMG), and net ankle joint torque was recorded. B-mode ultrasound imaging was used to confirm muscle fascicle stretch during stretch-hold contractions in a subset of participants., Results: Following stretch of the plantar flexors, an average rFE of 7% and 11% was observed for contractions with subthreshold and suprathreshold TMS, respectively. 41-46 ms following subthreshold TMS, triceps surae muscle activity was suppressed by 19-25%, but suppression was not significantly different between stretch-hold and fixed-end contractions. Similarly, the reduction in plantar flexion torque following subthreshold TMS was not significantly different between contraction conditions. Motor evoked potentials, silent periods and superimposed twitches following suprathreshold TMS were also not significantly different between contraction conditions., Discussion: As TMS of the motor cortex did not result in any differences between stretch-hold and fixed-end contractions, we conclude that rFE is not linked to changes in corticospinal excitability., Competing Interests: The authors declare there are no competing interests., (©2022 Frischholz et al.)

Purpose: Controversy remains about whether exercise-induced muscle damage (EIMD) and the subsequent repeated bout effect (RBE) are caused by the stretching of an activated muscle, or the production of high force at long, but constant, muscle lengths. The aim of this study was to determine the influence of muscle fascicle stretch elicited during different muscle contraction types on the magnitude of EIMD and the RBE., Methods: Fourteen participants performed an initial bout of lower limb exercise of the triceps surae. One leg performed sustained static contractions at a constant long muscle length (ISO), whereas the contralateral leg performed a bout of eccentric heel drop exercise (ECC). Time under tension was matched between the ECC and ISO conditions. Seven days later, both legs performed ECC. Plantar flexor twitch torque, medial gastrocnemius (MG) fascicle length and muscle soreness were assessed before, 2 h and 2 days after each exercise bout. MG fascicle length and triceps surae surface electromyography were examined across the bouts of exercise., Results: We found that both ECC and ISO conditions elicited EIMD and a RBE. ISO caused less damage 2 h after the initial bout (14% less drop in twitch torque, P = 0.03) and less protection from soreness 2 days after the repeated bout (56% higher soreness, P = 0.01). No differences were found when comparing neuromechanical properties across exercise bouts., Conclusion: For MG, the action of stretching an active muscle seems to be more important for causing damage than a sustained contraction at a long length., (© 2021. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Humans choose work-minimizing movement strategies when interacting with compliant surfaces. Our ankles are credited with stiffening our lower limbs and maintaining the excursion of our body's center of mass on a range of surface stiffnesses. We may also be able to stiffen our feet through an active contribution from our plantar intrinsic muscles (PIMs) on such surfaces. However, traditional modeling of the ankle joint has masked this contribution. We compared foot and ankle mechanics and muscle activation on low, medium, and high stiffness surfaces during bilateral hopping using a traditional and anatomical ankle model. The traditional ankle model overestimated work and underestimated stiffness compared with the anatomical model. Hopping on a low stiffness surface resulted in less longitudinal arch compression with respect to the high stiffness surface. However, because midfoot torque was also reduced, midfoot stiffness remained unchanged. We observed lower activation of the PIMs, soleus, and tibialis anterior on the low and medium stiffness conditions, which paralleled the pattern we saw in the work performed by the foot and ankle. Rather than performing unnecessary work, participants altered their landing posture to harness the energy stored by the sprung surface in the low and medium conditions. These findings highlight our preference to minimize mechanical work when transitioning to compliant surfaces and highlight the importance of considering the foot as an active, multiarticular, part of the human leg. NEW & NOTEWORTHY When seeking to understand how humans adapt their movement to changes in substrate, the role of the human foot has been neglected. Using multi-segment foot modeling, we highlight the importance of adaptable foot mechanics in adjusting to surfaces of different compliance. We also show, via electromyography, that the adaptations are under active muscular control.

The repeated bout effect (RBE) confers protection following exercise-induced muscle damage. Typical signs of this protective effect are significantly less muscle soreness and faster recovery of strength after the second bout. The aim of this study was to compare regional changes in medial gastrocnemius (MG) muscle activity and mechanical hyperalgesia after repeated bouts of eccentric exercise. Twelve healthy male participants performed two bouts of eccentric heel drop exercise (separated by 7 days) while wearing a vest equivalent to 20% of their body weight. High-density MG electromyographic amplitude maps and topographical pressure pain sensitivity maps were created before, two hours (2H), and two days (2D) after both exercise bouts. Statistical parametric mapping was used to identify RBE effects on muscle activity and mechanical hyperalgesia, using pixel-level statistics when comparing maps. The results revealed a RBE, as a lower strength loss (17% less; P < .01) and less soreness (50% less; P < .01) were found after the second bout. However, different muscle regions were activated 2H and 2D after the initial bout but not following the repeated bout. Further, no overall changes in EMG distribution or mechanical hyperalgesia were found between bouts. These results indicate that muscle activation is unevenly distributed during the initial bout, possibly to maintain muscle function during localized mechanical fatigue. However, this does not reflect a strategy to confer protection during the repeated bout by activating undamaged/non-fatigued muscle areas., (© 2021 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Unanticipated variations in terrain can destabilize the body. The foot is the primary interface with the ground and we know that cutaneous reflexes provide important sensory feedback. However, little is known about the contribution of stretch reflexes from the muscles within the foot to upright stability. We used intramuscular electromyography measurements of the foot muscles flexor digitorum brevis (FDB) and abductor hallucis (AH) to show for the first time how their short-latency stretch reflex response (SLR) may play an important role in responding to stepping perturbations. The SLR of FDB and AH was highest for downwards steps and lowest for upwards steps, with the response amplitude for level and compliant steps in between. When the type of terrain was unknown or unexpected to the participant, the SLR of AH and the ankle muscle soleus tended to decrease. We found significant relationships between the contact kinematics and forces of the leg and the SLR, but a person's expectation still had significant effects even after accounting for these relationships. Motor control models of short-latency body stabilization should not only include local muscle dynamics, but also predictions of terrain based on higher level information such as from vision or memory.

Squats are considered a useful basic exercise for trunk muscle activation. To gain knowledge about trunk muscle activity patterns depending on the barbell position in beginners, we examined squats with low weights in the back, front, and overhead position., Methods: Twelve healthy adults (6 women/6 men, age: 29.1 (SD 8.0) y, height: 173.4 (6.9) cm, body mass: 70.1 (9.1) kg) randomly performed the three barbell squats in normal and in forefoot standing. Surface electromyography from external (EO) and internal oblique, rectus abdominis, and erector spinae (ES) was recorded. The centre of pressure path length (CoP) and the motion of the lumbar spine were captured., Results: The overhead squat revealed the highest percent muscle activity, where EO (p = 0.009) and ES (p = 0.03) showed the greatest activity. Forefoot standing did not change overall trunk muscle activities (.05< Hedges' g <.29, 0.17 < p < 0.95) although longer CoP path length (.45 < g < 1.3, p < 0.05) was measured., Conclusions: Squat exercises with low weight are useful to activate trunk muscles. Activity increases with the difficulty of the squat by frontal or overhead loading, but not by standing on the forefoot. The low weighted squat can target well core muscle activity in training with beginners or in rehabilitation.

When cyclists ride off the saddle, their center of mass (CoM) appears to go through a rhythmic vertical oscillation during each crank cycle. Just like in walking and running, the pattern of CoM movement may have a significant effect on the mechanical power that needs to be generated and dissipated by muscle., Purpose: To date, neither the CoM movement strategies during nonseated cycling nor the limb mechanics that allow this phenomenon to occur have been quantified., Methods: Here we estimate how much power can be contributed by a rider's CoM at each instant during the crank cycle by combining a kinematic and kinetic approach to measure CoM movement and joint powers of 15 participants riding in a nonseated posture at three individualized power outputs (10%, 30%, and 50% of peak maximal power) and two different cadences (70 and 120 rpm)., Results: The peak-to-peak amplitude of vertical CoM displacement increased significantly with power output and with decreasing cadence. Accordingly, the greatest peak-to-peak amplitude of CoM displacement (0.06 ± 0.01 m) and change in total mechanical energy (0.54 ± 0.12 J·kg) occurred under the combination of high-power output and low cadence. At the same combination of high-power output and low cadence, we found that the peak rate of CoM energy loss (3.87 ± 0.93 W·kg) was equal to 18% of the peak crank power., Conclusion: Consequently, it appears that for a given power output, changes in CoM energy contribute to peak instantaneous power output at the crank, thus reducing the required muscular contribution. These findings suggest that the rise and fall of a rider's CoM acts as a mechanical amplifier during nonseated cycling, which has important implications for both rider and bicycle performance.

Cyclists frequently use a nonseated posture when accelerating, climbing steep hills, and sprinting; yet, the biomechanical difference between seated and nonseated cycling remains unclear., Purpose: This study aimed to test the effects of posture (seated and nonseated) and cadence (70 and 120 rpm) on joint power contributions, effective mechanical advantage, and muscle activations within the leg during very-high-power output cycling., Methods: Fifteen male participants rode on an instrumented ergometer at 50% of their individualized instantaneous maximal power (10.74 ± 1.99 W·kg; above the reported threshold for seated to nonseated transition) in different postures (seated and nonseated) and at different cadences (70 and 120 rpm) while leg muscle activity, full-body motion capture, and crank radial and tangential forces were recorded. A scaled, full-body model was used to solve inverse kinematics and inverse dynamics to determine joint displacements and net joint moments. Statistical comparisons were made using a two-way repeated-measures ANOVA (posture-cadence)., Results: There were significant main effects of posture and cadence on joint power contributions. A key finding was that the nonseated posture increased negative power at the knee, with an associated significant decrease of net power at the knee. The contribution of knee power decreased by 15% at both 70 and 120 rpm (~0.8 W·kg) when nonseated compared with seated. Subsequently, hip power and ankle power contributions were significantly higher when nonseated compared with seated at both cadences. In both postures, knee power was 9% lower at 120 rpm compared with 70 rpm (~0.4 W·kg)., Conclusion: These results evidenced that the contribution of knee joint power to leg power was reduced by switching from a seated to nonseated posture during very-high-power output cycling; however, the size of the reduction is cadence dependent.

Our current knowledge on the neurophysiological properties of intrinsic foot muscles is limited, especially at high forces. This study therefore aimed to investigate the discharge characteristics of single motor units in an intrinsic foot muscle, namely flexor hallucis brevis, during voluntary contractions up to 100% of maximal voluntary contraction. We measured the recruitment threshold and discharge rate of flexor hallucis brevis motor units using indwelling fine-wire electrodes. Ten participants followed a target ramp up to maximal voluntary contraction by applying a metatarso-phalangeal flexion torque. We observed motor unit recruitment thresholds across a wide range of isometric forces (ranging from 10 to 98% of maximal voluntary contraction) as well as across a wide range of discharge rates (ranging from 4.8 to 23.3 Hz for initial discharge rate and 9.5 to 34.2 Hz for peak discharge rate). We further observed patterns of high variability in recruitment threshold and discharge rate as well as crossover in discharge rate between motor units within the same participant. These findings suggest that the force output of a muscle is generated through a mechanism with substantial variability rather than relying on a rigid organization, which is in contrast to the proposed onion-skin theory. The demands placed on the plantar intrinsic foot muscles during high- and low-force tasks may explain these observed neurophysiological properties. NEW & NOTEWORTHY We recorded for the first time single motor unit action potential trains in the flexor hallucis brevis, a short toe muscle, over the full range of maximum voluntary contraction. Its motor units are recruited up to very high (98%) recruitment thresholds with a substantial range of discharge rates. We further show high variability with crossover of discharge rates as a function of recruitment threshold both between participants and between motor units within participants.

Measuring motion of the human foot presents a unique challenge due to the large number of closely packed bones with congruent articulating surfaces. Optical motion capture (OMC) and multi-segment models can be used to infer foot motion, but might be affected by soft tissue artifact (STA). Biplanar videoradiography (BVR) is a relatively new tool that allows direct, non-invasive measurement of bone motion using high-speed, dynamic x-ray images to track individual bones. It is unknown whether OMC and BVR can be used interchangeably to analyse multi-segment foot motion. Therefore, the aim of this study was to determine the agreement in kinematic measures of dynamic activities. Nine healthy participants performed three walking and three running trials while BVR was recorded with synchronous OMC. Bone position and orientation was determined through manual scientific-rotoscoping. The OMC and BVR kinematics were co-registered to the same coordinate system, and BVR tracking was used to create virtual markers for comparison to OMC during dynamic trials. Root mean square (RMS) differences in marker positions and joint angles as well as a linear fit method (LFM) was used to compare the outputs of both methods. When comparing BVR and OMC, sagittal plane angles were in good agreement (ankle: R 2 = 0.947, 0.939; Medial Longitudinal Arch (MLA) Angle: R 2 = 0.713, 0.703, walking and running, respectively). When examining the ankle, there was a moderate agreement between the systems in the frontal plane (R 2 = 0.322, 0.452, walking and running, respectively), with a weak to moderate correlation for the transverse plane (R 2 = 0.178, 0.326, walking and running, respectively). However, root mean squared error (RMSE) showed angular errors ranging from 1.06 to 8.31° across the planes (frontal: 3.57°, 3.67°, transverse: 4.28°, 4.70°, sagittal: 2.45°, 2.67°, walking and running, respectively). Root mean square (RMS) differences between OMC and BVR marker trajectories were task dependent with the largest differences in the shank (6.0 ± 2.01 mm) for running, and metatarsals (3.97 ± 0.81 mm) for walking. Based on the results, we suggest BVR and OMC provide comparable solutions to foot motion in the sagittal plane, however, interpretations of out-of-plane movement should be made carefully.

The subtalar joint (STJ) contributes to the absorption and generation of mechanical energy (and power) during walking to maintain frontal plane stability. Previous observational studies have suggested that there may be a relationship between step width and STJ supination moment. This study directly tests the hypothesis that walking with a step width greater than preferred would reduce STJ moments, energy absorption, and power generation requirements, while increasing energy absorption at the hip during initial contact. Participants (n = 12, 7 females) were asked to walk on an instrumented treadmill at a constant velocity and cadence at a range of fixed step widths ranging from 0.1 to 0.4 times leg length (L). Walking at step widths greater than preferred (0.149 ± 0.04 L) reduced peak STJ moments at initial contact and propulsion which subsequently reduced the negative and positive work performed at the STJ. There was a 43% reduction in energy absorption (negative work) and approximately 30% decrease in positive work at the STJ as step width increased from 0.1 L to 0.4 L. An increase in energy absorption at the knee and hip was evident with an increase in step width during initial contact, although minimal mechanical changes were observed at the proximal joints during propulsion. These results suggest an increase in step width reduces the forces generated by muscles at the STJ across stance and is therefore likely to be beneficial in the prevention and treatment of their injuries. In terms of rehabilitation, the increase in mechanical costs occurring due to an increase in energy absorption by the hip and knee is of minimal concern., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The elastic tendinous tissues of distal lower limb muscles can improve the economy of walking and running, amplify the power generated by a muscle and absorb energy. This paper explores the behaviour of the tibialis anterior (TA) muscle and its tendinous tissue during gait, as it absorbs energy during contact and controls foot position during swing. Simultaneous measurements of ultrasound, surface electromyography and 3D motion capture with musculoskeletal modelling from 12 healthy participants were recorded as they walked at preferred and fast walking speeds. We quantified the length changes and velocities of the TA muscle-tendon unit (MTU) and its fascicles across the stride at each speed. Fascicle length changes and velocities were relatively consistent across speeds, although the magnitude of fascicle length change differed between the deep and superficial regions. At contact, when the TA is actively generating force, the fascicles remained relatively isometric as the MTU actively lengthened, presumably stretching the TA tendinous tissue and absorbing energy. This potentially protects the muscle fibres from damage during weight acceptance and allows energy to be returned to the system later in the stride. During early swing, the fascicles and MTU both actively shortened to dorsiflex the foot, clearing the toes from the ground; however, at the fast walking velocity, the majority of shortening occurred through tendinous tissue recoil, highlighting its role in accelerating ankle dorsiflexion to power rapid foot clearance in swing., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2019. Published by The Company of Biologists Ltd.)

At a constant power output, cyclists prefer to use a higher cadence than those that minimize metabolic cost. The neuromuscular mechanism underpinning the preferred higher cadence remains unclear., Purpose: The aim of this study was to investigate the effect of cadence on joint level work and vastus lateralis (VL) fascicle mechanics while cycling at a constant, submaximal, power output. We hypothesized that preferred cycling cadence would enhance the power capacity of the VL muscle when compared with a more economical cadence. Furthermore, we predicted that the most economical cadence would coincide with minimal total electromyographic activity from the leg muscles., Methods: Metabolic cost, lower-limb kinematics, joint level work, VL fascicle mechanics, and muscle activation of the VL, rectus femoris, biceps femoris, gastrocnemius medialis, and soleus muscles were measured during cycling at a constant power output of 2.5 W·kg and cadences of 40, 60, 80, and 100 rpm. A preferred condition was also performed where cadence feedback was hidden from the participant., Results: Metabolic cost was lowest at 60 rpm, but the mean preferred cadence was 81 rpm. The distribution of joint work remained constant across cadences, with the majority of positive work being performed at the knee. The preferred cadence coincided with the highest VL power capacity, without a significant penalty to efficiency, based on fascicle shortening velocity., Conclusions: Cycling at a higher cadence is preferred to ensure that the muscle's ability to produce positive power remains high. Further investigations are required to examine what feedback mechanism could be responsible for the optimization of this motor pattern.

New Findings: What is the central question of this study? What factors influence the onset and magnitude of activation of the neck inspiratory muscles during inspiration? What is the main finding and its importance? Recruitment of the sternocleidomastoid and scalene muscles during inspiration, measured by means of surface EMG, was strongly correlated with maximal inspiratory pressure. This result indicates that muscle recruitment depends on the capacity of an individual to generate inspiratory pressure. Surface measurements of neck inspiratory muscle EMG activity might complement tests currently used for the screening of respiratory-related disease., Abstract: The aims of the present study were as follows: (i) to examine the relationship between the onset of recruitment of the neck inspiratory muscles and inspiratory muscle strength; and (ii) to clarify the effect of inspiratory resistance on neck inspiratory muscle activation during inspiration at specific flow rates and to specific lung volumes. Inspiratory muscle strength, as indicated by maximal inspiratory pressure (MIP), and peak inspiratory flow rate (PFR) were measured in healthy participants. Subsequently, participants inspired at target inspiratory flow rates between 20 and 100% of PFR as closely as possible, with and without artificial inspiratory resistance. Electromyographic activity (EMG RMS ) of the sternocleidomastoid and scalene muscles was measured from surface electrodes at each target flow rate for each 10% increment of forced vital capacity (FVC) between 20 and 50% of FVC. Recruitment onset for each muscle was determined from %PFR-EMG RMS curves at each lung volume (%FVC). Finally, linear regression analyses were performed for MIP and recruitment onset for each muscle at each %FVC. Recruitment onset during inspiration without inspiratory resistance was strongly correlated with MIP (r > 0.60, P < 0.040). Specifically, a lower MIP was associated with earlier muscle recruitment (i.e. recruitment at a lower flow rate), especially for the sternocleidomastoid muscle (r > 0.75, P < 0.005). Recruitment of both neck inspiratory muscles at a given flow rate was also earlier when inspiratory resistance was added (P = 0.002). These results indicate that the recruitment and activation of the neck inspiratory muscles depends on both inspiratory muscle strength and inspiratory resistance., (© 2019 The Authors. Experimental Physiology © 2019 The Physiological Society.)

Human feet have evolved to facilitate bipedal locomotion, losing an opposable digit that grasped branches in favor of a longitudinal arch (LA) that stiffens the foot and aids bipedal gait. Passive elastic structures are credited with supporting the LA, but recent evidence suggests that plantar intrinsic muscles (PIMs) within the foot actively contribute to foot stiffness. To test the functional significance of the PIMs, we compared foot and lower limb mechanics with and without a tibial nerve block that prevented contraction of these muscles. Comparisons were made during controlled limb loading, walking, and running in healthy humans. An inability to activate the PIMs caused slightly greater compression of the LA when controlled loads were applied to the lower limb by a linear actuator. However, when greater loads were experienced during ground contact in walking and running, the stiffness of the LA was not altered by the block, indicating that the PIMs' contribution to LA stiffness is minimal, probably because of their small size. With the PIMs blocked, the distal joints of the foot could not be stiffened sufficiently to provide normal push-off against the ground during late stance. This led to an increase in stride rate and compensatory power generated by the hip musculature, but no increase in the metabolic cost of transport. The results reveal that the PIMs have a minimal effect on the stiffness of the LA when absorbing high loads, but help stiffen the distal foot to aid push-off against the ground when walking or running bipedally., Competing Interests: The authors declare no conflict of interest., (Copyright © 2019 the Author(s). Published by PNAS.)

The human foot is uniquely stiff to enable forward propulsion, yet also possesses sufficient elasticity to act as an energy store, recycling mechanical energy during locomotion. Historically, this dichotomous function has been attributed to the passive contribution of the plantar aponeurosis. However, recent evidence highlights the potential for muscles to modulate the energetic function of the foot actively. Here, we test the hypothesis that the central nervous system can actively control the foot's energetic function, via activation of the muscles within the foot's longitudinal arch. We used a custom-built loading apparatus to deliver cyclical loads to human feet in vivo, to deform the arch in a manner similar to that observed in locomotion. We recorded foot motion and forces, alongside muscle activation and ultrasound images from flexor digitorum brevis (FDB), an intrinsic foot muscle that spans the arch. When active, the FDB muscle fascicles contracted in an isometric manner, facilitating elastic energy storage in the tendon, in addition to the energy stored within the plantar aponeurosis. We propose that the human foot is akin to an active suspension system for the human body, with mechanical and energetic properties that can be actively controlled by the central nervous system. NEW & NOTEWORTHY The human foot is renowned for its ability to recycle mechanical energy during locomotion, contributing up to 17% of the energy required to power a stride. This mechanism has long been considered passive in nature, facilitated by the elastic ligaments within the arch of the foot. In this paper, we present the first direct evidence that the intrinsic foot muscles also contribute to elastic energy storage and return within the human foot. Isometric contraction of the flexor digitorum brevis muscle tissue facilitates tendon stretch and recoil during controlled loading of the foot. The significance of these muscles has been greatly debated by evolutionary biologists seeking to understand the origins of upright posture and gait, as well as applied and clinical scientists. The data we present here show a potential function for these muscles in contributing to the energetic function of the human foot.

Changes in muscle fascicle mechanics have been postulated to underpin the repeated bout effect (RBE) observed following exercise-induced muscle damage (EIMD). However, in the medial gastrocnemius (MG), mixed evidence exists on whether fascicle stretch amplitude influences the level of EIMD, thus questioning whether changes in fascicle mechanics underpin the RBE. An alternative hypothesis is that neural adaptations contribute to the RBE in this muscle. The aim of this study was to investigate the neuromechanical adaptations during and after repeated bouts of a highly controlled muscle lengthening exercise that aimed to maximize EIMD in MG. In all, 20 subjects performed two bouts of 500 active lengthening contractions (70% of maximal activation) of the triceps surae, separated by 7 days. Ultrasound constructed fascicle length-torque (L-T) curves of MG, surface electromyography (EMG), maximum torque production, and muscle soreness were assessed before, 2 hours and 2 days after each exercise bout. The drop in maximum torque (4%) and the increase in muscle soreness (24%) following the repeated bout were significantly less than following the initial bout (8% and 59%, respectively), indicating a RBE. However, neither shift in the L-T curve nor changes in EMG parameters were present. Furthermore, muscle properties during the exercise were not related to the EIMD or RBE. Our results show that there are no global changes in gastrocnemius mechanical behavior or neural activation that could explain the observed RBE in this muscle. We suggest that adaptations in the non-contractile elements of the muscle are likely to explain the RBE in the triceps surae., (© 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

The human foot contains passive elastic tissues that have spring-like qualities, storing and returning mechanical energy and other tissues that behave as dampers, dissipating energy. Additionally the intrinsic and extrinsic foot muscles have the capacity to act as dampers and motors, dissipating and generating mechanical energy. It remains unknown as to how the contribution of all passive and active tissues combine to produce the overall energetic function of the foot during running. Therefore, the aim of this study was to determine if the foot behaves globally as an active spring-damper during running. Fourteen participants ran on a force-instrumented treadmill at 2.2 ms -1 , 3.3 ms -1 and 4.4 ms -1 , while foot segment motion was collected simultaneously with kinetic measurements. A unified deformable segment model was applied to quantify the instantaneous power of the foot segment during ground contact and mechanical work was calculated by integrating the foot power data. At all running speeds, the foot absorbed energy from early stance through to mid-stance and subsequently returned/generated a proportion of this energy in late stance. The magnitude of negative work performed increased with running speed, while the magnitude of positive work remained relatively constant across all running speeds. The proportion of energy dissipated relative to that absorbed (foot dissipation-ratio) was always greater than zero and increased with running speed, suggesting that the foot behaves as a viscous spring-damper.

Purpose: Foot orthoses maybe used in the management of musculoskeletal disorders related to abnormal subtalar joint (STJ) pronation. However, the precise mechanical benefits of foot orthoses for preventing injuries associated with the STJ are not well understood. The aim of this study was to investigate the immediate effect of foot orthoses on the energy absorption requirements of the STJ and subsequently tibialis posterior (TP) muscle function., Methods: Eighteen asymptomatic subjects with a pes planus foot posture were prescribed custom-made foot orthoses made from a plaster cast impression. Participants walked at preferred and fast velocities barefoot, with athletic footwear and with athletic footwear plus orthoses, as three-dimensional motion capture, force data, and intramuscular electromyography of the TP muscle were simultaneously collected. Statistical parametric mapping was used to identify time periods across the stride cycle during which footwear with foot orthoses significantly differed to barefoot and footwear only., Results: During early stance, footwear alone and footwear with orthoses significantly reduced TP muscle activation (1%-12%), supination moments (3%-21%), and energy absorption (5%-12%) at the STJ, but had no effect on STJ pronation displacement., Conclusions: The changes in TP muscle activation and STJ energy absorption were primarily attributed to footwear because the addition of foot orthoses provided little additional effect. We speculate that these results are most likely a result of the compliant material properties of footwear. These results suggest that athletic footwear may be sufficient to absorb energy in the frontal plane and potentially reducing any benefit associated with the addition of foot orthoses.

The factors that drive variable aponeurosis behaviors in active versus passive muscle may alter the longitudinal stiffness of the aponeurosis during contraction, which may change the fascicle strains for a given muscle force. However, it remains unknown whether these factors can drive variable aponeurosis behaviors across different muscle-tendon unit (MTU) lengths and influence the subsequent fascicle strains during contraction. Here, we used ultrasound and elastography techniques to examine in vivo muscle fascicle behavior and central aponeurosis deformations of human tibialis anterior (TA) during force-matched voluntary isometric dorsiflexion contractions at three MTU lengths. We found that increases in TA MTU length increased both the length and apparent longitudinal stiffness of the central aponeurosis at low and moderate muscle forces ( P < 0.01). We also found that increased aponeurosis stiffness was directly related to reduced magnitudes of TA muscle fascicle shortening for the same change in force ( P < 0.01). The increase in slope and shift to longer overall lengths of the active aponeurosis force-length relationship as MTU length increased was likely due to a combination of parallel lengthening of aponeurosis and greater transverse aponeurosis strains. This study provides in vivo evidence that human aponeurosis stiffness is increased from low to moderate forces and that the fascicle strains for a given muscle force are MTU length dependent. Further testing is warranted to determine whether MTU length-dependent stiffness is a fundamental property of the aponeurosis in pennate muscles and evaluate whether this property can enhance muscle performance., Competing Interests: The authors declare no conflict of interest.

The maximum force-generating capacity of a muscle is dependent on the lengths and velocities of its contractile apparatus. Muscle-tendon unit (MTU) length changes can be estimated from joint kinematics; however, contractile element length changes are more difficult to predict during dynamic contractions. The aim of this study was to compare vastus lateralis (VL) MTU and fascicle level force-length and force-velocity relationships, and dynamic muscle function while cycling at a constant submaximal power output (2.5 W/kg) with different cadences. We hypothesized that manipulating cadence at a constant power output would not affect VL MTU shortening, but significantly affect VL fascicle shortening. Furthermore, these differences would affect the predicted force capacity of the muscle. Using an isokinetic dynamometer and B-mode ultrasound (US), we determined the force-length and force-velocity properties of the VL MTU and its fascicles. In addition, three-dimensional kinematics and kinetics of the lower limb, as well as US images of VL fascicles were collected during submaximal cycling at cadences of 40, 60, 80, and 100 rotations per minute. Ultrasound measures revealed a significant increase in fascicle shortening as cadence decreased (84% increase across all conditions, P < 0.01), whereas there were no significant differences in MTU lengths across any of the cycling conditions (maximum of 6%). The MTU analysis resulted in greater predicted force capacity across all conditions relative to the force-velocity relationship ( P < 0.01). These results reinforce the need to determine muscle mechanics in terms of separate contractile element and connective tissue length changes during isokinetic contractions, as well as dynamic movements like cycling. NEW & NOTEWORTHY We demonstrate that vastus lateralis (VL) muscle tendon unit (MTU) length changes do not adequately reflect the underlying fascicle mechanics during cycling. When examined across different pedaling cadence conditions, the force-generating potential measured only at the level of MTU (or joint) overestimated the maximum force capacity of VL compared with analysis using fascicle level data.

Purpose: The aim of this study was to investigate the influence of foot-strike technique on longitudinal arch mechanics and intrinsic foot muscle function during running., Methods: Thirteen healthy participants ran barefoot on a force-instrumented treadmill at 2.8 ms with a forefoot (FFS) and rearfoot (RFS; habitual) running technique, whereas kinetic, kinematic, and electromyographic data from the intrinsic foot muscles were collected simultaneously. The longitudinal arch was modeled as a single "midfoot" joint representing motion of the rearfoot (calcaneus) relative to the forefoot (metatarsals). An inverse dynamic analysis was performed to estimate joint moments generated about the midfoot, as well as mechanical work and power., Results: The midfoot was more plantar flexed (higher arch) at foot contact when running with a forefoot running technique (RFS 0.2 ± 1.8 vs FFS 6.9 ± 3.0°, effect size (ES) = 2.7); however, there was no difference in peak midfoot dorsiflexion in stance (RFS -11.6 ± 3.0 vs FFS -11.4 ± 3.4°, ES = 0.63). When running with a forefoot technique, participants generated greater moments about the midfoot (27% increase, ES = 1.1) and performed more negative work (240% increase, ES = 2.2) and positive work (42% increase, ES = 1.1) about the midfoot. Average stance-phase muscle activation was greater for flexor digitorum brevis (20% increase, ES = 0.56) and abductor hallucis (17% increase, ES = 0.63) when running with a forefoot technique., Conclusions: Forefoot running increases loading about the longitudinal arch and also increases the mechanical work performed by the intrinsic foot muscles. These findings have substantial implications in terms of injury prevention and management for runners who transition from a rearfoot to a forefoot running technique.

During human walking, the tibialis posterior (TP) tendon absorbs energy in early stance as the subtalar joint (STJ) pronates. However, it remains unclear whether an increase in energy absorption between individuals, possibly a result of larger STJ pronation displacement, is fulfilled by greater magnitudes of TP tendon or muscle fascicle strain. By collecting direct measurements of muscle fascicle length (ultrasound), MTU length (3D motion capture and musculoskeletal modelling), and TP muscle activation (intramuscular electromyography) we endeavoured to illustrate that the TP tendinous tissue fulfils the requirements for energy absorption at the STJ as a result of an increase in muscle force production. While a significant relationship between TP tendon strain, energy absorption at the STJ (R 2  = 0.53, P = < 0.01) and STJ pronation (R 2  = 0.53, P = < 0.01) was evident, we failed to find any significant associations between tendon strain and surrogate measure of TP muscle force (TP muscle activation together with ankle and subtalar joint moments). These results suggest that TP tendon compliance may explain the variance in pronation and energy absorption at the STJ. Therefore, as the tendinous tissue of the TP is accountable for the absorption of energy at the STJ it may be predisposed to strain-induced injury.

Ultrasonography is a useful technique to study muscle contractions in vivo, however larger muscles like vastus lateralis may be difficult to visualise with smaller, commonly used transducers. Fascicle length is often estimated using linear trigonometry to extrapolate fascicle length to regions where the fascicle is not visible. However, this approach has not been compared to measurements made with a larger field of view for dynamic muscle contractions. Here we compared two different single-transducer extrapolation methods to measure VL muscle fascicle length to a direct measurement made using two synchronised, in-series transducers. The first method used pennation angle and muscle thickness to extrapolate fascicle length outside the image (extrapolate method). The second method determined fascicle length based on the extrapolated intercept between a fascicle and the aponeurosis (intercept method). Nine participants performed maximal effort, isometric, knee extension contractions on a dynamometer at 10° increments from 50 to 100° of knee flexion. Fascicle length and torque were simultaneously recorded for offline analysis. The dual transducer method showed similar patterns of fascicle length change (overall mean coefficient of multiple correlation was 0.76 and 0.71 compared to extrapolate and intercept methods respectively), but reached different absolute lengths during the contractions. This had the effect of producing force-length curves of the same shape, but each curve was shifted in terms of absolute length. We concluded that dual transducers are beneficial for studies that examine absolute fascicle lengths, whereas either of the single transducer methods may produce similar results for normalised length changes, and repeated measures experimental designs., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Introduction/aim: Foot structure has been implicated as a risk factor of numerous overuse injuries, however, the mechanism linking foot structure and the development of soft-tissue overuse injuries are not well understood. The aim of this study was to identify factors that could predict foot function during walking., Methods: A total of eleven variables (including measures of foot structure, anthropometry and spatiotemporal gait characteristics) were investigated for their predictive ability on identifying kinematic, kinetic and energetic components of the foot. Three-dimensional motion capture and force data were collected at preferred walking speed on an instrumented treadmill. Mechanical measures were subsequently assessed using a custom multi-segment foot model in Opensim. Factors with significant univariate associations were entered into multiple linear regression models to identify a group of factors independently associated with the mechanical measures., Results: Although no model could be created for any of the kinematic measures analysed, approximately 46% and 37% of the variance in the kinetic and energetic measures were associated with three or two factors respectively. Arch-height ratio, foot length and step width were associated with peak subtalar joint (STJ) moment, while greater STJ negative work was correlated to a low arch-height ratio and greater foot mobility., Conclusion: The models presented in this study suggest that the soft-tissue structures of a flat-arched, mobile foot are at a greater risk of injury as they have greater requirements to absorb energy and generate larger forces. However, as these associations are only moderate, other measures may also have an influence., (Copyright © 2017. Published by Elsevier B.V.)

Muscle spindles provide exquisitely sensitive proprioceptive information regarding joint position and movement. Through passively driven length changes in the muscle-tendon unit (MTU), muscle spindles detect joint rotations because of their in-parallel mechanical linkage to muscle fascicles. In human microneurography studies, muscle fascicles are assumed to follow the MTU and, as such, fascicle length is not measured in such studies. However, under certain mechanical conditions, compliant structures can act to decouple the fascicles, and, therefore, the spindles, from the MTU. Such decoupling may reduce the fidelity by which muscle spindles encode joint position and movement. The aim of the present study was to measure, for the first time, both the changes in firing of single muscle spindle afferents and changes in muscle fascicle length in vivo from the tibialis anterior muscle (TA) during passive rotations about the ankle. Unitary recordings were made from 15 muscle spindle afferents supplying TA via a microelectrode inserted into the common peroneal nerve. Ultrasonography was used to measure the length of an individual fascicle of TA. We saw a strong correlation between fascicle length and firing rate during passive ankle rotations of varying rates (0.1-0.5 Hz) and amplitudes (1-9°). In particular, we saw responses observed at relatively small changes in muscle length that highlight the sensitivity of the TA muscle to small length changes. This study is the first to measure spindle firing and fascicle dynamics in vivo and provides an experimental basis for further understanding the link between fascicle length, MTU length, and spindle firing patterns. NEW & NOTEWORTHY Muscle spindles are exquisitely sensitive to changes in muscle length, but recordings from human muscle spindle afferents are usually correlated with joint angle rather than muscle fascicle length. In this study, we monitored both muscle fascicle length and spindle firing from the human tibialis anterior muscle in vivo. Our findings are the first to measure these signals in vivo and provide an experimental basis for exploring this link further., (Copyright © 2017 the American Physiological Society.)

Humans naturally select a cadence that minimizes metabolic cost at a constant walking velocity. The aim of this study was to examine the effects of cadence on the medial gastrocnemius (MG) muscle and tendon interaction, and examine how this might influence lower limb energetics. We hypothesized that cadences higher than preferred would increase MG fascicle shortening velocity because of the reduced stride time. Furthermore, we hypothesized that cadences lower than preferred would require greater MG fascicle shortening to achieve increased muscle work requirements. We measured lower limb kinematics and kinetics, surface electromyography of the triceps surae and MG fascicle length, via ultrasonography, during walking at a constant velocity at the participants' preferred cadence and offsets of ±10%, ±20%, and ±30%. There was a significant increase in MG fascicle shortening with decreased cadence. However, there was no increase in the MG fascicle shortening velocity at cadences higher than preferred. Cumulative MG muscle activation per minute was significantly increased at higher cadences. We conclude that low cadence walking requires more MG shortening work, while MG muscle and tendon function changes little for each stride at higher cadences, driving up cumulative activation costs due to the increase in steps per minute., (© 2016 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

Lateral connections between muscles provide pathways for myofascial force transmission. To elucidate whether these pathways have functional roles in vivo, we examined whether activation could alter the shear between the soleus (SOL) and lateral gastrocnemius (LG) muscles. We hypothesized that selective activation of LG would decrease the stretch-induced shear between LG and SOL. Eleven volunteers underwent a series of knee joint manipulations where plantar flexion force, LG, and SOL muscle fascicle lengths and relative displacement of aponeuroses between the muscles were obtained. Data during a passive full range of motion were recorded, followed by 20° knee extension stretches in both passive conditions and with selective electrical stimulation of LG. During active stretch, plantar flexion force was 22% greater (P < 0.05) and relative displacement of aponeuroses was smaller than during passive stretch (P < 0.05). Soleus fascicle length changes did not differ between passive and active stretches but LG fascicles stretched less in the active than passive condition when the stretch began at angles of 70° and 90° of knee flexion (P < 0.05). The activity-induced decrease in the relative displacement of SOL and LG suggests stronger (stiffer) connectivity between the two muscles, at least at flexed knee joint angles, which may serve to facilitate myofascial force transmission., (© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)

There are high mechanical demands placed on skeletal muscles in movements requiring rapid acceleration of the body or its limbs. Tendons are responsible for transmitting muscle forces, but, because of their elasticity, can manipulate the mechanics of the internal contractile apparatus. Shortening of the contractile apparatus against the stretch of tendon affects force generation according to known mechanical properties; however, the extent to which differences in tendon compliance alter force development in response to a burst of electrical impulses is unclear. To establish the influence of series compliance on force summation, we studied electrically evoked doublet contractions in the cane toad peroneus muscle in the presence and absence of a compliant artificial tendon. Additional series compliance reduced tetanic force by two-thirds, a finding predicted based on the force-length property of skeletal muscle. Doublet force and force-time integral expressed relative to the twitch were also reduced by additional series compliance. Active shortening over a larger range of the ascending limb of the force-length curve and at a higher velocity, leading to a progressive reduction in force-generating potential, could be responsible. Muscle-tendon interaction may also explain the accelerated time course of force relaxation in the presence of additional compliance. Our findings suggest that a compliant tendon limits force summation under constant-length conditions. However, high series compliance can be mechanically advantageous when a muscle-tendon unit is actively stretched, permitting muscle fibres to generate force almost isometrically, as shown during stretch-shorten cycles in locomotor activities. Restricting active shortening would likely favour rapid force development., (© 2016. Published by The Company of Biologists Ltd.)

Compliant tendons permit mechanically unfavourable fascicle dynamics during fixed-end contractions. The purpose of this study was to reduce the effective compliance of tendon and investigate how small reductions in active shortening affect twitch kinetics and contractile performance in response to a second stimulus. The series elastic element (SEE) of the human triceps surae (N=15) was effectively stiffened by applying a 55 ms rotation to the ankle, through a range of 5 deg, at the onset of twitch and doublet [interstimulus interval (ISI) of 80 ms] stimulation. Ultrasonography was employed to quantify lateral gastrocnemius and soleus fascicle lengths. Rotation increased twitch torque (40-75%), rate of torque development (RTD; 124-154%) and torque-time integral (TTI; 70-110%) relative to constant-length contractions at the initial and final joint positions, yet caused only modest reductions in shortening amplitude and velocity. The torque contribution of the second pulse increased when stimulation was preceded by rotation, a finding unable to be explained on the basis of fascicle length or SEE stiffness during contraction post-rotation. A further increase in torque contribution was not demonstrated, nor was an increase in doublet TTI, when the second pulse was delivered during rotation and shortly after the initial pulse (ISI of 10 ms). The depressant effect of active shortening on subsequent torque generation suggests that compliant tendons, by affording large length changes, may limit torque summation. Our findings indicate that changes in tendon compliance shown to occur in response to resistance training or unloading are likely sufficient to considerably alter contractile performance, particularly maximal RTD., (© 2016. Published by The Company of Biologists Ltd.)

The tibialis posterior (TP) muscle is believed to provide mediolateral stability of the subtalar joint during the stance phase of walking as it actively lengthens to resist pronation at foot contact and then actively shortens later in stance to contribute to supination. Because of its anatomical structure of short muscle fibres and long series elastic tissue, we hypothesised that TP would be a strong candidate for energy storage and return. We investigated the potential elastic function of the TP muscle and tendon through simultaneous measurements of muscle fascicle length (ultrasound), muscle tendon unit length (musculoskeletal modelling) and muscle activation (intramuscular electromyography). In early stance, TP fascicles actively shortened as the entire muscle-tendon unit lengthened, resulting in the absorption of energy through stretch of the series elastic tissue. Energy stored in the tendinous tissue from early stance was maintained during mid-stance, although a small amount of energy may have been absorbed via minimal shortening in the series elastic elements and lengthening of TP fascicles. A significant amount of shortening occurred in both the fascicles and muscle-tendon unit in late stance, as the activation of TP decreased and power was generated. The majority of the shortening was attributable to shortening of the tendinous tissue. We conclude that the tendinous tissue of TP serves two primary functions during walking: 1) to buffer the stretch of its fascicles during early stance and 2) to enhance the efficiency of the TP through absorption and return of elastic strain energy., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Introduction: The repeated bout effect characterizes the protective adaptation after a single bout of unaccustomed eccentric exercise that induces muscle damage. Sarcomerogenesis and increased tendon compliance have been suggested as potential mechanisms for the repeated bout effect by preventing muscle fascicles from being stretched onto the descending limb of the length-tension curve (the region where sarcomere damage is thought to occur). In this study, evidence was sought for three possible mechanical changes that would support either the sarcomerogenesis or the increased tendon compliance hypotheses: a sustained rightward shift in the fascicle length-tension relationship, reduced fascicle strain amplitude, and reduced starting fascicle length., Methods: Subjects (n = 10) walked backward downhill (5 km·h, 20% incline) on a treadmill for 30 min on two occasions separated by 7 d. Kinematic data and medial gastrocnemius fascicle lengths (ultrasonography) were recorded at 10-min intervals to compare fascicle strains between bouts. Fascicle length-torque curves from supramaximal tibial nerve stimulation were constructed before, 2 h after, and 2 d after each exercise bout., Results: Maximum torque decrement and elevated muscle soreness were present after the first, but not the second, backward downhill walking bout signifying a protective repeated bout effect. There was no sustained rightward shift in the length-torque relationship between exercise bouts, nor decreases in fascicle strain amplitude or shortening of the starting fascicle length., Conclusions: Protection from a repeated bout of eccentric exercise was conferred without changes in muscle fascicle strain behavior, indicating that sarcomerogenesis and increased tendon compliance were unlikely to be responsible. As fascicle strains are relatively small in humans, we suggest that changes to connective tissue structures, such as extracellular matrix remodeling, are better able to explain the repeated bout effect observed here.

Background. Muscles not only shorten during contraction to perform mechanical work, but they also bulge radially because of the isovolumetric constraint on muscle fibres. Muscle bulging may have important implications for muscle performance, however quantifying three-dimensional (3D) muscle shape changes in human muscle is problematic because of difficulties with sustaining contractions for the duration of an in vivo scan. Although two-dimensional ultrasound imaging is useful for measuring local muscle deformations, assumptions must be made about global muscle shape changes, which could lead to errors in fully understanding the mechanical behaviour of muscle and its surrounding connective tissues, such as aponeurosis. Therefore, the aims of this investigation were (a) to determine the intra-session reliability of a novel 3D ultrasound (3DUS) imaging method for measuring in vivo human muscle and aponeurosis deformations and (b) to examine how contraction intensity influences in vivo human muscle and aponeurosis strains during isometric contractions. Methods. Participants (n = 12) were seated in a reclined position with their left knee extended and ankle at 90° and performed isometric dorsiflexion contractions up to 50% of maximal voluntary contraction. 3DUS scans of the tibialis anterior (TA) muscle belly were performed during the contractions and at rest to assess muscle volume, muscle length, muscle cross-sectional area, muscle thickness and width, fascicle length and pennation angle, and central aponeurosis width and length. The 3DUS scan involved synchronous B-mode ultrasound imaging and 3D motion capture of the position and orientation of the ultrasound transducer, while successive cross-sectional slices were captured by sweeping the transducer along the muscle. Results. 3DUS was shown to be highly reliable across measures of muscle volume, muscle length, fascicle length and central aponeurosis length (ICC ≥ 0.98, CV < 1%). The TA remained isovolumetric across contraction conditions and progressively shortened along its line of action as contraction intensity increased. This caused the muscle to bulge centrally, predominantly in thickness, while muscle fascicles shortened and pennation angle increased as a function of contraction intensity. This resulted in central aponeurosis strains in both the transverse and longitudinal directions increasing with contraction intensity. Discussion. 3DUS is a reliable and viable method for quantifying multidirectional muscle and aponeurosis strains during isometric contractions within the same session. Contracting muscle fibres do work in directions along and orthogonal to the muscle's line of action and central aponeurosis length and width appear to be a function of muscle fascicle shortening and transverse expansion of the muscle fibres, which is dependent on contraction intensity. How factors other than muscle force change the elastic mechanical behaviour of the aponeurosis requires further investigation.

Generating high leg power outputs is important for executing rapid movements. Squats are commonly used to increase leg strength and power. Therefore, it is useful to understand factors affecting power output in squatting. We aimed to deconstruct the mechanisms behind why power is maximized at certain resistances in squatting. Ten male rowers (age = 20 ± 2.2 years; height = 1.82 ± 0.03 m; mass = 86 ± 11 kg) performed maximal power squats with resistances ranging from body weight to 80% of their one repetition maximum (1RM). Three-dimensional kinematics was combined with ground reaction force (GRF) data in an inverse dynamics analysis to calculate leg joint moments and powers. System center of mass (COM) velocity and power were computed from GRF data. COM power was maximized across a range of resistances from 40% to 60% 1RM. This range was identified because a trade-off in hip and knee joint powers existed across this range, with maximal knee joint power occurring at 40% 1RM and maximal hip joint power at 60% 1RM. A non-linear system force-velocity relationship was observed that dictated large reductions in COM power below 20% 1RM and above 60% 1RM. These reductions were due to constraints on the control of the movement., (© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.)