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"Super-shoes" are designed to improve endurance running performance by reducing the metabolic demands of running. Although the research on "super-shoes" is still developing and has mostly been studied in male runners, it is not clear how possible mediating factors, such as foot strike pattern, influence the metabolic cost and joint mechanics in competitive female runners. Purpose: The objective of this study is to assess the influence of "super-shoes" on metabolic cost and joint mechanics in competitive female runners and to understand how foot strike patter may influence the footwear effects. Methods: Eighteen competitive female runners ran four 5-min bouts on a force instrumented treadmill at 12.9 km⋅h-1 in 1) Nike Vaporfly Next% 2™(SUPER) and 2) Nike Pegasus 38™(CON) in a randomized and mirrored order. Results: Metabolic power was improved by 4.2% (P < 0.001; d = 0.43) and metatarsophalangeal (MTP) negative work (P < 0.001; d = 1.22), ankle negative work (P = 0.001; d = 0.67), and ankle positive work (P < 0.001; d = 0.97) were all smaller when running in SUPER compared with CON. There was no correlation between foot strike pattern and the between-shoe (CON to SUPER) percentage change for metabolic power (r = 0.093, P = 0.715). Conclusions: Metabolic power improved by 4.2% in "super-shoes" (but only by ~3.2% if controlling for shoe mass differences) in this cohort of competitive female runners, which is a smaller improvement than previously observed in men. The reduced mechanical demand at the MTP and ankle in "super-shoes" are consistent with previous literature and may explain or contribute to the metabolic improvements observed in "super-shoes"; however, foot strike pattern was not a moderating factor for the metabolic improvements of "super-shoes." Future studies should directly compare the metabolic response among different types of "super-shoes" between men and women. [ABSTRACT FROM AUTHOR]

Introduction/Purpose: Shoe longitudinal bending stiffness (LBS) is often considered to influence running economy (RE) and thus, running performance. However, previous results are mixed and LBS levels have not been studied in advanced footwear technology (AFT). The purpose of this study was to evaluate the effects of increased LBS from curved carbon fiber plates embedded within an AFT midsole compared to a traditional running shoe on RE and spatiotemporal parameters. Methods: Twenty‐one male trained runners completed three times 4 min at 13 km/h with two experimental shoe models with a curved carbon fiber plate embedded in an AFT midsole with different LBS values (Stiff: 35.5 N/mm and Stiffest: 43.1 N/mm), and a Control condition (no carbon fiber plate: 20.1 N/mm). We measured energy cost of running (W/kg) and spatiotemporal parameters in one visit. Results: RE improved for the Stiff shoe condition (15.71 ± 0.95 W/kg; p < 0.001; n2 = 0.374) compared to the Control condition (16.13 ± 1.08 W/kg; 2.56%) and Stiffest condition (16.03 ± 1.19 W/kg; 1.98%). However, we found no significant differences between the Stiffest and Control conditions. Moreover, there were no spatiotemporal differences between shoe conditions. Conclusion: Changes in LBS in AFT influences RE suggesting that moderately stiff shoes have the most effective LBS to improve RE in AFT compared to very stiff shoes and traditional, flexible shoe conditions while running at 13 km/h. [ABSTRACT FROM AUTHOR]

The wide variation in muscle fibre type distribution across individuals, along with the very different energy consumption rates in slow versus fast muscle fibres, suggests that muscle fibre typology contributes to inter‐individual differences in metabolic rate during exercise. However, this has been hard to demonstrate due to the gap between a single muscle fibre and full‐body exercises. We investigated the isolated effect of triceps surae muscle contraction velocity on whole‐body metabolic rate during cyclic contractions in individuals a priori selected for their predominantly slow (n = 11) or fast (n = 10) muscle fibre typology by means of proton magnetic resonance spectroscopy (1H‐MRS). Subsequently, we examined their whole‐body metabolic rate during walking and running at 2 m/s, exercises with comparable metabolic rates but distinct triceps surae muscle force and velocity demands (walking: low force, high velocity; running: high force, low velocity). Increasing triceps surae contraction velocity during cyclic contractions elevated net whole‐body metabolic rate for both typology groups. However, the slow group consumed substantially less net metabolic energy at the slowest contraction velocity, but the metabolic difference between groups diminished at faster velocities. Consistent with the more economic force production during slow contractions, the slow group exhibited lower metabolic rates than the fast group while running, whereas metabolic rates were similar during walking. These findings provide important insights into the influence of muscle fibre typology on whole‐body metabolic rate and emphasize the importance of considering muscle mechanical demands to understand muscle fibre typology related differences in whole‐body metabolic rates. Key points: Muscle fibre typology is often suggested to affect whole‐body metabolic rate, yet convincing in vivo evidence is lacking.Using isolated plantar flexor muscle contractions in individuals a priori selected for their predominantly slow or fast muscle fibre typology, we demonstrated that having predominantly slow muscle fibres provides a metabolic advantage during slow muscle contractions, but this benefit disappeared at faster contractions.We extended these results to full‐body exercises, where we demonstrated that higher proportions of slow fibres associated with better economy during running but not when walking.These findings provide important insights into the influence of muscle fibre typology on whole‐body metabolic rate and emphasize the importance of considering muscle mechanical demands to understand muscle fibre typology related differences in whole‐body metabolic rate. [ABSTRACT FROM AUTHOR]

The zero-velocity update (ZUPT) method has become a popular approach to estimate foot kinematics from foot worn inertial measurement units (IMUs) during walking and running. However, the accuracy of the ZUPT method for stride parameters at sprinting speeds remains unknown, specifically when using sensors with characteristics well suited for sprinting (i.e., high accelerometer and gyroscope ranges and sampling rates). Seventeen participants performed 80-meter track sprints while wearing a Blue Trident IMeasureU IMU. Two cameras, at 20 and 70 meters from the start, were used to validate the ZUPT method on a stride-by-stride and on a cumulative distance basis. In particular, the validity of the ZUPT method was assessed for: (1) estimating a single stride length attained near the end of an 80m sprint (i.e., stride at 70m); (2) estimating cumulative distance from ∼20 to ∼70 m; and (3) estimating total distance traveled for an 80-meter track sprint. Individual stride length errors at the 70-meter mark were within -6% to 3%, with a bias of -0.27%. Cumulative distance errors were within -4 to 2%, with biases ranging from -0.85 to -1.22%. The results of this study demonstrate the ZUPT method provides accurate estimates of stride length and cumulative distance traveled for sprinting speeds. [ABSTRACT FROM AUTHOR]

Background: Ethylene and vinyl acetate (EVA) and polyether block amide (PEBA) are recently the most widely used materials for advanced footwear technology (AFT) that has been shown to improve running economy (RE). This study investigated the effects of these midsole materials on RE and biomechanics, in both fresh and worn state (after 450 km). Methods: Twenty‐two male trained runners participated in this study. Subjects ran four 4‐min trials at 13 km‧h−1 with both fresh EVA and PEBA AFT and with the same models with 450 km of wear using a randomized crossover experimental design. We measured energy cost of running (W/kg), spatiotemporal, and neuromuscular parameters. Results: There were significant differences in RE between conditions (p = 0.01; n2 = 0.17). There was a significant increase in energy cost in the worn PEBA condition compared with new (15.21 ± 1.01 and 14.87 ± 0.99 W/kg; p < 0.05; ES = 0.54), without differences between worn EVA (15.13 ± 1.14 W/kg; p > 0.05), and new EVA (15.15 ± 1.13 w/kg; ES = 0.02). The increase in energy cost between new and worn was significantly higher for the PEBA shoes (0.32 ± 0.38 W/kg) but without significant increase for the EVA shoes (0.06 ± 0.58 W/kg) (p < 0.01; ES = 0.51) with changes in step frequency and step length. The new PEBA shoes had lower energy cost than the new EVA shoes (p < 0.05; ES = 0.27) with significant differences between conditions in contact time. Conclusion: There is a clear RE advantage of incorporating PEBA versus EVA in an AFT when the models are new. However, after 450 km of use, the PEBA and EVA shoes had similar RE. [ABSTRACT FROM AUTHOR]

Introduction/purpose: Previous results about shoe longitudinal bending stiffness (LBS) and running economy (RE) show high variability. This study aimed to assess the effects of shoes with increased LBS on RE and performance in trained and national runners. Methods: Twenty-eight male runners were divided into two groups according to their 10-km performance times (trained, 38-45 min and national runners, <34 min). Subjects ran 2 - 3 min (at 9 and 13 km·h-1 for trained, and 13 and 17 km·h-1 for national runners) with an experimental shoe with carbon fiber plate to increase the LBS (Increased LBS) and a control shoe (without carbon fiber plate). We measured energy cost of running (W·kg-1) and spatiotemporal parameters in visit one and participants performed a 3000 m time trial (TT) in two successive visits. Results: Increased LBS improved RE in the trained group at slow (11.41 ± 0.93 W·kg-1 vs 11.86 ± 0.93 W·kg-1) and fast velocity (15.89 ± 1.24 W·kg--1 vs 16.39 ± 1.24 W·kg-1) and only at the fast velocity in the national group (20.35 ± 1.45 W·kg-1 vs 20.78 ± 1.18 W·kg-1). The improvements in RE were accompanied by different changes in biomechanical variables between groups. There were a similar improvement in the 3000 m TT test in Increased LBS for trained (639 ± 59 vs 644 ± 61 s in control shoes) and national runners (569 ± 21 vs 574 ± 21 s in control shoes)withmore constant pace in increased LBS compared with control shoes in both groups. Conclusions: Increasing shoe LBS improved RE at slow and fast velocities in trained runners and only at fast velocity in national runners. However, the 3000m TT test improved similarly in both levels of runners with increased LBS. The improvements in RE are accompanied by small modifications in running kinematics that could explain the difference between the different levels of runners. [ABSTRACT FROM AUTHOR]

While it is well recognized that the preferred stride frequency (PSF) in running closely corresponds to the metabolically optimal frequency, the underlying mechanisms are still unclear. Changes in joint kinematics when altering stride frequency will affect the muscle‐tendon unit lengths and potentially the efficiency of muscles crossing these joints. Here, we investigated how fascicle kinematics and forces of the triceps surae muscle, a highly energy consuming muscle, are affected when running at different stride frequencies. Twelve runners ran on a force measuring treadmill, adopting five different frequencies (PSF; PSF ± 8%; PSF ± 15%), while we measured joint kinematics, whole‐body energy expenditure, triceps surae muscle activity, and soleus (SOL; N = 10) and gastrocnemius medialis (GM; N = 12) fascicle kinematics. In addition, we used dynamic optimization to estimate SOL and GM muscle forces. We found that SOL and GM mean muscle fascicle length during stance followed an inverted U‐relationship with the longest fascicle lengths occurring at PSF. Fascicle lengths were shortest at frequencies lower than PSF. In addition, average SOL force was greater at PSF−15% compared with PSF. Overall, our results suggest that reduced SOL and GM muscle fascicle lengths, associated with reduced muscle force potential, together with greater SOL force demand, contribute to the increased whole‐body energy expenditure when running at lower than PSF. At higher stride frequencies, triceps surae muscle kinematics and force production were less affected suggesting that increased energy expenditure is rather related to higher cost of leg swing and greater cost of force production. [ABSTRACT FROM AUTHOR]

The benefits of drafting for elite marathon runners are intuitive, but the quantitative energetic and time savings are still unclear due to the different methods used for converting aerodynamic drag force reductions to gross metabolic power savings. Further, we lack a mechanistic understanding of the relationship between aerodynamic drag forces and ground reaction forces (GRFs) over a range of running velocities. Here, we quantified how small horizontal impeding forces affect gross metabolic power and GRF over a range of velocities in competitive runners. In three sessions, 12 runners completed six 5-min trials with 5 min of recovery in-between. We tested one velocity per session (12, 14, and 16 km/h), at three horizontal impeding force conditions (0, 4, and 8 N) applied at the waist of the runners. On average, gross metabolic power increased by 6.13% per 1% body weight of horizontal impeding force but the increases varied considerably between individuals (4.17%-8.14%). With greater horizontal impeding force, braking GRF impulses decreased, whereas propulsive GRF impulses increased, but the impulses were not related to individual changes in gross metabolic power. Combining our findings with those of previous aerodynamics studies, we estimate that for a solo runner (52 kg) at 2-h marathon pace, overcoming aerodynamic drag force (1.39% BW) comprises 7.8% of their gross metabolic power and drafting can save between 3 min 42 s and 5 min 29 s. NEW & NOTEWORTHY We measured the metabolic and biomechanical effects of small horizontal impeding forces (representing realistic aerodynamic drag forces) on high-caliber runners across a range of velocities. Combining our metabolic results with existing aerodynamic models indicates that at 2-h marathon pace, optimal drafting likely allows a marathoner to run between 3 min 42 s and 5 min 29 s faster. Our rule-of-thumb (~6% increase in gross metabolic power per 1% body weight of horizontal impeding force) will allow others to estimate the performance enhancement of different drafting formations. [ABSTRACT FROM AUTHOR]

Older adults and neurological populations tend to walk with slower speeds, more gait variability, and a higher metabolic cost. This higher metabolic cost could be related to their increased gait variability, but this relationship is still unclear. The purpose of this study was to determine how increased step length variability affects the metabolic cost of waking. Eighteen healthy young adults completed a set of 5-minute trials of treadmill walking at 1.20 m/s while we manipulated their step length variability. Illuminated rectangles were projected onto the surface of a treadmill to cue step length variabilities of 0, 5 and 10% (coefficient of variation). Actual step lengths and their variability were tracked with reflective markers on the feet, while metabolic cost was measured using indirect calorimetry. Changes in metabolic cost across habitual walking (no projections) and the three variability conditions were analyzed using a linear mixed effects model. Metabolic power was largest in the 10% condition (4.30 ± 0.23 W/kg) compared to 0% (4.16 ± 0.18 W/kg) and habitual (3.98 ± 0.25 W/kg). The participant's actual step length variability did not match projected conditions for 0% (3.10%) and 10% (7.03%). For every 1% increase in step length variability, there is an 0.7% increase in metabolic cost. Our results demonstrate an association between the metabolic cost of walking and gait step length variability. This suggests that increased gait variability contributes to a portion of the increased cost of walking seen in older adults and neurological populations., Competing Interests: Competing interests The authors declare no competing or financial interests.

Wearable exoskeletons show significant potential for improving gait impairments, such as interlimb asymmetry. However, a more profound understanding of whether exoskeletons are capable of eliciting neural adaptation is needed. This study aimed to characterize how individuals adapt to bilateral asymmetric joint stiffness applied by a hip exoskeleton, similar to split-belt treadmill training. Thirteen unimpaired individuals performed a walking trial on the treadmill while wearing the exoskeleton. The right side of the exoskeleton acted as a positive stiffness torsional spring, pulling the thigh towards the neutral standing position, while the left acted as a negative stiffness spring pulling the thigh away from the neutral standing position. The results showed that this intervention applied by a hip exoskeleton elicited adaptation in spatiotemporal and kinetic gait measures similar to split-belt treadmill training. These results demonstrate the potential of the proposed intervention for retraining symmetric gait.

Abstract: Looney, DP, Hoogkamer, W, Kram, R, Arellano, CJ, and Spiering, BA. Estimating metabolic energy expenditure during level running in healthy, military-age women and men. J Strength Cond Res 37(12): 2496-2503, 2023-Quantifying the rate of metabolic energy expenditure (Ṁ) of varied aerobic exercise modalities is important for optimizing fueling and performance and maintaining safety in military personnel operating in extreme conditions. However, although equations exist for estimating oxygen uptake during running, surprisingly, there are no general equations that estimate Ṁ. Our purpose was to generate a general equation for estimating Ṁ during level running in healthy, military-age (18-44 years) women and men. We compiled indirect calorimetry data collected during treadmill running from 3 types of sources: original individual subject data (n = 45), published individual subject data (30 studies; n = 421), and published group mean data (20 studies, n = 619). Linear and quadratic equations were fit on the aggregated data set using a mixed-effects modeling approach. A chi-squared (χ2) difference test was conducted to determine whether the more complex quadratic equation was justified (p < 0.05). Our primary indicator of model goodness-of-fit was the root-mean-square deviation (RMSD). We also examined whether individual characteristics (age, height, body mass, and maximal oxygen uptake [V̇O2max]) could minimize prediction errors. The compiled data set exhibited considerable variability in Ṁ (14.54 ± 3.52 W·kg-1), respiratory exchange ratios (0.89 ± 0.06), and running speeds (3.50 ± 0.86 m·s-1). The quadratic regression equation had reduced residual sum of squares compared with the linear fit (χ2, 3,484; p < 0.001), with higher combined accuracy and precision (RMSD, 1.31 vs. 1.33 W·kg-1). Age (p = 0.034), height (p = 0.026), and body mass (p = 0.019) were associated with the magnitude of under and overestimation, which was not the case for V̇O2max (p = 0.898). The newly derived running energy expenditure estimation (RE3) model accurately predicts level running Ṁ at speeds from 1.78 to 5.70 m·s-1 in healthy, military-age women and men. Users can rely on the following equations for improved predictions of running Ṁ as a function of running speed (S, m·s-1) in either watts (W·kg-1 = 4.43 + 1.51·S + 0.37·S2) or kilocalories per minute (kcal·kg-1·min-1 = 308.8 + 105.2·S + 25.58·S2)., (Copyright © 2023 Written work prepared by employees of the Federal Government as part of their official duties is, under the U.S. Copyright Act, a “work of the United States Government” for which copyright protection under Title 17 of the United States Code is not available. As such, copyright does not extend to the contributions of employees of the Federal Government.)

Walking on a split-belt treadmill elicits an adaptation response that changes baseline step length asymmetry. The underlying causes of this adaptation, however, are difficult to determine. It has been proposed that effort minimization may drive this adaptation, based on the idea that adopting longer steps on the fast belt, or positive step length asymmetry (SLA), can cause the treadmill to exert net-positive mechanical work on a bipedal walker. However, humans walking on split-belt treadmills have not been observed to reproduce this behavior when allowed to freely adapt. To determine if an effort-minimization motor control strategy would result in experimentally observed adaptation patterns, we conducted simulations of walking on different combinations of belt speeds with a human musculoskeletal model that minimized muscle excitations and metabolic rate. The model adopted increasing amounts of positive SLA and decreased its net metabolic rate with increasing belt speed difference, reaching +42.4% SLA and -5.7% metabolic rate relative to tied-belt walking at our maximum belt speed ratio of 3:1. These gains were primarily enabled by an increase in braking work and a reduction in propulsion work on the fast belt. The results suggest that a purely effort minimization driven split-belt walking strategy would involve substantial positive SLA, and that the lack of this characteristic in human behavior points to additional factors influencing the motor control strategy, such as aversion to excessive joint loads, asymmetry, or instability. NEW & NOTEWORTHY Behavioral observations of split-belt treadmill adaptation have been inconclusive toward its underlying causes. To estimate gait patterns when driven exclusively by one of these possible underlying causes, we simulated split-belt treadmill walking with a musculoskeletal model that minimized its summed muscle excitations. Our model took significantly longer steps on the fast belt and reduced its metabolic rate below tied-belt walking, unlike experimental observations. This suggests that asymmetry is energetically optimal, but human adaptation involves additional factors.

Hundred years ago, Fenn demonstrated that when a muscle shortens faster, its energy liberation increases. Fenn's results were the first of many that led to the general understanding that isometric muscle contractions are energetically cheaper than concentric contractions. However, this evidence is still primarily based on single fiber or isolated (ex vivo) muscle studies and it remains unknown whether this translates to whole body metabolic rate. In this study, we specifically changed the contraction velocity of the ankle plantar flexors and quantified the effects on triceps surae muscle activity and whole body metabolic rate during cyclic plantar flexion (PF) contractions. Fifteen participants performed submaximal ankle plantar flexions (∼1/3 s activation and ∼2/3 s relaxation) on a dynamometer at three different ankle angular velocities: isometric (10° PF), isokinetic at 30°/s (5-15° PF), and isokinetic at 60°/s (0-20° PF) while target torque (25% MVC) and cycle frequency were kept constant. In addition, to directly determine the effect of ankle angular velocity on muscle kinematics we collected gastrocnemius medialis muscle fascicle ultrasound data. As expected, increasing ankle angular velocity increased gastrocnemius medialis muscle fascicle contraction velocity and positive mechanical work ( P < 0.01), increased mean and peak triceps surae muscle activity ( P < 0.01), and considerably increased net whole body metabolic rate ( P < 0.01). Interestingly, the increase in triceps surae muscle activity with fast ankle angular velocities was most pronounced in the gastrocnemius lateralis ( P < 0.05). Overall, our results support the original findings from Fenn in 1923 and we demonstrated that greater triceps surae muscle contraction velocities translate to increased whole body metabolic rate. NEW & NOTEWORTHY Single muscle fiber studies or research on isolated (ex vivo) muscles demonstrated that faster concentric muscle contractions yield increased energy consumption. Here we translated this knowledge to muscle activation and whole body metabolic rate. Increasing ankle angular velocity increased triceps surae contraction velocity and mechanical work, increasing triceps surae muscle activity and substantially elevating whole body metabolic rate. Additionally, we demonstrated that triceps surae muscle activation strategy depends on the mechanical demands of the task.