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Purpose The purpose of this study was to determine whether net metabolic cost of walking is affected by age per se. Methods We selected 10 healthy, active older adults (mean age 75 years) and 10 young adults (mean age 26 years), and determined their preferred overground walking speed. On the same day, in a morning and afternoon session, we had them walk at that speed overground and on a treadmill while we measured oxygen consumption rate. From the latter we subtracted the rate in sitting and calculated net metabolic cost. Results Anthropometrics were not different between the groups nor was preferred walking speed (1.27 m s−1 both groups). There was no difference in net metabolic cost of overground walking between older and young adults (e.g., in the morning 2.64 and 2.56 J kg−1 m−1, respectively, p > 0.05). In the morning session, net metabolic cost of walking was higher on the treadmill than overground in our older adults by 0.6 J kg−1 m−1 (p Conclusion First, there is no effect of age per se on metabolic cost of overground walking. Second, older adults tend to have higher metabolic cost of walking on a treadmill than walking overground at preferred speed, and adaptation may take a long time. The commonly reported age-related elevation of metabolic cost of walking may be due to confounding factors causing preferred walking speed to be lower in older adults, and/or due to older adults reacting differently to treadmill walking than young adults.

Fascicle length of m. vastus lateralis in cyclists has been shown to correlate positively with peak sprint cycling power normalized for lean body mass. We investigated whether vasti morphology affects sprint cycling power via force-length and force-velocity relationships. We simulated isokinetic sprint cycling at pedaling rates ranging from 40 to 150 rpm with a forward dynamic model of the human musculoskeletal system actuated by eight leg muscles. Input of the model was muscle stimulation over time, which was optimized to maximize the average power output over a pedal cycle. This was done for a reference model and for models in which the vasti had equal volume but different morphology. It was found that models with longer muscle fibers but a reduced physiological cross-sectional area of the vasti produced a higher sprint cycling power. This was partly explained by better alignment of the peak power-pedaling rate curve of the vasti with the corresponding curves of the other leg muscles. The highest sprint cycling power was achieved in a model in which the increase in muscle fiber length of the vasti was accompanied by a concomitant shift in optimum knee angle. It was concluded that muscle mechanics can partly explain the positive correlations between fascicle length of m. vastus lateralis and normalized peak sprint cycling power. It should be investigated whether muscle fiber length of the vasti and optimum knee angle are suitable training targets for athletes who want to concurrently improve their sprint and endurance cycling performance. NEW & NOTEWORTHY We simulated isokinetic sprint cycling at pedaling rates ranging from 40 to 150 rpm with a forward dynamic model of the human musculoskeletal system actuated by eight leg muscles. We selectively modified vasti morphology: we lengthened the muscle fibers and reduced the physiological cross-sectional area. The modified model was able to produce a higher sprint cycling power.

Trunk motion is related to the performance and risk of injuries during dynamic sports motions. Optical motion capture is traditionally used to measure trunk motion during dynamic sports motions, but these systems are typically constrained to a laboratory environment. Inertial measurement units (IMUs) might provide a suitable alternative for measuring the trunk orientation during dynamic sports motions. The objective of the present study was to assess the accuracy of the three-dimensional trunk orientation measured using IMUs during dynamic sports motions and isolated anatomical trunk motions. The motions were recorded with two IMUs and an optical motion capture system (gold standard). Ten participants performed a total of 71 sports motions (19 golf swings, 15 one-handed ball throws, 19 tennis serves, and 18 baseball swings) and 125 anatomical trunk motions (42, 41, and 42 trials of lateral flexion, axial rotation, and flexion/extension, respectively). The root-mean-square differences between the IMU- and optical motion capture-based trunk angles were less than 5 degrees, and the similarity between the methods was on average across all trials “very good” to “excellent” (R ≥ 0.85; R2 ≥ 0.80). Across the dynamic sports motions, even higher measures of similarity were found (R ≥ 0.90; R2 ≥ 0.82). When aligned to the relevant segment, the current IMUs are a promising alternative to optical motion capture and previous presented IMU-based systems for the field-based measurement of the three-dimensional trunk orientation during dynamic sports motions and the anatomical trunk motions.

Contains fulltext : 191305.pdf (Publisher’s version ) (Closed access) Walking is one of the preferred exercises among elderly, but could a prolonged walking increase gait variability, a risk factor for a fall in the elderly? Here we determine whether 30 min of treadmill walking increases coefficient of variation of gait in elderly. Because gait responses to exercise depend on fitness level, we included 15 sedentary and 15 active elderly. Sedentary participants preferred a lower gait speed and made smaller steps than the actives. Step length coefficient of variation decreased ~16.9% by the end of the exercise in both the groups. Stride length coefficient of variation decreased ~9% after 10 minutes of walking, and sedentary elderly showed a slightly larger step width coefficient of variation (~2%) at 10 min than active elderly. Active elderly showed higher walk ratio (step length/cadence) than sedentary in all times of walking, but the times did not differ in both the groups. In conclusion, treadmill gait kinematics differ between sedentary and active elderly, but changes over time are similar in sedentary and active elderly. As a practical implication, 30 min of walking might be a good strategy of exercise for elderly, independently of the fitness level, because it did not increase variability in step and stride kinematics, which is considered a risk of fall in this population.

Background:In negotiating stairs, low foot clearance increases the risk of tripping and a fall. Foot clearance may be related to physical fitness, which differs between active and sedentary participants, and be acutely affected by exercise. Impaired stair negotiation could be an acute response to exercise. Here we determined acute changes in foot clearances during stair walking in sedentary (n = 15) and physically active older adults (n = 15) after prolonged exercise.Methods:Kinematic data were acquired during negotiation with a 3-steps staircase while participants walked at preferred speed, before and after 30 min walking at preferred speed and using a treadmill. Foot clearances were compared before and after exercise and between the groups.Results:Sedentary older adults presented larger (0.5 cm for lead and 2 cm for trail leg) toe clearances in ascent, smaller (0.7 cm) heel clearance in the leading foot in descent, and larger (1 cm) heel clearance in the trailing foot in descent than physically active.Conclusion:Sedentary older adults negotiate stairs in a slightly different way than active older adults, and 30 min walking at preferred speed does not affect clearance in stair negotiation.

Background The sit-to-stand task, which involves rising unassisted from sitting on a chair to standing, is important in daily life. Many people with muscle weakness, reduced range of motion or loading-related pain in a particular joint have difficulty performing the task. How should a person suffering from such impairment best perform the sit-to-stand task and, in the case of pain in a particular joint, with reduced loading of that joint? Methods We developed a musculoskeletal model with reference parameter values based on properties of healthy strong subjects. The model's muscle stimulation-time input was optimized using direct collocation to find strategies that yielded successful sit-to-stand task performance with minimum ‘control effort’ for the reference set and modified sets of parameter values, and with constraints on tibiofemoral compression force. Findings The sit-to-stand task could be performed successfully and realistically by the reference model, by a model with isometric knee extensor forces reduced to 40% of reference, by a model with isometric forces of all muscles reduced to 45% of reference, and by the reference model with the tibiofemoral compression force constrained during optimization to 65% of the peak value in the reference condition. Interpretation The strategies found by the model in conditions other than reference could be interpreted well on the basis of cost function and task biomechanics. The question remains whether it is feasible to teach patients with musculoskeletal impairments or joint pain to perform the sit-to-stand task according to strategies that are optimal according to the simulation model.

Purpose: To document the magnitude and time course of human Achilles tendon adaptations (i.e. changes in tendon morphological and mechanical properties) during a 12-week high-load plantar flexion training program. Methods: Ultrasound was used to determine Achilles tendon cross-sectional area (CSA), length and elongation as a function of plantar flexion torque during voluntary plantar flexion. Tendon force–elongation and stress–strain relationships were determined before the start of training (pre-training) and after 4 (post-4), 8 (post-8) and 12 (post-12) training weeks. Results: At the end of the training program, maximum isometric force had increased by 49% and tendon CSA by 17%, but tendon length, maximal tendon elongation and maximal strain were unchanged. Hence, tendon stiffness had increased by 82%, and so had Young’s modulus, by 86%. Significant changes were first detected at post-4 in stiffness (51% increase) and Young’s modulus (87% increase), and at post-8 in CSA (15% increase). Conclusions: Achilles tendon material properties already improved after 4 weeks of high-load training: stiffness increased while CSA remained unchanged. Tendon hypertrophy (increased CSA) was observed after 8 training weeks and contributed to a further increase in Achilles tendon stiffness, but tendon stiffness increases were mostly caused by adaptations in tissue properties.

We examined the effect of boundary conditions in imitation ski jumping on movement dynamics and coordination. We compared imitation ski jumps with – and without – the possibility to generate shear propulsion forces. Six elite ski jumpers performed imitation jumps by jumping from a fixed surface and from a rolling platform. The ground reaction force vector, kinematics of body segments, and EMG of eight lower limb muscles were recorded. Net joint dynamics were calculated using inverse dynamics. The two imitation jumps differed considerably from each other with regard to the dynamics (moments, forces), whereas the kinematics were very similar. Knee power was higher and hip power was lower on the rolling platform than on the fixed surface. Mean EMG levels were very similar for both conditions, but differences in the development of muscle activity were indicated for seven of eight muscles. These differences are reflected in a subtle difference of the alignment of ground reaction force with centre of mass: the ground reaction force runs continuously close to but behind the centre of mass on the rolling platform and fluctuates around it on the fixed surface. This likely reflects a different strategy for controlling angular momentum.

The muscle mass specific mean power output (PMMS,mean) during push-off in jumping is in marmosets (Callithrix jacchus) more than twice that in humans. In the present study it was tested whether this is attributable to differences in muscle contractile properties. In biopsies of marmoset m. vastus lateralis (VL) and m. gastrocnemius medialis (GM) (n=4) fiber type distribution was assessed using fluorescent immunohistochemistry. In single fibers from four marmoset and nine human VL biopsies the force-velocity characteristics were determined. Marmoset VL contained almost exclusively fast muscle fibers (>99.0%), of which 63% were type IIB and 37% hybrid fibers, fibers containing multiple myosin heavy chains. GM contained 9% type I fibers, 44% type IIB and 47% hybrid muscle fibers. The proportions of fast muscle fibers in marmoset VL and GM were substantially larger than those reported in the corresponding human muscles. The curvature of the force-velocity relationships of marmoset type IIB and hybrid fibers was substantially flatter than that of human type I, IIA, IIX and hybrid fibers resulting in substantially higher muscle fiber mass specific peak powers (PFMS,peak). Muscle mass specific peak power output (PMMS,peak) of marmoset whole VL and GM, estimated from their fiber type distributions and force-velocity characteristics were more than twice the estimates for the corresponding human muscles. Since the relative difference in estimated PMMS,peak between marmoset and human is similar to that PMMS,mean during push-off in jumping, it is likely that the difference in in vivo mechanical output between humans and marmosets is attributable to differences in muscle contractile properties.

We investigated adjustments of control to initial posture in squat jumping. Eleven male subjects jumped from three initial postures: preferred initial posture (PP), a posture in which the trunk was rotated 18° more backward (BP) and a posture in which it was rotated 15° more forward (FP) than in PP. Kinematics, ground reaction forces and electromyograms (EMG) were collected. EMG was rectified and smoothed to obtain smoothed rectified EMG ( srEMG ). Subjects showed adjustments in srEMG histories, most conspicuously a shift in srEMG -onset of rectus femoris (REC): from early in BP to late in FP. Jumps from the subjects’ initial postures were simulated with a musculoskeletal model comprising four segments and six Hill-type muscles, which had muscle stimulation ( STIM ) over time as input. STIM of each muscle changed from initial to maximal at STIM -onset, and STIM -onsets were optimized using jump height as criterion. Optimal simulated jumps from BP, PP and FP were similar to jumps of the subjects. Optimal solutions primarily differed in STIM -onset of REC: from early in BP to late in FP. Because the subjects’ adjustments in srEMG -onsets were similar to adjustments of the model’s optimal STIM -onsets, it was concluded that the former were near-optimal. With the model we also showed that near-maximum jumps from BP, PP and FP could be achieved when STIM -onset of REC depended on initial hip joint angle and STIM -onsets of the other muscles were posture-independent. A control theory that relies on a mapping from initial posture to STIM -onsets seems a parsimonious alternative to theories relying on internal optimal control models.

Jump height, defined as vertical displacement in the airborne phase, depends on vertical takeoff velocity. For centuries, researchers have speculated on how jump height is affected by body size and many have adhered to what has come to be known as Borelli's law, which states that jump height does not depend on body size per se. The underlying assumption is that the amount of work produced per kg body mass during the push-off is independent of size. However, if a big body is isometrically downscaled to a small body, the latter requires higher joint angular velocities to achieve a given takeoff velocity and work production will be more impaired by the force-velocity relationship of muscle. In the present study, the effects of pure isometric scaling on vertical jumping performance were investigated using a biologically realistic model of the human musculoskeletal system. The input of the model, muscle stimulation over time, was optimized using jump height as criterion. It was found that when the human model was miniaturized to the size of a mouse lemur, with a mass of about one-thousandth that of a human, jump height dropped from 40 cm to only 6 cm, mainly because of the force-velocity relationship. In reality, mouse lemurs achieve jump heights of about 33 cm. By implication, the unfavourable effects of the small body size of mouse lemurs on jumping performance must be counteracted by favourable effects of morphological and physiological adaptations. The same holds true for other small jumping animals. The simulations for the first time expose and explain the sheer magnitude of the isolated effects of isometric downscaling on jumping performance, to be counteracted by morphological and physiological adaptations. © 2013 Maarten Frank Bobbert.

The purpose of this study was to understand how humans regulate their ‘leg stiffness’ in hopping, and to determine whether this regulation is intended to minimize energy expenditure. ‘Leg stiffness’ is the slope of the relationship between ground reaction force and displacement of the centre of mass (CM). Variations in leg stiffness were achieved in six subjects by having them hop at maximum and submaximum heights at a frequency of 1.7 Hz. Kinematics, ground reaction forces and electromyograms were measured. Leg stiffness decreased with hopping height, from 350 N m−1kg−1at 26 cm to 150 N m−1kg−1at 14 cm. Subjects reduced hopping height primarily by reducing the amplitude of muscle activation. Experimental results were reproduced with a model of the musculoskeletal system comprising four body segments and nine Hill-type muscles, with muscle stimulation STIM(t) as only input. Correspondence between simulated hops and experimental hops was poor when STIM(t) was optimized to minimize mechanical energy expenditure, but good when an objective function was used that penalized jerk of CM motion, suggesting that hopping subjects are not minimizing energy expenditure. Instead, we speculated, subjects are using a simple control strategy that results in smooth movements and a decrease in leg stiffness with hopping height.

When human hoppers are surprised by a change in surface stiffness, they adapt almost instantly by changing leg stiffness, implying that neural feedback is not necessary. The goal of this simulation study was first to investigate whether leg stiffness can change without neural control adjustment when landing on an unexpected hard or unexpected compliant (soft) surface, and second to determine what underlying mechanisms are responsible for this change in leg stiffness. The muscle stimulation pattern of a forward dynamic musculoskeletal model was optimized to make the model match experimental hopping kinematics on hard and soft surfaces. Next, only surface stiffness was changed to determine how the mechanical interaction of the musculoskeletal model with the unexpected surface affected leg stiffness. It was found that leg stiffness adapted passively to both unexpected surfaces. On the unexpected hard surface, leg stiffness was lower than on the soft surface, resulting in close-to-normal center of mass displacement. This reduction in leg stiffness was a result of reduced joint stiffness caused by lower effective muscle stiffness. Faster flexion of the joints due to the interaction with the hard surface led to larger changes in muscle length, while the prescribed increase in active state and resulting muscle force remained nearly constant in time. Opposite effects were found on the unexpected soft surface, demonstrating the bidirectional stabilizing properties of passive dynamics. These passive adaptations to unexpected surfaces may be critical when negotiating disturbances during locomotion across variable terrain. Copyright © 2009 the American Physiological Society.

This paper presents a simulation study that was conducted to investigate whether the stereotyped motion pattern observed in human sub-maximal jumping can be interpreted from the perspective of energy expenditure. Human sub-maximal vertical countermovement jumps were compared to jumps simulated with a forward dynamic musculo-skeletal model. This model consisted of four interconnected rigid segments, actuated by six Hill-type muscle actuators. The only independent input of the model was the stimulation of muscles as a function of time. This input was optimized using an objective function, in which targeting a specific sub-maximal height value was combined with minimizing the amount of muscle work produced. The characteristic changes in motion pattern observed in humans jumping to different target heights were reproduced by the model. As the target height was lowered, two major changes occurred in the motion pattern. First, the countermovement amplitude was reduced; this helped to save energy because of reduced dissipation and regeneration of energy in the contractile elements. Second, the contribution of rotation of the heavy proximal segments of the lower limbs to the vertical velocity of the centre of gravity at take-off was less; this helped to save energy because of reduced ineffective rotational energies at take-off. The simulations also revealed that, with the observed movement adaptations, muscle work was reduced through improved relative use of the muscle's elastic properties in sub-maximal jumping. According to the results of the simulations, the stereotyped motion pattern observed in sub-maximal jumping is consistent with the idea that in sub-maximal jumping, subjects are trying to achieve the targeted jump height with minimal energy expenditure. © 2006 Elsevier Ltd. All rights reserved.

A goal of biomechanics and motor control is to understand the design of the human musculoskeletal system. Here we investigated human functional morphology by making predictions about the muscle volume distribution that is optimal for a specific motor task. We examined a well-studied and relatively simple human movement, vertical jumping. We investigated how high a human could jump if muscle volume were optimized for jumping, and determined how the optimal parameters improve performance. We used a four-link inverted pendulum model of human vertical jumping actuated by Hill-type muscles, that well-approximates skilled human performance. We optimized muscle volume by allowing the cross-sectional area and muscle fiber optimum length to be changed for each muscle, while maintaining constant total muscle volume. We observed, perhaps surprisingly, that the reference model, based on human anthropometric data, is relatively good for vertical jumping; it achieves 90% of the jump height predicted by a model with muscles designed specifically for jumping. Alteration of cross-sectional areas-which determine the maximum force deliverable by the muscles-constitutes the majority of improvement to jump height. The optimal distribution results in large vastus, gastrocnemius and hamstrings muscles that deliver more work, while producing a kinematic pattern essentially identical to the reference model. Work output is increased by removing muscle from rectus femoris, which cannot do work on the skeleton given its moment arm at the hip and the joint excursions during push-off. The gluteus composes a disproportionate amount of muscle volume and jump height is improved by moving it to other muscles. This approach represents a way to test hypotheses about optimal human functional morphology. Future studies may extend this approach to address other morphological questions in ethological tasks such as locomotion, and feature other sets of parameters such as properties of the skeletal segments.

AB Purpose: Relationships between tangential pedal force and crank angular velocity in sprint cycling tend to be linear. We set out to understand why they are not hyperbolic, like the intrinsic force-velocity relationship of muscles. Methods: We simulated isokinetic sprint cycling at crank angular velocities ranging from 30 to 150 rpm with a forward dynamic model of the human musculoskeletal system actuated by eight lower extremity muscle groups. The input of the model was muscle stimulation over time, which we optimized to maximize average power output over a cycle. Results: Peak tangential pedal force was found to drop more with crank angular velocity than expected based on intrinsic muscle properties. This linearizing effect was not due to segmental dynamics but rather due to active state dynamics. Maximizing average power in cycling requires muscles to bring their active state from as high as possible during shortening to as low as possible during lengthening. Reducing the active state is a relatively slow process, and hence must be initiated a certain amount of time before lengthening starts. As crank angular velocity goes up, this amount of time corresponds to a greater angular displacement, so the instant of switching off extensor muscle stimulation must occur earlier relative to the angle at which pedal force was extracted for the force-velocity relationship. Conclusion: Relationships between pedal force and crank angular velocity in sprint cycling do not reflect solely the intrinsic force-velocity relationship of muscles but also the consequences of activation dynamics.

Vertical jumping was used to assess muscle mechanical output in bonobos and comparisons were drawn to human jumping. Jump height, defined as the vertical displacement of the body centre of mass during the airborne phase, was determined for three bonobos of varying age and sex. All bonobos reached jump heights above 0.7 m, which greatly exceeds typical human maximal performance (0.3–0.4 m). Jumps by one male bonobo (34 kg) and one human male (61.5 kg) were analysed using an inverse dynamics approach. Despite the difference in size, the mechanical output delivered by the bonobo and the human jumper during the push-off was similar: about 450 J, with a peak power output close to 3000 W. In the bonobo, most of the mechanical output was generated at the hips. To account for the mechanical output, the muscles actuating the bonobo's hips (directly and indirectly) must deliver muscle-mass-specific power and work output of 615 W kg−1and 92 J kg−1, respectively. This was twice the output expected on the basis of muscle mass specific work and power in other jumping animals but seems physiologically possible. We suggest that the difference is due to a higher specific force (force per unit of cross-sectional area) in the bonobo.

Prediction of speed skating performance with a power balance model requires assumptions about the kinetics of energy production, skating efficiency, and skating technique. The purpose of this study was to evaluate these parameters during competitive imitations for the purpose of improving model predictions. Elite speed skaters ( n = 8) performed races and submaximal efficiency tests. External power output (Po) was calculated from movement analysis and aerodynamic models and ice friction measurements. Aerobic kinetics was calculated from breath-by-breath oxygen uptake (V̇o2). Aerobic power (Paer) was calculated from measured skating efficiency. Anaerobic power (Pan) kinetics was determined by subtracting Paer from Po. We found gross skating efficiency to be 15.8% (1.8%). In the 1,500-m event, the kinetics of Pan was characterized by a first-order system as Pan = 88 + 556 e−0.0494 t (in W, where t is time). The rate constant for the increase in Paer was −0.153 s−1, the time delay was 8.7 s, and the peak Paer was 234 W; Paer was equal to 234[1 − e−0.153(t−8.7)] (in W). Skating position changed with preextension knee angle increasing and trunk angle decreasing throughout the event. We concluded the pattern of Paer to be quite similar to that reported during other competitive imitations, with the exception that the increase in Paer was more rapid. The pattern of Pan does not appear to fit an “all-out” pattern, with near zero values during the last portion of the event, as assumed in our previous model (De Koning JJ, de Groot G, and van Ingen Schenau GJ. J Biomech 25: 573–580, 1992). Skating position changed in ways different from those assumed in our previous model. In addition to allowing improved predictions, the results demonstrate the importance of observations in unique subjects to the process of model construction.

A ski jumper tries to maintain an aerodynamic position in the in-run during changing environmental forces. The purpose of this study was to analyze the mechanical demands on a ski jumper taking the in-run in a static position. We simulated the in-run in ski jumping with a 4-segment forward dynamic model (foot, leg, thigh, and upper body). The curved path of the in-run was used as kinematic constraint, and drag, lift, and snow friction were incorporated. Drag and snow friction created a forward rotating moment that had to be counteracted by a plantar flexion moment and caused the line of action of the normal force to pass anteriorly to the center of mass continuously. The normal force increased from 0.88G on the first straight to 1.65G in the curve. The required knee joint moment increased more because of an altered center of pressure. During the transition from the straight to the curve there was a rapid forward shift of the center of pressure under the foot, reflecting a short but high angular acceleration. Because unrealistically high rates of change of moment are required, an athlete cannot do this without changing body configuration which reduces the required rate of moment changes.

Older people fall more often after tripping than young people due to a slower development of mechanical responses. This might be due to age-related changes in muscle properties, but also to changes in motor control. The purpose of the present study was to determine whether (a) timing and sequencing of muscle activation and (b) the magnitude and rate of development of muscle activation in recovery after a trip differs between young and older subjects. We focused on the support limb, as it contributes to recovery after tripping by counteracting the forward angular momentum. Ten young (25 years) and seven older (68 years) men and women walked over a platform and were tripped several times at different points in the gait cycle. Kinematics and EMG of the support limb muscles were measured. After tripping, rapid EMG responses (60-80 ms) were observed in hamstring and triceps surae muscles in both young and older subjects. A slightly increased delay (11 ms) was found only in the soleus muscle of the older subjects. The muscle activity patterns (timing and sequencing) were similar in young and older subjects, but the magnitude and rate of development of muscle activity were significantly lower in older subjects. Especially the lower rate of development of muscle activation in the support limb of older subjects is likely to reduce the rate of force generation, which can contribute to inadequate recovery responses and falls. © Springer-Verlag 2004.

Concepts of how athletes should expend their aerobic and anaerobic energetic reserves are generally based on results of tests where an "all out" strategy is imposed on/required from the athlete. We sought to determine how athletes spontaneously expend their energetic reserves when the only instruction was to finish the event in minimal time, as in competition. Well trained, and task habituated, road cyclists (N = 14) completed randomly ordered laboratory time trials of 500 m, 1000 m, 1500 m and 3000 m on a windload braked cycle ergometer. The pattern of aerobic and anaerobic energy use was calculated from total work accomplished and V.O (2) during the trials. The events were completed in 40.3 +/- 0.6 s, 87.4 +/- 4.1 s, 133.8 +/- 6.6 s and 296.0 +/- 7.2 s. The peak V.O (2) during the terminal 200 m of all events was similar (2.72 +/- 0.22, 3.01 +/- 0.34, 3.23 +/- 0.44 and 3.12 +/- 0.13 l x min (-1)). In all events, the initial power output and anaerobic energy use was high, and decreased to a more or less constant value over the remainder of the event. However, the subjects seemed to reserve some ability to expend energy anaerobically for a terminal acceleration which is contrary to predictions of an "all out" starting strategy. Although the total work accomplished increased with distance (23.14 +/- 4.24, 34.14 +/- 6.37, 43.54 +/- 6.12 and 78.22 +/- 8.28 kJ), the energy attributable to anaerobic sources was not significantly different between the rides (17.29 +/- 3.82, 18.68 +/- 8.51, 20.60 +/- 6.99 and 23.28 +/- 9.04 kJ). The results are consistent with the concept that athletes monitor their energetic resources and regulate their energetic output over time in a manner designed to optimize performance.

It has been shown in previous research that the initial phase of EMG for a punching movement remained almost unchanged regardless of whether an external force was applied to the arm. The purpose of the present study was to explain this finding with the help of simulations. A two-dimensional model of me arm actuated by 6 Hill-type muscles was used to simulate a punching movement in the horizontal plane from a prescribed starting position with 90° elbow flexion. Input to the model was the stimulation of me muscles, and output were, among others, muscle forces and segmental accelerations. A genetic algorithm was used to determine the muscle onset times mat minimized movement duration and targeting error. In a subsequent forward simulation, the optimized muscle onset times for an unloaded punching movement were superimposed on the isometric stimulation necessary to hold me arm in the starting position while an external force was applied to the arm. The resulting movement was only slightly different from the unloaded movement. It appeared that because of the low level of isometric muscle force prior to the movement, and the high level of stimulation during the movement, muscle force was increased at a rate mat was almost independent of the prior force level. These results confirmed the suggestion that the initial phase of EMG in ballistic movements is more related to the rate of change of force than to the absolute force level. It is hypothesized mat this may simplify the task of the nervous system in the choice of initial muscle activity in ballistic arm movements because no adjustments to varying external forces are required.

We have investigated, in fast movements, the hypothesis that bi-articular muscles are preferentially selected to control me direction of force exerted on the environment, while mono-articular muscles are selected to control both this exerted force direction as well as the movement direction. Fourteen subjects performed ballistic arm movements involving shoulder and elbow rotations in the horizontal plane, either with or without an external force applied at the wrist. Joint torques required to counteract the external force were in the same order of magnitude as those required to overcome the inertial load during movements. EMG was recorded from mono- and bi-articular flexors and extensors of me elbow and shoulder. Signals were rectified and integrated (IREMG) over 100 ms following the first detected activity. MANOVA revealed mat, contrary to the hypothesis, IREMG of bi-articular muscles varied with movement direction just as that of the mono-articular muscles. It was concluded that the present data do not support me hypothesis mentioned above. A second finding was that movement effects on IREMG were much stronger than external force effects. This could not be explained using Hill's force-velocity relationship. It may be an indication that in the initiation of fast movements, IREMG is not only tuned to movement dynamics and muscle contractile properties, but also to me dynamics of the build up of an active state of the muscle.

HOUDIJK, H., A. J. WIJKER, J. J. DE KONING, M. F. BOBBERT, and G. DE GROOT. Ice friction in speed skating: can klapskates reduce ice frictional loss? Med. Sci. Sports Exerc., Vol. 33, No. 3, 2001, pp. 499 ‐504. Purpose: Reducing ice friction was one of the motives for developing the klapskate. However, the magnitude of power dissipation that occurs with conventional skates when a skater plantar flexes his ankle and the tip of the blade is pressed into the ice has not been quantified previously. In this study, we examine how ice friction varies during a single stroke with conventional skates and estimate the reduction in ice friction that might be obtained with klapskates. Methods: Five elite speed skaters performed a series of trials at constant velocity and a series of maximal accelerations. Energy dissipated to ice friction during a stroke with conventional skates was analyzed using an instrumented skate and high-speed 3D kinematic analysis. The energy that would be dissipated when klapskates were used was estimated from the collected data with conventional skates. Results: The estimated difference in power loss between conventional and klapskates was less dramatic than has been suggested frequently. Pressing the tip of the blade into the ice comprises only 0.84 W of the total power dissipated by ice friction (54 W) during constant velocity speed skating. During an all-out acceleration, this power loss reached 4.55 W. Conclusion: We conclude that only a minor part of the benefit of klapskates can be attributed to a reduction in ice friction. It is shown that this relatively small increase in ice friction is related to the large length of the skate blade. Key Words: WINTER SPORTS, EQUIPMENT, TECHNIQUE, BIOMECHANICS, LOCOMOTION

The purposes of this study were to determine the dependence of human squat jump performance on the compliance of series elastic elements (SEEs) of the triceps surae (consisting of the soleus and gastrocnemius) and to explain this dependence. Vertical squat jumps were simulated using an optimal control model of the human musculo-skeletal system. Maximum jump height was found for several values of triceps surae SEE strain at maximum isometric force (ε0). When ε0 was increased from 1 to 10 %, maximum jump height increased by 8 cm. This was partly due to a higher work output of contractile elements (CEs) of the muscles, primarily of the soleus, and also partly to an increased efficacy of converting muscle work to energy contributing to jump height. The soleus produced more work at ε0=10 % because, as a result of SEE recoil, the CE covered its shortening range at lower velocity and hence produced more force. Efficacy was higher at ε0=10 % because a higher vertical velocity at take-off was achieved with a lower rotational energy of the body segments. This apparent discrepancy was explained by increased angular velocities of the shanks and feet, which have small moments of inertia, and decreased angular velocities of the thighs and trunk, which have larger moments of inertia. This redistribution of segmental contributions to the vertical velocity of the centre of mass was possible because the increased compliance of the triceps surae SEE enhanced the energy-buffering capacity of this muscle group and, thereby, allowed for a higher power output at the ankles. It seems that long compliant tendons in the plantar flexors are an elegant solution to the problem of maximizing jumping performance.

In order to assess the significance of the dynamics of neural control signals for the rise time of muscle moment, simulations of isometric and dynamic plantar flexion contractions were performed using electromyographic signals (EMG signals) of m. triceps surae as input. When excitation dynamics of the muscle model was optimized for an M-wave of the medial head of m. gastrocnemius (GM), the model was able to make reasonable predictions of the rise time of muscle moment during voluntary isometric plantar flexion contractions on the basis of voluntary GM EMG signals. The rise time of muscle moment in the model was for the greater part determined by the amplitude of the first EMG burst. For dynamic jumplike movements of the ankle joint, however, no relationship between rise time of muscle moment in the experiment and muscle moment predicted by the model on the basis of GM EMG signals was found. Since rise time of muscle moment varied over a small range for this movement, it cannot be completely excluded that stimulation dynamics plays a role in control of these simple single-joint movements. [S0148-0731(00)01104-3]

VAN ZANDWIJK, J. P., M. F. BOBBERT, M. MUNNEKE, and P. PAS. Control of maximal and submaximal vertical jumps.Med. Sci. Sports Exerc., Vol. 32, No. 2, pp. 477‐ 485, 2000. Purpose: It was investigated to what extent control signals used by human subjects to perform submaximal vertical jumps are related to control signals used to perform maximal vertical jumps. Methods: Eight subjects performed both maximal and submaximal height jumps from a static squatting position. Kinematic and kinetic data were recorded as well as electromyographic (EMG) signals from eight leg muscles. Principal component analysis was used analyze the shape of smoothed rectified EMG (SREMG) histories. Jumps were also simulated with a forward dynamic model of the musculoskeletal system, comprising four segments and six muscles. First, a maximal height jump was simulated by finding the optimal stimulation pattern, i.e., the pattern resulting in a maximum height of the mass center of the body. Subsequently, submaximal jumps were simulated by adapting the optimal stimulation pattern using strategies derived from the experimental SREMG histories. Results: SREMG histories of maximal and submaximal jumps revealed only minor differences in relative timing of the muscles between maximal and submaximal jumps, but SREMG amplitude was reduced in the biarticular muscles. The shape of the SREMG recordings was not much different between the two conditions, even for the biarticular muscles. The simulated submaximal jump resembled to some extent the submaximal jumps found in the experiment, suggesting that differences in control signals as inferred from the experimental data could indeed be sufficient to get the observed behavior. Conclusions: The results fit in with theories on the existence of generalized motor programs within the central nervous system, the output of which is determined by the setting of parameters such as amplitude and

The development of the new skate design specifically the klapskate in a historical and scientific perspective is described. Reasons why it took so long for top athletes to use the new skate design is explained. The klapskate demonstrated its advantage over conventional skates and proved its benefits.

The purpose of this study was to investigate the effect of pacing strategies on performance times in the 1000 m time trial event and the 4000 m pursuit event in track cycling. For this purpose, we simulated these events with a model based on the flow of energy in cycling. Different strategies in distributing the available anaerobic energy were evaluated and we compared model predictions of split times and final times with values achieved by cyclists during championships. The best result at the 1000 m time trial was obtained when the cyclist had the highest anaerobic peak power output and used an 'all-out' strategy. The fastest time on the 4000 m pursuit was achieved with an 'all-out' start at a high level of initial power output, followed by a constant anaerobic power output after 12 seconds, resulting in an evenly paced race. The results show that even small variations in pacing strategy may have substantial effects on performance. There seems to be an opportunity to gain a competitive advantage when individual athletes experiment with small variations in pacing strategy to find the precise individual strategy that works best under specific conditions.

The effect of muscle stimulation dynamics on the sensitivity of jumping achievement to variations in timing of muscle stimulation onsets was investigated. Vertical squat jumps were simulated using a forward dynamic model of the human musculoskeletal system. The model calculates the motion of body segments corresponding to STIM(t) of six major muscle groups of the lower extremity, where STIM is muscle stimulation level. For each muscle, STIM was allowed to switch "on" only once. The subsequent rise of STIM to its maximum was described using a sigmoidal curve, the dynamics of which was expressed as rise time (RT). For different values of stimulation RT, the optimal set of onset times was determined using dynamic optimization with height reached by the center of mass as performance criterion. Subsequently, 200 jumps were simulated in which the optimal muscle stimulation onset times were perturbed by adding to each a small number taken from a Gaussian-distributed set of pseudo-random numbers. The distribution of heights achieved in these perturbed jumps was used to quantify the sensitivity of jump height to variations in timing of muscle stimulation onsets. It was found that with increasing RT, the sensitivity of jump height to timing of stimulation onset times decreased. To try and understand this finding, a post-hoc analysis was performed on the perturbed jumps. Jump height was most sensitive to errors in the delay between stimulation onset times of proximal muscles and stimulation onset times of plantar flexors. It is explained how errors in this delay cause aberrations in the configuration of the system, which are regenerative and lead to relatively large jump height deficits. With increasing RT, the initial aberrations due to erroneous timing of muscle stimulation are smaller, and the regeneration is less pronounced. Finally, it is speculated that human subjects decrease or increase RT depending on the relative importance of different performance criteria.

PURPOSE: The purpose of this study was to gain insight into the importance of stimulation dynamics for force development in human vertical jumping.METHODS: Maximum height squat jumps were performed by 21 male subjects. As a measure of signal dynamics, rise time (RT) was used, i.e., the time taken by the signal to increase from 10% to 90% of its peak value. RT were calculated for time histories of smoothed rectified electromyograms (SREMG) of seven lower extremity muscles, net moments about hip, knee, and ankle joints, and components of the ground reaction force vector.RESULTS: Average RT values were 105-143 ms for SREMG signals, 90-112 ms for joint moments, and 120 ms for the vertical component of the ground reaction force (Fz). A coefficient of linear correlation of 0.88 was found between RT of SREMG of m. gluteus maximus (GLU) and RT of Fz. To explain this correlation, it was speculated that for an effective transfer from joint extensions to vertical motion of the center of mass (CM), the motion of CM needs a forward component during the push-off. Given the starting position, only the hip extensor muscles are able to generate such a forward acceleration of CM. To preserve the forward motion of CM, RT of knee and ankle joint moments need to be adjusted to RT of the hip joint moment. Thus, the greater RT of the hip joint moment and RT of GLU-SREMG, the greater RT of Fz.CONCLUSIONS: Overall, it was concluded that the time it takes to develop muscle stimulation has a substantial effect on the dynamics of force development in vertical jumping, and that this effect should not be neglected in studies of the control of explosive movements.

In models describing the excitation of muscle by the central nervous system, it is often assumed that excitation during a tetanic contraction can be obtained by the linear summation of responses to individual stimuli, from which the active state of the muscle is calculated. We investigate here the extent to which such a model describes the excitation of human muscle in vivo. For this purpose, experiments were performed on the calf muscles of four healthy subjects. Values of parameters in the model describing the behaviour of the contractile element (CE) and the series elastic element (SEE) of this muscle group were derived on the basis of a set of isokinetic release contractions performed on a special-purpose dynamometer as well as on the basis of morphological data. Parameter values describing the excitation of the calf muscles were optimized such that the model correctly predicted plantar flexion moment histories in an isometric twitch, elicited by stimulation of the tibial nerve. For all subjects, the model using these muscle parameters was able to make reasonable predictions of isometric moment histories at higher stimulation frequencies. These results suggest that the linear summation of responses to individual stimuli can indeed give an adequate description of the process of human muscle excitation in vivo.

This study investigates the influence of parameter values of the human triceps surae muscle on the torque-angle relationship. The model used consisted of three units, each containing a contractile, a series elastic and a parallel elastic element. Parameter values were based on morphological characteristics, which made it possible to model individual units. However, for a number of parameters the values reported in the literature vary considerably. It was investigated how sensitive model results were for variation of these parameters. Slack length of the series elastic element, mean moment arm, maximum force, and length of the contractile element appeared to be the most important determinants of the behavior. For mean moment arm and contractile element length, morphology-based methods of estimation could be recommended. Slack length and maximum force were obtained through optimization. It was concluded that the model does not contain parameters on which its output depends strongly and which are difficult to estimate as well, with two exceptions: slack length of the series elastic element and maximum force.

In experiments investigating vertical squat jumping, the HAT segment is typically defined as a line drawn from the hip to some point proximally on the upper body (eg, the neck, the acromion), and the hip joint as the angle between this line and the upper legs (θUL-HAT). In reality, the hip joint is the angle between the pelvis and the upper legs (θUL-pelvis). This study aimed to estimate to what extent hip joint definition affects hip joint work in maximal squat jumping. Moreover, the initial pelvic tilt was manipulated to maximize the difference in hip joint work as a function of hip joint definition. Twenty-two male athletes performed maximum effort squat jumps in three different initial pelvic tilt conditions: backward (pelvisB), neutral (pelvisN), and forward (pelvisF). Hip joint work was calculated by integrating the hip net joint torque with respect to θUL-HAT(WUL-HAT) or with respect to θUL-pelvis(WUL-pelvis). θUL-HATwas greater than θUL-pelvisin all conditions. WUL-HAToverestimated WUL-pelvisby 33%, 39%, and 49% in conditions pelvisF, pelvisN, and pelvisB, respectively. It was concluded that θUL-pelvisshould be measured when the mechanical output of hip extensor muscles is estimated.

Force-velocity relationships reported in the literature for functional tasks involving a combination of joint rotations tend to be quasilinear. The purpose of this study was to explain why they are not hyperbolic, like Hill's relationship. For this purpose, a leg press task was simulated with a musculoskeletal model of the human leg, which had stimulation of knee extensor muscles as only independent input. In the task the ankles moved linearly, away from the hips, against an imposed external force that was reduced over contractions from 95 to 5% of the maximum isometric value. Contractions started at 70% of leg length, and force and velocity values were extracted when 80% of leg length was reached. It was shown that the relationship between leg extension velocity and external force was quasi-linear, while the relationship between leg extension velocity and muscle force was hyperbolic. The discrepancy was explained by the fact that segmental dynamics canceled more and more of the muscle force as the external force was further reduced and velocity became higher. External power output peaked when the imposed external force was ∼50% of maximum, while muscle power output peaked when the imposed force was only ∼15% of maximum; in the latter case ∼70% of muscle power was buffered by the leg segments. According to the results of this study, there is no need to appeal to neural mechanisms to explain why, in leg press tasks, the force-velocity relationship is quasilinear rather than hyperbolic. Copyright © 2012 the American Physiological Society.

In this study, we aim to investigate whether motor commands, emanating from movement planning, are customized to movement orientation relative to gravity from the first trial on. Participants made fast point-to-point elbow flexions and extensions in the transverse plane. We compared movements that had been practiced in reclined orientation either against or with gravity with the same movement relative to the body axis made in the upright orientation (neutral compared to gravity). For each movement type, five rotations from reclined to upright orientation were made. For each rotation, we analyzed the first trial in upright orientation and the directly preceding trial in reclined orientation. Additionally, we analyzed the last five trials of a 30-trial block in upright position and compared these trials with the first trials in upright orientation. Although participants moved fast, gravitational torques were substantial. The change in body orientation affected movement planning: we found a decrease in peak angular velocity and a decrease in amplitude for the first trials made in the upright orientation, regardless of whether the previous movements in reclined orientation were made against or with gravity. We found that these decreases disappeared after participants familiarized themselves with moving in upright position in a 30-trial block. These results indicate that participants used a general strategy, corresponding to the strategy observed in situations with unreliable or limited information on external conditions. From this, we conclude that during movement planning, a priori knowledge of gravity was not used to specifically customize motor commands for the neutral gravity condition. © The Author(s) 2011.

For repeated point-to-point arm movements it is often assumed that motor commands are customized in a trial-to-trial manner, based on previous endpoint error. To test this assumption, we perturbed movement execution without affecting the endpoint error by using a modest manipulation of inertia. Participants made point-to-point elbow flexion and extension movements in the horizontal plane, under the instruction to move as fast as possible from one target area to another. In selected trials the moment of inertia of the lower arm was increased or decreased by 25%. First, we found that an unexpected increase or decrease of inertia did not affect the open loop controlled part of the movement path (and thus endpoint error was not affected). Second, we found that when the increased or decreased inertia was presented repeatedly, after 5-11 trials motor commands were customized: the first 100. ms of agonistic muscle activity in the smoothed and rectified electromyographic signal of agonistic muscles was higher for the high inertia compared to the low inertia. We conclude that endpoint error is not the only parameter that is used to evaluate if motor commands lead to movements as planned. © 2011 Elsevier Ltd.

In this study, a forward dynamic simulation model of the human musculoskeletal system was used to explore various strategies of generating muscle stimulation patterns for vertical squat jumping. It was shown that a simple mapping from joint angles to muscle stimulation onsets yielded successful control, albeit not optimal control, for jumps from different initial squat depths. Furthermore, it was shown that this mapping could be implemented in a straightforward way in a simple network of Hodgkin-Huxley type neurons.

Maximal voluntary isometric torque-angle relationships of elbow extensors and flexors in the transverse plane (humerus elevation angle of 90 degrees ) were measured at two different horizontal adduction angles of the humerus compared to thorax: 20 degrees and 45 degrees . For both elbow flexors and extensors, the torque-angle relationship was insensitive to this 25 degrees horizontal adduction of the humerus. The peak in torque-angle relationship of elbow extensors was found at 55 degrees (0 degrees is full extension). This is closer to full elbow extension than reported by researchers who investigated this relationship in the sagittal plane. Using actual elbow angles during contraction, as we did in this study, instead of angles set by the dynamometer, as others have done, can partly explain this difference. We also measured electromyographic activity of the biceps and triceps muscles with pairs of surface electrodes and found that electromyographic activity level of the agonistic muscles was correlated to measured net torque (elbow flexion torque: Pearson's r=0.21 and extension torque: Pearson's r=0.53). We conclude that the isometric torque-angle relationship of the elbow extensors found in this study provides a good representation of the force-length relationship and the moment arm-angle relationship of the elbow extensors, but angle dependency of neural input gives an overestimation of the steepness.

Results of mechanical analyses of running may be helpful in the search for the etiology of running injuries. In this study a mechanical analysis was made of the landing phase of three trained heel-toe runners, running at their preferred speed and style. The body was modeled as a system of seven linked rigid segments, and the positions of markers defining these segments were monitored using 200 Hz video analysis. Information about the ground reaction force vector was collected using a force plate. Segment kinematics were combined with ground reaction force data for calculation of the net intersegmental forces and moments. The vertical component of the ground reaction force vector Fz was found to reach a first peak approximately 25 ms after touch-down. This peak occurs because, in the support leg, the vertical acoeleration of the knee joint is not reduced relative to that of the ankle joint by rotation of the lower leg, so that the support leg segments collide with the floor. Rotation of the support upper leg, however, reduces the vertical acceleration of the hip joint relative to that of the knee joint, and thereby plays an important role in limiting the vertical forces during the first 40 ms. Between 40 and 100 ms after touch-down, the vertical forces are mainly limited by rotation of the support lower leg. At the instant that Fz reaches its first peak, net moments about ankle, knee and hip joints of the support leg are virtually zero. The net moment about the knee joint changed from -100 Nm (flexion) at touch-down to +200 Nm (extension) 50 ms after touch-down. These changes are too rapid to be explained by variations in the muscle activation levels and were ascribed to spring-like behavior of pre-activated knee flexor and knee extensor muscles. These results imply that the runners investigated had no opportunity to control the rotations of body segments during the first part of the contact phase, other than by selecting a certain geometry of the body and muscular (co-)activation levels prior to touch-down.

The purpose of this study was to calculate, as a function of time, segmental contributions to the vertical ground reaction force Fz from positional data for the landing phase in running. In order to evaluate the accuracy of the method, time histories of the sum of the segmental contributions were compared to Fz(t) measured directly by a force plate. The human body was modeled as a system of seven rigid segments. During running the positions of markers defining these segments were monitored using a video analysis system operating at 200 Hz. Special care was taken to minimize marker movement relative to the mass centers of segments, and low-pass cutoff frequencies of 50 Hz (markers defining leg segments) and 15-20 Hz (markers defining upper body) were used in filtering the position time histories so as to ensure that high signal frequencies were preserved. The magnitude of the high-frequency peak in Fz, also known as 'impact force peak', was estimated with errors z(t) revealed that the first peak in Fz has its origin in the contribution of support leg segments, while its magnitude is determined primarily by the contribution of the rest of the body. These contributions could be varied independently by changing running style. It follows that if the possible relationship between 'impact force peaks' and injuries is to be investigated, or if the effects of running shoe and surface construction on these force peaks are to be evaluated, the calculation of segmental contributions to Fz(t) is a more suitable approach than measuring only Fz(t).

SUMMARY Better running economy (i.e. a lower rate of energy consumption at a given speed) is correlated with superior distance running performance. There is substantial variation in running economy, even among elite runners. This variation might be due to variation in the storage and reutilization of elastic energy in tendons. Using a simple musculoskeletal model, it was predicted that the amount of energy stored in a tendon during a given movement depends more critically on moment arm than on mechanical properties of the tendon, with the amount of stored energy increasing as the moment arm gets smaller. Assuming a link between elastic energy reutilization and overall metabolic cost of running, a smaller moment arm should therefore be associated with superior running economy. This prediction was confirmed experimentally in a group of 15 highly trained runners. The moment arm of the Achilles tendon was determined from standardized photographs of the ankle, using the position of anatomical landmarks. Running economy was measured as the rate of metabolic energy consumption during level treadmill running at a speed of 16 km h–1. A strong correlation was found between the moment arm of the Achilles tendon and running economy. Smaller muscle moment arms correlated with lower rates of metabolic energy consumption (r2=0.75, P

The purpose of this study was to gain insight into the control strategy that humans use in jumping. Eight male gymnasts performed vertical squat jumps from five initial postures that differed in squat depth (P1-P5) while kinematic data, ground reaction forces, and electromyograms (EMGs) of leg muscles were collected; the latter were rectified and smoothed to obtain SREMGs. P3 was the preferred initial posture; in P1, P2, P4, and P5 height of the mass center was +13, +7, -7 and -14 cm, respectively, relative to that in P3. Furthermore, maximum-height jumps from the initial postures observed in the subjects were simulated with a model comprising four body segments and six Hill-type muscles. The only input was the onset of stimulation of each of the muscles (Stim). The subjects were able to perform well-coordinated squat jumps from all postures. Peak SREMG levels did not vary among P1-P5, but SREMG onset of plantarflexors occurred before that of gluteus maximus in P1 and >90 ms after that in P5 (P < 0.05). In the simulation study, similar systematic shifts occurred in Stim onsets across the optimal control solutions for jumps from P1-P5. Because the adjustments in SREMG onsets to initial posture observed in the subjects were very similar to the adjustments in optimal Stim onsets of the model, it was concluded that the SREMG adjustments were functional, in the sense that they contributed to achieving the greatest jump height possible from each initial posture. For the model, we were able to develop a mapping from initial posture to Stim onsets that generated successful jumps from P1-P5. It appears that to explain how subjects adjust their control to initial posture there is no need to assume that the brain contains an internal dynamics model of the musculoskeletal system. Copyright © 2008 the American Physiological Society.

Lengths of muscle tendon complexes of the quadriceps femoris muscle and some of its heads, biceps femoris and gastrocnemius muscles, were measured for six limbs of human cadavers as a function of knee and hip-joint angles. Length-angle curves were fitted using second degree polynomials. Using these polynomials the relationships between knee and hip-joint angles and moment arms were calculated. The effect of changing the hip angle on the biceps femoris muscle length is much larger than that of changing the knee angle. For the rectus femoris muscle the reverse was found. The moment arm of the biceps femoris muscle was found to remain constant throughout the whole range of knee flexion as was the case for the medial part of the vastus medialis muscle. Changes in the length of the lateral part of the vastus medialis muscle as well as the medial part of the vastus lateralis muscle are very similar to those of vastus intermedius muscle to which they are adjacent, while those changes in the length of the medial part of the vastus medialis muscle and the lateral part of the vastus lateralis muscle, which are similar to each other, differ substantially from those of the vastus intermedius muscle. Application of the results to jumping showed that bi-articular rectus femoris and biceps femoris muscles, which are antagonists, both contract eccentrically early in the push off phase and concentrically in last part of this phase.

The purpose of this study was to compare for a group of ten subjects the mechanical output about the ankle during isokinetic plantar flexion with that during one-legged vertical jumps. For evaluation of the mechanical output the plantar flexion moment of force was related to the angular velocity of plantar flexion. The relationship for isokinetic plantar flexion was obtained using an isokinetic dynamometer; that for plantar flexion in jumping was obtained by combining kinematics and ground reaction forces. It was found that, at any given angular velocity of plantar flexion above 1 rad.s-1, the subjects produced much larger moments during jumping than during isokinetic plantar flexion. In order to explain the observed differences in mechanical output about the ankle, a model was used to simulate isokinetic plantar flexion and plantar flexion during jumping. The model represented both m. soleus and m. gastrocnemius as a complex composed of elastic tissue in series with muscle fibers. The force of the muscle fibers depended on fiber length, shortening velocity (Vfibers), and active state. The input variables of the model were histories of shortening velocities of the complexes, determined from kinematics, and active state. Among the output variables were Vfibers and plantar flexion moment. The simulation results were very similar to the experimental findings. According to the simulation results there are two reasons why at the same angular velocity of plantar flexion larger moments were produced during jumping than during isokinetic plantar flexion.(ABSTRACT TRUNCATED AT 250 WORDS)

Vertical jumping ability is of importance for good performance in sports such as basketball and volleyball. Coaches are in need of exercises that consume only little time and still help to improve their players’ jumping ability, without involving a high risk of injury. Drop jumping is assumed to satisfy these requirements. This assumption is supported by a review of results of training studies. However, it appears that regular jumping exercises can be just as helpful. The same holds for exercises with weights, provided the subjects have no weight-training history. In fact, for unskilled jumpers who have no weight-training history, the effects of training programmes utilising these different exercises are additive. The most effective, efficient and safe way for a coach to improve the jumping achievement of his athletes may well be to submit them first to a training programme utilising regular jumps, then to a weight-training programme and finally to a drop jump training programme. In drop jump training programmes themselves, the improvement in jumping height varies greatly among studies. This variation cannot be explained satisfactorily with the information available on subjects and training programmes. Given the current state of knowledge, coaches seem to have no other option than to strictly copy a programme which has proved to be very effective. Obviously there is a need for more systematic research of the relationship between design and effect of drop jump training programmes. The most important variable to be controlled is drop jumping technique. From a review of biomechanical studies of drop jumping, it becomes clear that jumping technique strongly affects the mechanical output of muscles. The biomechanics of 2 techniques are discussed. In the bounce drop jump the downward movement after the drop is reversed as soon as possible into an upward push-off, while in the countermovement drop jump this is done more gradually by increasing the amplitude of the downward movement after landing. It is speculated that the bounce drop jump might trigger improvement of the power output capacity of muscles, whereas the repetition of the countermovement drop jump may help to improve coordination. Future training studies are needed to determine whether drop jumping technique really affects the outcome of the training, and if so, which technique should be preferred. Also, further biomechanical research is needed to determine kinematics and kinetics of other drop jumping techniques, and to trace potential dangers. The author urges for a close cooperation between coaches and scientists in future research.

In isokinetic experiments on human subjects, conducted to determine moments that can be exerted about a joint at different angular velocities, joint rotation starts as soon as the moment increases above the resting level. This contraction history differs from the one in experiments on isolated muscle, where the force is allowed to increase to an isometric level before shortening is initiated. The purpose of the present study was to determine the influence of contraction history on plantar flexing moments found during maximal voluntary plantar flexion on an isokinetic dynamometer. In ten subjects, plantar flexing moments were measured as a function of ankle angle at different angular velocities. They were also calculated using a model of the muscle-tendon complex of the human triceps surae. The model incorporates elastic tendinous tissue in series with muscle fibers. The input of the model consists of time histories of active state (the force generating capacity of contractile elements) and shortening velocity of the muscle-tendon complex. Different time courses of active state were offered at fixed length of the muscle-tendon complex. The time course yielding a close match between the calculated rise of plantar flexing moment and the rise measured during fixed angle contractions was used to calculate moment-angle curves for isokinetic plantar flexion. The active state value reached when a peak occurred in calculated moment-angle curves was found to be lower if the angular velocity was made higher. Comparing measured and calculated results, it was concluded that moment-angular velocity diagrams determined in studies of isokinetic plantar flexion in human subjects reflect not only the influence of shortening velocity of contractile elements on the force which can be produced by plantar flexors.

the purpose of this paper is to discuss various approaches used in load analysis with special consideration of whether there is some evidence that these approaches have actually contributed to a reduction of sport injuries and whether these approaches have the theoretical potential to reduce the frequency of injuries at all. Two possible approaches are compared. The 'cause-effect approach' uses the estimation of internal stress. It is proposed that a comparison of the internal stress with the critical stress limits for an anatomical structure of interest may not help to reduce the frequency of injuries for two reasons. Firstly, the estimation of the critical stress limits and the internal stress may have significant errors which inhibit possible conclusions. Secondly, the information provided by the comparison of internal stress and critical stress limits was implicitly available in the injury frequencies and no new knowledge has been added. It is proposed that stresses estimated from model calculations can successfully be used to reduce the frequency of injuries if they are compared (comparison technique) with stresses estimated for comparable situations (e.g. shoes with and without orthotics). The 'empirical approach' is based on the assumption that the knowledge of the mechanisms of an injury is not a prerequisite for the reduction of the frequency of injuries. The approach establishes statistical correlations between factors related to injuries on one side and specific injuries or groups of injuries on the other side. This approach has frequently been used in the development of sport shoes. It is speculated that studies using an empirical approach have the potential to actually reduce the frequency of injuries. The conclusive evidence that biomechanical research in load analysis has in fact contributed to a reduction of the frequency of injuries is still missing for both discussed approaches.

In the literature, it has been hotly debated whether the brain uses internal models or equilibrium point (EP) control to generate arm movements. EP control involves specification of EP trajectories, time series of arm configurations in which internal forces and external forces are in equilibrium; if the arm is not in a specified EP, it is driven toward this EP by muscle forces arising due to central drive, reflexes, and muscle mechanics. EP control has been refuted by researchers claiming that EP trajectories underlying movements of subjects were complex. These researchers used an approach that involves applying force perturbations during movements of subjects and fitting a mass-spring-damper model to the kinematic responses, and then reconstructing the EP trajectory using the estimated stiffness, damping, and measured kinematics. In this study, we examined the validity of this approach using an EP-controlled musculoskeletal model of the arm. We used the latter model to simulate unperturbed and perturbed maximally fast movements and optimized the parameter values of a mass-spring-damper model to make it reproduce as best as possible the kinematic responses. It was shown that estimated stiffness not only depended on the "true" stiffness of the musculoskeletal model but on all of its dynamical parameters. Furthermore it was shown that reconstructed EP trajectories were in agreement with those presented in the literature but did not resemble the simple EP trajectories that had been used to generate the movement of the model. It was concluded that the refutation of EP control on the basis of results obtained with mass-spring-damper models was unjust. Copyright © 2007 The American Physiological Society.

According to the equilibrium point theory, the control of posture and movement involves the setting of equilibrium joint positions (EP) and the independent modulation of stiffness. One model of EP control, the alpha-model, posits that stable EPs and stiffness are set open-loop, i.e. without the aid of feedback. The purpose of the present study was to explore for the elbow joint the range over which stable EPs can be set open-loop and to investigate the effect of co-contraction on intrinsic low-frequency elbow joint stiffness (K (ilf)). For this purpose, a model of the upper and lower arm was constructed, equipped with Hill-type muscles. At a constant neural input, the isometric force of the contractile element of the muscles depended on both the myofilamentary overlap and the effect of sarcomere length on the sensitivity of myofilaments to [Ca2+] (LDCS). The musculoskeletal model, for which the parameters were chosen carefully on the basis of physiological literature, captured the salient isometric properties of the muscles spanning the elbow joint. It was found that stable open-loop EPs could be achieved over the whole range of motion of the elbow joint and that K (ilf), which ranged from 18 to 42 N m.rad(-1), could be independently controlled. In the model, LDCS contributed substantially to K (ilf) (up to 25 N m.rad(-1)) and caused K (ilf) to peak at a sub-maximal level of co-contraction.