A.J. van Soest

Effects of muscle strengthening on vertical jump height: a simulation study

In this study the effects of systematic manipulations of control and muscle strength on vertical jump height were investigated. Forward dynamic simulations of vertical squat jumps were performed with a model of the human musculoskeletal system. Model input was STIM(t), stimulation of six lower extremity muscles as function of time; model output was body motion. The model incorporated all features of the musculoskeletal system of human test subjects considered salient for vertical jumping, and the initial body configuration was set equal to that of the test subjects. First, optimal STIM(t) was found for a standard version of the model (experiment A). A satisfactory correspondence was found between simulation results and kinematics, kinetics and electromyograms of the test subjects. Subsequently, optimal STIM(t) for the standard model was used to drive a model with strengthened muscles (experiment B). Jump height was now lower than that found in experiment A. Finally, optimal STIM(t) was found for the model with strengthened muscles (experiment C). Jump height was now higher than that found in experiment A. These results suggest that in order to take full benefit of an increase in muscle strength, control needs to be adapted. It is speculated that in training programs aimed at improving jumping achievement, muscle training exercises should be accompanied by exercises that allow athletes to practice with their changed muscles.

A comparison of one-legged and two-legged countermovement jumps.

Ten well-trained male volleyball players performed one-legged and two-legged vertical countermovement jumps. Ground reaction forces, cinematographic data, and electromyographic data were recorded. Jumping height in one-legged jumps was 58.5% of that reached in two-legged jumps. Mean net torques in hip and ankle joints were higher in one-legged jumps. Net power output in the ankle joint was extremely high in one-legged jumps. This high power output was explained by a higher level of activation in both heads of m. gastrocnemius in the one-legged jump. A higher level of activation was also found in m. vastus medialis. These differences between unilateral and bilateral performance of the complex movement jumping were shown to be in agreement with differences reported in literature based on isometric and isokinetic experiments.

Why do people jump the way they do

BOBBERT, M.F., and A.J. “KNOEK” van SOEST. Why do people jump the way they do? Exerc. Sport Sci. Rev., Vol. 29, No. 3, pp 95–102, 2001. When humans perform maximum height squat jumps, their segmental rotations contribute in a proximodistal sequence to the vertical acceleration of the center of gravity. The same kinematic pattern occurs in a forward dynamic model of the musculoskeletal system when muscle stimulation is optimized to maximize jump height. This paper examines why this kinematic pattern maximizes jump height in humans, given the design of the human musculoskeletal system.

The Dynamics of Postural Sway Cannot Be Captured Using a One-Segment Inverted Pendulum Model: A PCA on Segment Rotations During Unperturbed Stance

Research on unperturbed stance is largely based on a one-segment inverted pendulum model. Recently, an increasing number of studies report a contribution of other major joints to postural control. Therefore this study evaluates whether the conclusions originating from the research based on a one-segment model adequately capture postural sway during unperturbed stance. High-pass filtered kinematic data (cutoff frequency 1/30 Hz) obtained over 3 min of unperturbed stance were analyzed in different ways. Variance of joint angles was analyzed. Principal-component analysis (PCA) was performed on the variance of lower leg, upper leg, and head-arms-trunk (HAT) angles, as well as on lower leg and COM angle (the orientation of the line from ankle joint to center of mass). It was found that the variance in knee and hip joint angles did not differ from the variance found in the ankle angle. The first PCA component indicated that, generally, the upper leg and HAT segments move in the same direction as the lower leg with a somewhat larger amplitude. The first PCA component relating ankle angle variance and COM angle variance indicated that the ankle joint angle displacement gives a good estimate of the COM angle displacement. The second PCA component on the segment angles partly explains the apparent discrepancy between these findings because this component points to a countermovement of the HAT relative to the ankle joint angle. It is concluded that postural control during unperturbed stance should be analyzed in terms of a multiple inverted pendulum model.

The control of multi-joint movements relies on detailed internal representations

Abstract This paper addresses the question what level of detail is required in internal representations used in control of multi-joint movements, focusing on contact control tasks. Following Bernstein, we define the central problem to be which strategies are used in the nervous system in order to control the vastly redundant musculoskeletal system. Simplifications based on equilibrium point theories are rejected on the basis that when they are simple they do not lead to adequate behaviour, whereas when they are complex they implicitly introduce the detailed internal representations that they were meant to dispense with. Based on both experimental data and on simulation results, it is argued that timing of muscle activation needs to be precisely tuned to the task at hand and the environmental conditions. It is argued that it is impossible to achieve this without detailed internal representations of the properties of the effector system in relation to the environment. It is attempted to link Bernsteins notion of a hierarchical organization of the nervous system in which tasks are delegated to subsystems as low as possible in the hierarchical structure of the central nervous system, to recent advances in neuroscience.

Rowing skill affects power loss on a modified rowing ergometer.

PURPOSE In rowing, the athlete has to maximize power output and to minimize energy losses to processes unrelated to average shell velocity. The contribution of velocity efficiency (evelocity; the fraction of mechanical power not lost to velocity fluctuations) to rowing performance in relation to the contributions of maximum oxygen uptake (V[spacing dot above]O2max) and gross efficiency (egross) was investigated. Relationships between evelocity and movement execution were determined. METHODS Twenty-two well-trained female rowers participated in two testing sessions. In the first session, they performed a 2000-m time trial on a modified rowing ergometer that allowed for power losses due to velocity fluctuations. The V[spacing dot above]O2max, the evelocity, and the amount of rower-induced impulse fluctuations (RIIF) due to horizontal handle and foot stretcher forces were determined in a steady state part of the time trial. RIIF was used as a measure of movement execution. In the second session, egross was determined at submaximal intensity. RESULTS As expected, V[spacing dot above]O2max accounted for the major part of explained variance in the 2000-m time (53%, P < 0.001). Velocity efficiency accounted for a further 14%, egross for 11% (P < 0.05). Negative correlations were found between evelocity and RIIF values of several discreet intervals within a stroke cycle. The results suggest that optimal timing of forces applied to the ergometer will help minimizing power loss to velocity fluctuations. CONCLUSIONS This study indicates that a relationship exists between performance and evelocity. Furthermore, evelocity appears to be related to movement execution, in particular the timing of handle and foot stretcher forces.

Consequences of Ankle Joint Fixation on Fes Cycling Power Output: A Simulation Study

INTRODUCTION During fixed-ankle FES cycling in paraplegics, in which the leg position is completely determined by the crank angle, mechanical power output is low. This low power output limits the cardiovascular load that could be realized during FES ergometer cycling, and limits possibilities for FES cycling as a means of locomotion. Stimulation of ankle musculature in a released-ankle setup might increase power output. However, releasing the ankle joint introduces a degree of freedom in the leg that has to be controlled, which imposes constraints on the stimulation pattern. METHODS In this study, a forward dynamics modeling/simulation approach was used to assess the potential effect of releasing the ankle on the maximal mechanical power output. RESULTS For the released-ankle setup, the optimal stimulation pattern was found to be less tightly related to muscle shortening/lengthening than for the fixed-ankle setup, which indicates the importance of the constraints introduced by releasing the ankle. As a result, the maximal power output for 45-RPM cycling in the released-ankle setup was found to be about 10% lower than with a fixed ankle, despite the additional muscle mass available for stimulation. Power output for the released-ankle setup can be improved by tuning the point of contact between the foot and pedal to the relative strength of the ankle plantar flexors. For the model used, power output was 14% higher than for the fixed-ankle setup when this point of contact was moved posteriorly by 0.075 m. CONCLUSION Releasing the ankle joint and stimulating the triceps surae and tibialis anterior is expected to result in a modest increase in power output at best.

COORDINATION OF MULTI-JOINT MOVEMENTS: AN INTRODUCTION TO EMERGING VIEWS

In this introductory paper, an overview is provided of the topics addressed in this special issue. These topics center around what is often referred to as Bernsteins problem. The first two topics both offer partial solutions to the indeterminacy problem. The first by identification of constraints acting on the neuro-musculo-skeletal system, in its interaction with the environment; the second by expanding the number of variables that are to be controlled. Regarding the first, it is argued that a distinction should be made between holonomic and nonholonomic constraints. Regarding the second, the necessity of independent control of in particular stiffness is a recurrent theme. The third and fourth topics concern choices to be made when modelling motor behavior. In particular, the level of detail at which the neuro-musculo-skeletal system is to be modelled in studies of coordination, and the merits of descriptive models offered by nonlinear dynamics are discussed. Apart from refining our models of the nervous system, the models of which are currently identified as a weak link, a major challenge for the coming years is concluded to lie in linking neuro-musculo-skeletal models to the behavioral models generated by nonlinear dynamics.

Planning of human motions: how simple must it be?

Human beings are able to perform a tremendously large set of motions. These motions vary from cyclic (e.g., walking, biking), to explosive (e.g., jumping, hitting, throwing) to positioning tasks (e.g., reaching). The way these motions are accomplished is still the subject of debate among psychologists, neurophysiologists, biomechanists, and others.