Is the effect of a countermovement on jump height due to active state development?

Purpose: To investigate whether the difference in jump height between countermovement jumps (CMJ) and squat jumps (SJ) could be explained by a difference in active state during propulsion. Methods: Simulations were performed with a model of the human musculoskeletal system comprising four body segments and six muscles. The model's only input was STIM, the stimulation of muscles, which could be switched "off" or "on." After switching "on," STIM increased to its maximum at a fixed rate of change (dSTIM/dt). For various values of dSTIM/dt, stimulation switch times were optimized to produce a maximum height CMJ. From this CMJ, the configuration at the lowest height of the center of gravity (CG) was selected and used as static starting configuration for simulation of SJ. Next, STIM-switch times were optimized to find the maximum height SJ. Results: Simulated CMJ and SJ closely resembled jumps of human subjects. Maximum jump height of the model was greater in CMJ than in SJ, with the difference ranging from 0.4 cm at infinitely high dSTIM/dt to about 2.5 cm at the lowest dSTIM/dt investigated. The greater jump height in CMJ was due to a greater work output of the hip extensor muscles. These muscles could produce more force and work over the first 30% of their shortening range in CMJ, due to the fact that they had a higher active state in CMJ than in SJ. Conclusion: The greater jump height in CMJ than in SJ could be explained by the fact that in CMJ active state developed during the preparatory countermovement, whereas in SJ it inevitably developed during the propulsion phase, so that the muscles could produce more force and work during shortening in CMJ. Copyright © 2005 by the American College of Sports Medicine

Contribution of the forelimbs and hindlimbs of the horse to mechanical energy changes in jumping

The purpose of the present study was to gain more insight into the contribution of the forelimbs and hindlimbs of the horse to energy changes during the push-off for a jump. For this purpose, we collected kinematic data at 240 Hz from 23 5-year-old Warmbloods (average mass: 595 kg) performing free jumps over a 1.15 m high fence. From these data, we calculated the changes in mechanical energy and the changes in limb length and joint angles. The force carried by the forelimbs and the amount of energy stored was estimated from the distance between elbow and hoof, assuming that this part of the leg behaved as a linear spring. During the forelimb push, the total energy first decreased by 3.2 J k

Dynamics of the in-run in ski jumping: a simulation study

Utilisation d'un modèle de simulation à 4 segments, intégrant les forces aérodynamiques et de friction de la neige pour étudier la dynamique du skieur sur la piste d'élan depuis un départ statique

Control of support limb muscles in recovery after tripping in young and older subjects

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

Is equilibrium point control feasible for fast goal-directed single-joint movements?

Several types of equilibrium point (EP) controllers have been proposed for the control of posture and movement. EP controllers are appealing from a computational perspective because they do not require solving the "inverse dynamic problem" (i.e., computation of the torques required to move a system along a desired trajectory). It has been argued that EP controllers are not capable of controlling fast single-joint movements. To refute this statement, several extensions have been proposed, although these have been tested using models in which only the tendon compliance, force-length-velocity relation, and mechanical interaction between tendon and contractile element were not adequately represented. In the present study, fast elbow-joint movements were measured and an attempt was made to reproduce these using a realistic musculoskeletal model of the human arm. Three types of EP controllers were evaluated: an open-loop α-controller, a closed-loop λ-controller, and a hybrid open- and closed-loop controller. For each controller we considered a continuous version and a version in which the control signals were sent out intermittently. Only the intermittent hybrid EP controller was capable of generating movements that were as fast as those of the subjects. As a result of the nonlinear muscle properties, the hybrid EP controller requires a more detailed representation of static muscle properties than generally assumed in the context of EP control. In sum, this study shows that fast single-joint movements can be realized without explicitly solving the inverse dynamics problem, but in a less straightforward manner than implied by proponents of conventional EP controllers. Copyright © 2006 The American Physiological Society