Effect of vasti morphology on peak sprint cycling power of a human musculoskeletal simulation model.

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.

The Relationship between Pedal Force and Crank Angular Velocity in Sprint Cycling.

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.

Humans adjust control to initial squat depth in vertical squat jumping.

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.

Is energy expenditure taken into account in human sub-maximal jumping?--A simulation study.

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.

Explanation of the bilateral deficit in human vertical squat jumping.

In the literature, it has been reported that the mechanical output per leg is less in two-leg jumps than in one-leg jumps. This so-called bilateral deficit has been attributed to a reduced neural drive to muscles in two-leg jumps. The purpose of the present study was to investigate the possible contribution of nonneural factors to the bilateral deficit in jumping. We collected kinematics, ground reaction forces, and electromyograms of eight human subjects performing two-leg and one-leg (right leg) squat jumps and calculated mechanical output per leg. We also used a model of the human musculoskeletal system to simulate two-leg and one-leg jumps, starting from the initial position observed in the subjects. The model had muscle stimulation as input, which was optimized using jump height as performance criterion. The model did not incorporate a reduced maximal neural drive in the two-leg jump. Both in the subjects and in the model, the work of the right leg was more than 20% less in the two-leg jump than in the one-leg jump. Peak electromyogram levels in the two-leg jump were reduced on average by 5%, but the reduction was only statistically significant in m. rectus femoris. In the model, approximately 75% of the bilateral deficit in work per leg was explained by higher shortening velocities in the two-leg jump, and the remainder was explained by lower active state of muscles. It was concluded that the bilateral deficit in jumping is primarily caused by the force-velocity relationship rather than by a reduction of neural drive.

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.

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.