It was hypothesized that a tight integration of feed-forward and feedback-driven muscle activation with the characteristic intrinsic muscle properties is a key feature of locomotion in challenging environments. In this simulation study it was investigated whether a combination of feed-forward and feedback signals improves hopping stability compared with those simulations with one individual type of activation. In a reduced one-dimensional hopping model with a Hill-type muscle (one contractile element, neither serial nor parallel elastic elements), the level of detail of the muscle's force–length–velocity relation and the type of activation generation (feed-forward, feedback and combination of both) were varied to test their influence on periodic hopping. The stability of the hopping patterns was evaluated by return map analysis. It was found that the combination of feed-forward and proprioceptive feedback improved hopping stability. Furthermore, the nonlinear Hill-type representation of intrinsic muscle properties led to a faster reduction of perturbations than a linear approximation, independent of the type of activation. The results emphasize the ability of organisms to exploit the stabilizing properties of intrinsic muscle characteristics.

The spring-loaded inverted pendulum (SLIP) model is a well established model for describing bouncy gaits like human running. The notion of spring-like leg behavior has led many researchers to compute the corresponding parameters, predominantly stiffness, in various experimental setups and in various ways. However, different methods yield different results, making the comparison between studies difficult. Further, a model simulation with experimentally obtained leg parameters typically results in comparatively large differences between model and experimental center of mass trajectories. Here, we pursue the opposite approach which is calculating model parameters that allow reproduction of an experimental sequence of steps. In addition, to capture energy fluctuations, an extension of the SLIP (ESLIP) is required and presented. The excellent match of the models with the experiment validates the description of human running by the SLIP with the obtained parameters which we hence call dynamical leg parameters.

A reductionist approach was presented to investigate which level of detail of the physiological muscle is required for stable locomotion. Periodic movements of a simplified one-dimensional hopping model with a Hill-type muscle (one contractile element, neither serial nor parallel elastic elements) were analyzed. Force-length and force-velocity relations of the muscle were varied in three levels of approximation (constant, linear and Hill-shaped nonlinear) resulting in nine different hopping models of different complexity. Stability of these models was evaluated by return map analysis and the performance by the maximum hopping height. The simplest model (constant force-length and constant force-velocity relations) outperformed all others in the maximum hopping height but was unstable. Stable hopping was achieved with linear and Hill-shaped nonlinear characteristic of the force-velocity relation. The characteristics of the force-length relation marginally influenced hopping stability. The results of this approach indicate that the intrinsic properties of the contractile element are responsible for stabilization of periodic movements. This connotes that (a) complex movements like legged locomotion could benefit from stabilizing effects of muscle properties, and (b) technical systems could benefit from the emerging stability when implementing biological characteristics into artificial muscles.

The purpose of this study was to: (1) establish the reliability of a new unilateral concentric only horizontal jump assessment (HSJ) then compare the reliability of this test to other types of unilateral vertical and horizontal jumps; (2) compare the tests to whether they differ in their ability to determine limb asymmetries; and (3) investigate the relationship between these jumps and sprint running.Eighteen sportsmen performed unilateral jump assessments involving the horizontal squat jump, horizontal countermovement jump, horizontal repetitive jump, vertical squat jump, vertical countermovement jump, and vertical repetitive jump.Reliability for the new test was found to be the equal if not better than the other more established tests of leg power, with the within trial variation (CV=1.1–1.9%) and test–retest reliability (ICC=0.89–0.90). None of the tests were found to have greater discriminative ability in determining limb asymmetries. Stretch shorten cycle enhancement was greater in the vertical tests (12.1%) compared to the horizontal tests (1.3%). Horizontal jump assessments (r=−0.73 to −0.86) were found better predictors of 20-m sprint performance than the vertical assessments (r=−0.52 to −0.73), with the horizontal cyclic assessment being the best predictor (r=−0.86).Horizontal leg power assessment appears an inexpensive, easy to administer, reliable and valid method to assess unilateral leg power.