Human hopping on very soft elastic surfaces: implications for muscle pre-stretch and elastic energy storage in locomotion

Abstract

During hopping in place and running, humans maintain similar center of mass dynamics by precisely adjusting leg mechanics to compensate for moderate changes in surface stiffness. We investigated the limits of this precise control by asking humans to hop in place on extremely soft elastic surfaces. We found that hoppers drastically altered leg mechanics and maintained similar center of mass dynamics despite a sevenfold change in surface stiffness (11-81·kN·m -1 ). On the stiffest surfaces, the legs compressed in early stance and then extended in late stance in the pattern that is typical for normal bouncing gaits. On the softest surfaces, however, subjects reversed this pattern so that the legs extended up to 8·cm in early stance and then compressed by a similar distance in late stance. Consequently, the center of mass moved downward during stance by 5-7·cm less than the surface compressed and by a similar distance as on the stiffest surfaces. This unique leg action probably reduced extensor muscle pre-stretch because the joints first extended and then flexed during stance. This interpretation is supported by the observation that hoppers increased muscle activation by 50% on the softest surface despite similar joint moments and mechanical leg work as on the stiffest surface. Thus, the extreme adjustment to leg mechanics for very soft surfaces helps maintain normal center of mass dynamics but requires high muscle activation levels due to the loss of the normal extensor muscle stretch-shorten cycle.

Effects of Two Plyometric Protocols at Different Surfaces on Mechanical Properties of Achilles Tendon in Children

“…Crowther et al (29) also have shown that landing on a trampoline is accompanied by "stiffer" joint technique via optimization of the lower impact forces and the elasticity of trampoline. This may improve the transfer of training gains to maximal plantar flexor performance within a specific ankle range of motion (28). Our results indicate that despite any differences in plyometric exercise technique between minitrampoline and ground surface, AT strain adaptations did not differ after a 4-week program in children.…”

“…To our knowledge, the effects of plyometric training on an elastic surface on mechanical properties of AT in children have not been previously investigated. Studies in adults reported that subjects who exercise on elastic surfaces first extend the legs and then compress exactly out of phase with surface deformation, increasing leg stiffness and rebound gain (28). Crowther et al (29) also have shown that landing on a trampoline is accompanied by "stiffer" joint technique via optimization of the lower impact forces and the elasticity of trampoline.…”

“…It has been suggested that the training surface form during plyometric workout could affect the SSC kind being performed (extended technique), indicating different responses (28). For example, Moritz et al (28) reported that the ankle joint assumes a more flexed posture during hopping on elastic surfaces that increase joint mechanical work output during takeoff by changing the timing of peak muscle moments.…”

“…For example, Moritz et al (28) reported that the ankle joint assumes a more flexed posture during hopping on elastic surfaces that increase joint mechanical work output during takeoff by changing the timing of peak muscle moments. Therefore, it may be proposed that depending on the type of jumping stimulus, one may expect analogous plyometric training adaptations in peak torque and RTD.…”

Non-visually-guided distance perception depends on matching torso fluctuations between training and test

“…Of course, the signals travel through neural tracts in humans, but postural adaptations occur faster than neural transmission allows in multicellular organisms much smaller than humans posing shorter neural distances to traverse (e.g., Endlein & Federle, 2013) but also in humans, supporting quiet standing (Marsden, Merton, & Morton, 1983), speech (Kelso, Tuller, Vatikiotis-Bateson, & Fowler, 1984), and hopping (Moritz & Farley, 2004). The connective tissues constitute a hierarchically organized network of small prestressed structures (e.g., at the level of actin filaments composing a single cell's skeleton) nested within larger prestressed structures (e.g., extracellular matrix) nested within yet larger pre-stressed structures (e.g., the fascial encasings around muscles and joints).…”