D26 Elasticity and Efficiency

Topic

This assessment of the musculotendinous system's elasticity measures the system's capacity to store and subsequently release elastic energy—specifically, the ability to store and return mechanical energy, a parameter also addressed in another model within this framework regarding tendon elastic failure.

Tendon stiffness, conversely, is estimated via dynamic ultrasonography that records the displacement of the muscle-tendon junction during a progressive isometric muscle contraction, while the muscle force is simultaneously measured using an isokinetic dynamometer. The slope of the force-elongation curve represents the tendon stiffness—a metric also utilized in another model within this framework concerning chronic tendinosis. If the tendon elongates less for a given applied force, the slope of the force-elongation curve becomes steeper, thereby indicating greater tendon stiffness.

The analysis of the stretch-shortening cycle—the same cycle addressed in another model regarding the athlete's elastic-reactive capabilities—compares the height of a static jump with that of a countermovement jump and a reactive jump (drop jump). The difference between the countermovement jump height and the static jump height quantifies the benefit the stretch-shortening cycle provides to the athlete's performance. The athlete's final leg stiffness—specifically the same leg stiffness defined in another model within this set based on a collapsed spring-mass system—is calculated using that same spring-mass system applied to the ground reaction force data and the athlete's center-of-mass kinematics (parameters also used in the other distinct models of this set).

Consequently, this model—characterized by the elastic energy storage and release capabilities with which it began—allows for the estimation of the tendon's final hysteresis (i.e., the energy loss between loading and unloading on the force-elongation curve) based on the area enclosed by the loading-unloading loop; this is the same tendon hysteresis defined in another model of this set regarding a chronically degenerated tendon. Based on the ultrasound-measured displacement of the myotendinous junction, dynamometry-measured muscle force, and ground reaction force combined with kinematics, this diagnostic approach determines the tendon stiffness and leg stiffness, the final stored elastic energy, the recovery coefficient (defined in another model of this set), the benefit of the stretch-shortening cycle, and the reactive strength index (defined in yet another distinct model of this set), thereby closing the loop between the initial elastic capacity and the final values ​​that quantify it.

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