The influence of in vivo mechanical behaviour of the Achilles tendon on the mechanics, energetics and apparent efficiency of bouncing gaits

In this study, we used kinematic, kinetic, metabolic and ultrasound analysis to investigate the role of elastic energy utilization on the mechanical and physiological demands of a movement task (hopping) that primarily involves the plantar-flexor muscles to determine the contribution of tendon work to total mechanical work and its relationship with apparent efficiency (AE) in bouncing gaits. Metabolic power (P-MET) and (positive) mechanical power at the whole-body level (P-MEC) were measured during hopping at different frequencies (2, 2.5, 3 and 3.5 Hz). The (positive) mechanical power produced during the Achilles tendon recoil phase (P-TEN) was obtained by integrating ultrasound data with an inverse dynamic approach. As a function of hopping frequency, P-MEC decreased steadily and P-MET exhibited a U-shape behaviour, with a minimum at about 3 Hz. AE (P-MEC/P-MET) showed an opposite trend and was maximal (about 0.50) at the same frequency when P-TEN was also highest. Positive correlations were observed: (i) between P-TEN and AE (AE=0.22+0.15P(TEN), R-2=0.67, P<0.001) and the intercept of this relationship indicates the value of AE that should be expected when tendon work is nil; (ii) between AE and tendon gearing (Gt=Delta muscle-tendon unit length/Delta muscle belly length; R-2=0.50, P<0.001), where a high Gt indicates that the muscle is contracting more isometrically, thus allowing the movement to be more economical (and efficient); (iii) between Gt and P-TEN (R-2=0.73, P<0.001), which indicates that Gt could play an important role in the tendon's capability to store and release mechanical power.