The influence of Achilles tendon mechanical behaviour on "apparent" efficiency during running at different speeds

Purpose: We investigated the role of elastic strain energy on the "apparent" efficiency of locomotion (AE), a parameter that is known to increase as a function of running speed (up to 0.5-0.7) well above the values of "pure" muscle efficiency (about 0.25-0.30). Methods: In vivo ultrasound measurements of the gastrocnemius medialis (GM) muscle-tendon unit (MTU) were combined with kinematic, kinetic and metabolic measurements to investigate the possible influence of the Achilles tendon mechanical behaviour on the mechanics (total mechanical work, WTOT) and energetics (net energy cost, Cnet) of running at different speeds (10, 13 and 16 km h-1); AE was calculated as WTOT/Cnet. Results: GM fascicles shortened during the entire stance phase, the more so the higher the speed, but the majority of the MTU displacement was accommodated by the Achilles tendon. Tendon strain and recoil increased as a function of running speed (P < 0.01 and P < 0.001, respectively). The contribution of elastic energy to the positive work generated by the MTU also increased with speed (from 0.09 to 0.16 J kg-1 m-1). Significant negative correlations (P < 0.01) were observed between tendon work and metabolic energy at each running speed (the higher the tendon work the lower the metabolic demand) and significant positive correlations were observed between tendon work and AE (P < 0.001) at each running speed (the higher the tendon work the higher the efficiency). Conclusion: These results support the notion that the dynamic function of tendons is integral in reducing energy expenditure and increasing the "apparent" efficiency of running.