Abstract
PurposeWe 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).MethodsIn 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.ResultsGM 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).ConclusionThese results support the notion that the dynamic function of tendons is integral in reducing energy expenditure and increasing the “apparent” efficiency of running.
Highlights
Human locomotion entails the motion of the body through an environment: air in terrestrial locomotion whilst in contact with the ground, and water in aquatic locomotion
In the above-mentioned studies, “locomotion” efficiency was, calculated as the ratio between mechanical work per unit distance (WTOT) and net energy cost (Cnet, the metabolic energy expended per unit distance); in turn, Cnet was calculated as the ratio between net oxygen uptake and locomotion velocity (V O2net/v) and WTOT was calculated as the sum of two components: WEXT and WINT
We investigated the role of elastic strain energy on locomotion (“apparent”) efficiency at increasing running speeds Our results reveal that the work provided by the recoil of the Achilles tendon at each speed is linked to: (1) a reduction in the energy cost of running, (2) an increase in the mechanical work at Regarding the underpinning mechanisms, a possible explanation for the increase in “apparent” efficiency with speed is that the plantar flexor muscles favour the use of tendon elastic strain energy over muscle fibre work (Lichtwark et al 2007), and that this energy is enhanced when running speed advances towards maximum running velocity (Cavagna and Kaneko 1977)
Summary
Human locomotion entails the motion of the body through an environment: air in terrestrial locomotion whilst in contact with the ground, and water in aquatic locomotion. The efficiency of the locomotor apparatus can be expressed as the ratio between the work necessary to maintain motion and the chemical energy transformed by the muscles This “locomotion” efficiency (i.e. the total mechanical work generated at whole-body level as a proportion of metabolic cost), has been investigated in several forms of terrestrial and aquatic locomotion, such as swimming “locomotion” efficiency approximates “pure” muscle efficiency values (about 0.25–0.30, as reported by Woledge et al 1985) in the forms of locomotion where elastic recoil is negligible [e.g. swimming or cycling, as reported by Zamparo et al (2002) and Minetti et al (2001)] whereas in the case of running, the efficiency calculated in this manner can reach far larger values [e.g. up to 0.5–0.7, as reported by Cavagna and Kaneko (1977)]
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