The human ankle plantar-flexors, soleus (SOL) and gastrocnemius (GAS), utilize tendon elastic strain energy to reduce muscle fiber work and optimize contractile conditions during running. However, studies to date have considered only slow to moderate running speeds up to 5 m/s. Little is known about how the human ankle plantar-flexors utilize tendon elastic strain energy as running speed is advanced towards maximum sprinting. We used data obtained from gait experiments in conjunction with musculoskeletal modeling and optimization techniques to calculate muscle-tendon unit (MTU) work, tendon elastic strain energy and muscle fiber work for the ankle plantar-flexors as participants ran at five discrete steady-state speeds ranging from jogging (~2 m/s) to sprinting (≥8 m/s). As running speed progressed from jogging to sprinting, the contribution of tendon elastic strain energy to the positive work generated by the MTU increased from 53% to 74% for SOL and from 62% to 75% for GAS. This increase was facilitated by greater muscle activation and the relatively isometric behavior of the SOL and GAS muscle fibers. Both of these characteristics enhanced tendon stretch and recoil, which contributed to the bulk of the change in MTU length. Our results suggest that as steady-state running speed is advanced towards maximum sprinting, the human ankle plantar-flexors continue to prioritize the storage and recovery of tendon elastic strain energy over muscle fiber work.
|Lai, A., Schache, A.G., Lin, Y-C, Pandy, M.G. (2014) Tendon elastic strain energy in the human ankle plantar-flexors and its role with increased running speed, Journal of Experimental Biology, 217, 3159-3168. (2014) View|
|Lai, A., Lichtwark, G.A., Schache, A.G., Lin, Y-C, Brown, N.A., Pandy, M.G. (2015) In vivo behaviour of the human soleus muscle with increasing walking and running speeds, Journal of Applied Physiology, 118, 1266-1275 (2015) View|
The interaction between the muscle-fascicle and tendon components of the human soleus (SO) muscle influences the capacity of the muscle to generate force and mechanical work during walking and running. In the present study, ultrasound-based measurements of in-vivo SO muscle fascicle behavior were combined with an inverse dynamics analysis to investigate the interaction between the muscle-fascicle and tendon components over a broad range of steady-state walking and running speeds: slow-paced walking (0.7 m s-1) through to moderate-paced running (5.0 m s-1). Irrespective of a change in locomotion mode (i.e. walking vs. running) or an increase in steady-state speed, SO muscle fascicles were found to exhibit minimal shortening compared to the muscle-tendon unit (MTU) throughout stance. During walking and running, the muscle fascicles contributed only 35% and 20% of the overall MTU length change and shortening velocity, respectively. Greater levels of muscle activation resulted in increasingly shorter SO muscle fascicles as locomotion speed increased, both of which facilitated greater tendon stretch and recoil. Thus, the elastic tendon contributed the majority of the MTU length change during walking and running. When transitioning from walking to running near the preferred transition speed (2.0 m s-1), greater, more economical ankle torque development is likely explained by the SO muscle fascicles shortening slower and operating on a more favorable portion (i.e. closer to the plateau) of the force-length curve.