Human medial gastrocnemius force–velocity behavior shifts with locomotion speed and gait

Abstract

Humans walk and run over a wide range of speeds with remarkable efficiency. For steady locomotion, moving at different speeds requires the muscle-tendon units of the leg to modulate the amount of mechanical power the limb absorbs and outputs in each step. How individual muscles adapt their behavior to modulate limb power output has been examined using computer simulation and animal models, but has not been studied in vivo in humans. In this study, we used a combination of ultrasound imaging and motion analysis to examine how medial gastrocnemius (MG) muscletendon unit behavior is adjusted to meet the varying mechanical demands of different locomotor speeds during walking and running in humans. The results highlighted key differences in MG fascicle-shortening velocity with both locomotor speed and gait. Fascicle-shortening velocity at the time of peak muscle force production increased with walking speed, impairing the ability of the muscle to produce high peak forces. Switching to a running gait at 2.0 m·s −1 caused fascicle shortening at the time of peak force production to shift to much slower velocities. This velocity shift facilitated a large increase in peak muscle force and an increase in MG power output. MG fascicle velocity may be a key factor that limits the speeds humans choose to walk at, and may explain the transition from walking to running. This finding is consistent with previous modeling studies.muscle mechanics | biomechanics | preferred transition speed A nkle plantar-flexor muscles are a vital source of mechanical power for human locomotion (1, 2). During walking, plantar-flexor muscles provide body weight support, contribute to propulsion, and accelerate the limb into swing (3). In running, the ankle acts in a spring-like manner, absorbing energy in plantarflexor muscle-tendon units during early stance and providing energy to accelerate the body in late stance (1). The mechanical work required to produce whole-body movement during walking and running varies between gaits and across speeds (4, 5). Thus, the plantar-flexors may need to adjust their mechanical work output with gait and speed to meet the changing demands on their contribution to total mechanical work.A recent experimental study in humans used an inverse-dynamics approach to examine how the mechanical power outputs of muscles acting at the hip, knee, and ankle joint were modulated for walking and running at a range of speeds (6). It was found that positive power output at the ankle, in conjunction with the knee and hip, increased with walking speed. Also, Hansen et al. (7) showed that at walking speeds above those preferred, the net positive work done at the ankle increased. When switching from walking to running gait, the relative contribution of ankle positive power output to total positive power output also increased (6). It was inferred from these data that plantar-flexor muscle mechanics were adjusted to accommodate faster walking speeds and then again with the switch to running gait. If this is truly the case, then it may hav...

The human iliotibial band is specialized for elastic energy storage compared with the chimp fascia lata

“…Biewener and Roberts, 2000). Consistent with this, ultrasound-based studies of human gastrocnemius function show limited shortening until push-off (Lichtwark et al, 2007;Farris and Sawicki, 2012), and Arnold et al's simulations of human walking and running (2013) showed that the GMax muscle fiber velocities were low during walking. In Arnold et al's model, GMax inserts on the femur via a relatively stiff tendon, and this assumption likely amplifies the muscle's estimated shortening velocity.…”

Slower Walking Speed in Older Men Improves Triceps Surae Force Generation Ability

“…In support of the idea that plantarflexor force generation is an important factor in the selection of preferred walking speed in older adults, we observed a significant correlation between preferred walking speed and body weight-normalized plantarflexion strength. Previously, contractile conditions for force generation in triceps surae muscles have been linked to preferred walk to run transition speed in young adults in both experimental and modeling studies (11,33), and a similar mechanism could be involved in the selection of preferred walking speed of older adults. Improved contractile conditions for force generation could also play a role in the selection of preferred walking speed via energy cost of walking.…”

“…Tendinous tissues (tendon and aponeurosis, TT) elasticity greatly affects muscle function of the main plantarflexor, triceps surae, due to a large tendon to muscle fiber length ratio (41). Using ultrasound imaging, it has been revealed that TT elasticity is utilized in triceps surae to enhance power generation during the push-off phase of walking (11,21).…”

Tendon elastic strain energy in the human ankle plantar-flexors and its role with increased running speed

“…Comparison of in vivo measured and model-based gastrocnemius muscle fiber lengths for walking and slow speeds of running. In vivo measurements from ultrasound of gastrocnemius muscle fiber lengths were reported elsewhere (Farris and Sawicki, 2012). Note that the data reported by Farris and Sawicki for running correspond to rear-foot strikers.…”

“…The function of the tendon can influence that of the muscle fibers in such a way that muscle fiber behavior is decoupled from the overall behavior of the MTU, thereby optimizing muscle fiber contractile conditions and minimizing mechanical energy expenditure (Roberts and Azizi, 2011). For instance, in vivo ultrasonography studies of the ankle plantar-flexors have demonstrated that tendon stretch can optimize the regions in which muscle fibers operate on their force-length-velocity relationship during walking and slow speeds of running (Fukunaga et al, 2001;Ishikawa et al, 2005;Lichtwark et al, 2007;Farris and Sawicki, 2012). Furthermore, musculoskeletal modeling studies investigating the function of the ankle plantar-flexors during walking and running have calculated the contributions of muscle fiber work and tendon elastic strain energy to the mechanical energetics of the MTU (Hof et al, 2002;Sasaki and Neptune, 2006).…”