Abstract
Human running features a spring-like interaction of body and ground, enabled by elastic tendons that store mechanical energy and facilitate muscle operating conditions to minimize the metabolic cost. By experimentally assessing the operating conditions of two important muscles for running, the soleus and vastus lateralis, we investigated physiological mechanisms of muscle work production and muscle force generation. We found that the soleus continuously shortened throughout the stance phase, operating as work generator under conditions that are considered optimal for work production: high force-length potential and high enthalpy efficiency. The vastus lateralis promoted tendon energy storage and contracted nearly isometrically close to optimal length, resulting in a high force-length-velocity potential beneficial for economical force generation. The favorable operating conditions of both muscles were a result of an effective length and velocity-decoupling of fascicles and muscle-tendon unit, mostly due to tendon compliance and, in the soleus, marginally by fascicle rotation.
Highlights
During locomotion, muscles generate force and perform work in order to support and accelerate the body, and the activity of the lower-limb muscles accounts for most of the metabolic energy cost needed to walk or run (Kram and Taylor, 1990; Kram, 2000; Dickinson et al, 2000)
The EMG comparison showed that the soleus was active throughout the entire stance phase of running while the vastus lateralis was mainly active in the first part of the stance and with an earlier peak of activation
The soleus and the vastus lateralis fascicle length were clearly decoupled from the muscle-tendon unit (MTU) length with smaller operating length ranges throughout the whole stance phase (Figure 1)
Summary
Muscles generate force and perform work in order to support and accelerate the body, and the activity of the lower-limb muscles accounts for most of the metabolic energy cost needed to walk or run (Kram and Taylor, 1990; Kram, 2000; Dickinson et al, 2000). For a given muscle force, the metabolic cost depends on the muscle’s operating force-length and force-v elocity potential (Bohm et al, 2019; Bohm et al, 2018; Nikolaidou et al, 2017) (fraction of maximum force according to the force-length [Gordon et al, 1966] and force-velocity [Hill, 1938] curves) because it determines the number of recruited muscle fibers and the active muscle volume (Roberts, 2002). Muscular work by active muscle shortening is needed to maintain the running movement, yet it increases the metabolic cost a) due to the reduced force-v elocity potential, which will increase the active muscle volume for a given force (Roberts and Azizi, 2011), and b) due to the higher metabolic energy consumption
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