Abstract
BackgroundDuring the stance phase of running, the elasticity of the Achilles tendon enables the utilisation of elastic energy and allows beneficial contractile conditions for the triceps surae muscles. However, the effect of changes in tendon mechanical properties induced by chronic loading is still poorly understood. We tested the hypothesis that a training-induced increase in Achilles tendon stiffness would result in reduced tendon strain during the stance phase of running, which would reduce fascicle strains in the triceps surae muscles, particularly in the mono-articular soleus.MethodsEleven subjects were assigned to a training group performing isometric single-leg plantarflexion contractions three times per week for ten weeks, and another ten subjects formed a control group. Before and after the training period, Achilles tendon stiffness was estimated, and muscle-tendon mechanics were assessed during running at preferred speed using ultrasonography, kinematics and kinetics.ResultsAchilles tendon stiffness increased by 18% (P < 0.01) in the training group, but the associated reduction in strain seen during isometric contractions was not statistically significant. Tendon elongation during the stance phase of running was similar after training, but tendon recoil was reduced by 30% (P < 0.01), while estimated tendon force remained unchanged. Neither gastrocnemius medialis nor soleus fascicle shortening during stance was affected by training.DiscussionThese results show that a training-induced increase in Achilles tendon stiffness altered tendon behaviour during running. Despite training-induced changes in tendon mechanical properties and recoil behaviour, the data suggest that fascicle shortening patterns were preserved for the running speed that we examined. The asymmetrical changes in tendon strain patterns supports the notion that simple in-series models do not fully explain the mechanical output of the muscle-tendon unit during a complex task like running.
Highlights
The integrated function of tendon and muscle plays an important role for force production during locomotion
While lengthening of the gastrocnemius muscle–tendon unit (MTU) is partly offset by knee flexion during the first part of the stance phase (i.e., 0–40%), the relatively greater lengthening of the soleus MTU and series elastic elements (SEE) (Lai et al, 2018) caused by ankle dorsiflexion suggests that length changes in this muscle may be more affected by a training-induced increase in tendon stiffness
Tendon stiffness increased by 18% (P < 0.01), tendon strain did not decrease significantly (P = 0.18)
Summary
The integrated function of tendon and muscle plays an important role for force production during locomotion. On the basis of a two-dimensional model of the Achilles tendon in series with the gastrocnemius muscle, a change in tendon stiffness would affect the functional range of fascicles (i.e., fascicle operating length and shortening velocity), which would in turn alter the conditions of muscle work production. Contrary to this assumption, Albracht & Arampatzis (2013) did not find any alteration in gastrocnemius fascicle behaviour during running after a training-induced increase in Achilles tendon stiffness. The asymmetrical changes in tendon strain patterns supports the notion that simple inseries models do not fully explain the mechanical output of the muscle-tendon unit during a complex task like running
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