Assessment and modelling of the activation-dependent shift in optimal length of the human soleus muscle in vivo

Previous in vitro and in situ studies have reported a shift in optimal muscle fibre length for force generation (L-0) towards longer length at decreasing activation levels (also referred to as length-dependent activation), yet the relevance for in vivo human muscle contractions with a variable activation pattern remains largely unclear. By a combination of dynamometry, ultrasound and electromyography (EMG), we experimentally obtained muscle force-fascicle length curves of the human soleus at 100%, 60% and 30% EMG(max) levels from 15 participants aiming to investigate activation-dependent shifts in L(0)in vivo. The results showed a significant increase in L-0 of 6.5 +/- 6.0% from 100% to 60% EMG(max) and of 9.1 +/- 7.2% from 100% to 30% EMG(max) (both P < 0.001), respectively, providing evidence of a moderate in vivo activation dependence of the soleus force-length relationship. Based on the experimental results, an approximation model of an activation-dependent force-length relationship was defined for each individual separately and for the collective data of all participants, both with sufficiently high accuracy (R-2 of 0.899 +/- 0.056 and R-2 = 0.858). This individual approximation approach and the general approximation model outcome are freely accessible and may be used to integrate activation-dependent shifts in L-0 in experimental and musculoskeletal modelling studies to improve muscle force predictions.