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

Abstract 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. image Key points The phenomenon of the activation‐dependent shift in optimal muscle fibre length for force generation (length‐dependent activation) is poorly understood for human muscle in vivo dynamic contractions. We experimentally observed a moderate shift in optimal fascicle length towards longer length at decreasing electromyographic activity levels for the human soleus muscle in vivo . Based on the experimental results, we developed a freely accessible approximation model that allows the consideration of activation‐dependent shifts in optimal length in future experimental and musculoskeletal modelling studies to improve muscle force predictions.