Additional in-series compliance does not affect the length dependence of activation in rat medial gastrocnemius

What is the central question of this study? The length dependence of activation (LDA) is typically explained by a length-dependent increase in calcium sensitivity, but recently calcium-independent mechanisms have been suggested: does active muscle shortening provided by a compliant in-series component impact the muscle length at which force output is maximized, thus contributing to LDA? What is the main finding and its importance? Using an in situ rat medial gastrocnemius set-up and varying the magnitude of muscle shortening via an artificial compliant series-elastic component, we were unable to observe any change in optimal length between conditions, contrary to some previous findings. More research is therefore required to explain these discrepancies.

The force-length relationship dictates the amount of force a muscle can produce as a function of its length, during maximal isometric contractions. When activation is submaximal, it has been shown that the length at which force production is highest (the optimal length) is longer. This is typically explained by a length-dependent increase in Ca²⁺ sensitivity, known as the 'length dependence of activation'. Recent reports have implicated shortening against in-series compliance to be a potential factor in the observed optimal length (L₀ ) of muscle, via the phenomenon of shortening-induced force depression (a phenomenon which describes the relative reduction in muscle force when a muscle is actively shortening to a given length compared to contracting isometrically at that same length). In the current study, rat medial gastrocnemius was stimulated with single and triple pulses (200 Hz) over a range of lengths, both with and without additional in-series compliance provided by a small piece of silicon tubing in series with the muscle, which allowed greater fascicle shortening upon activation. Fascicle length was measured using sonomicrometry crystals, and peak force (F_peak ) and L₀ were estimated by curve-fitting of the force-length data. The additional in-series compliance significantly reduced F_peak by approximately 14% and 25% for the single and triple pulses, respectively (P = 0.003, P < 0.001), yet L₀ remained unchanged (P = 0.405), suggesting that in our model, shortening against in-series compliance does not affect L₀ . We offer potential explanations for the discrepancies seen and discuss whether the velocity of shortening may have a role in the length dependence of force.