Starting length and temperature dependence of eccentric muscle force-velocity behaviour.

The force-velocity relationship underpins much of our understanding of muscle force generation during dynamic movements and is a component of musculoskeletal models. This relationship comprises a concentric (shortening) component and an eccentric (lengthening) component. While the concentric force-velocity relationship has been comprehensively described, and the importance of experimental conditions (e.g. temperature, starting length and fitting equations) aptly determined, there is no standardised approach to describe and quantify the complex relationship during eccentric contractions. Despite more than five decades of research, inconsistent starting lengths and temperature protocols limit the generalisation of findings across studies, constrain understanding of underlying mechanisms and hinder accurate integration of eccentric contractions into musculoskeletal models. Here, we have investigated the functional implications of different starting lengths and temperatures on the dynamic force-velocity relationship in the mouse soleus muscle. We show that the initial rapid rise in force (phase-1, cross-bridge dependent) is highly sensitive to the starting position on the force-length relationship, suggesting that the rate of force development is linked to the degree of actin-myosin overlap. The phase-1 response is also significantly affected by temperature, with lower temperatures reducing the rate of force development. Further, our data highlight that the second shallower force response (phase-2, non-contractile serial elastic component) is also significantly impacted by temperature, again reducing the rate of force development, probably through reduced/slower Ca2+ activation of titin. Together, these findings establish a framework for representing the dynamic behaviour of muscle during eccentric contractions, enabling the incorporation of physiologically realistic eccentric muscle properties into musculoskeletal models.