Improved muscle gearing in the medial gastrocnemius of pregnant and lactating rats.

Pennate muscles typically contract with complex three-dimensional shape changes that vary in magnitude across muscle force levels, allowing improved muscle fiber force-velocity dynamics across a whole muscle's range of forces (i.e. variable gearing ability). Previous work on the underlying mechanisms that dictate variable gearing suggests that the mechanical properties of intramuscular and extracellular connective tissue can act to constrain muscle bulging and gearing ability. However, it remains unclear whether increased muscle compliance can confer improved gearing ability. Recent work in pregnant and lactating rats suggest adaptations in the medial gastrocnemius (MG) muscle that may influence variable gearing ability and overall function. Here, we used in situ muscle preparations in rats to characterize the range of architectural gear ratios (AGR = muscle velocity/fascicle velocity), force-length properties, and force-velocity properties of isolated gastrocnemii to test the hypothesis that gearing ability improves during pregnancy and lactation. We found that pregnant and lactating rats exhibit a significantly wider range of AGRs and steeper linear relationship between AGR and muscle force, suggesting an improved ability to fine-tune fiber contractile dynamics to force level during dynamic muscle contractions. This improved gearing ability is underpinned by longer muscle fascicles in pregnant and lactating rats, which have broader implications on in vivo force-velocity dynamics during locomotion. We conclude that young female rats are a viable biological system that naturally varies in gearing ability across different reproductive stages without differences in relative muscle stiffness.