Juvenile mammals must often compete in the same ecological niche as adults, despite smaller absolute body size and muscle force capacity. However, because limb joints operate as levers, greater muscle mechanical advantage (i.e., ‘leverage’) in juveniles may permit greater force production despite these disadvantages. Previous research on mammals has shown that bony proxies for extensor muscle leverage are greater early in development and decrease throughout growth. However, no previous study of mammalian musculoskeletal development has collected the locomotor data needed to examine changes in actual (i.e., “effective”) mechanical advantage.
We used high-speed videography and force plate measurements, combined with anatomical data on muscle lever arm lengths, to calculate effective mechanical advantage (EMA), a measure of limb muscle leverage, exploring the relationship between body size (proxy for age) and EMA. We predicted that if rabbits maintain proportionality during growth, then the length variables that determine muscle leverage should scale to the cube root of overall volume, and therefore the cube root of overall body mass (i.e., Mb0.33). However, we found that muscle lever arms scaled to Mb0.27, whereas the lever arms of the loading forces that the limb muscles had to resist scaled to Mb0.45. Therefore EMA, as the quotient of these two measures, scaled to Mb-0.18, such that muscle leverage decreased by 36% over the course of rabbit growth. Greater leverage early in life may permit young rabbits to attain adult-like levels of locomotor performance, despite smaller size and reduced muscle mass.
Dr. Jesse W. Young
Dr. Adam D. Foster