MUSCLE FUNCTION DURING JUMPING IN FROGS .2. MECHANICAL-PROPERTIES OF MUSCLE - IMPLICATIONS FOR SYSTEM-DESIGN

We characterized the design of the frog muscular system for jumping by comparing the properties of isolated muscle with the operating condit ions of muscle measured during maximal jumps. During jumping, the semi membranosus muscle (SM) shortened with a V/V-max (where V is shortenin g velocity and V-max is maximal shortening velocity) where 90 and 100% of maximal power would be generated at 15 and 25 degrees C, respectiv ely. To assess the level of activation during jumping, the SM was driv en through the in vivo length change and stimulus conditions while the resulting force was measured. The force generated under the in vivo c onditions at both temperatures was at least 90% of the force generated at that same V under maximally activated conditions. Thus the SM was nearly maximally activated, and shortening deactivation was minimal. T he initial sarcomere length and duration of the stimulus before shorte ning were important factors that minimized shortening deactivation dur ing jumping. Thus the frog muscular system appears to be designed to m eet the three necessary conditions for maximal power generation during jumping: optimal myofilament overlap, optimal V/V-max, and maximal ac tivation.