MUSCLE FUNCTION DURING JUMPING IN FROGS .1. SARCOMERE-LENGTH CHANGE, EMG PATTERN, AND JUMPING PERFORMANCE

We determined the influence of temperature on muscle function during j umping to better understand how the frog muscular system is designed t o generate a high level of mechanical power. Maximal jumping performan ce and the in vivo operating conditions of the semimembranosus muscle (SM), a hip extensor, were measured and related to the mechanical prop erties of the isolated SM in the accompanying paper [Muscle function d uring jumping in frogs. II. Mechanical properties of muscle: implicati on for system design. Am. J. Physiol. 271 (Cell Physiol. 40): C571-C57 8, 1996]. Reducing temperature from 25 to 15 degrees C caused a 1.75-f old decline in peak mechanical power generation and a proportional dec line in aerial jump distance. The hip and knee joint excursions were n early the same at both temperatures. Accordingly, sarcomeres shortened over the same range (2.4 to 1.9 mu m) at both temperatures, correspon ding to myofilament overlap at least 90% of maximal. At the low temper ature, however, movements were made more slowly. Angular velocities we re 1.2- to 1.4-fold lower, and ground contact time was increased by 1. 33-fold at 15 degrees C. Average shortening velocity of the SM was onl y 1.2-fold lower at 15 degrees C than at 25 degrees C. The low Q(10) o f velocity is in agreement with that predicted for muscles shortening against an inertial load.