Muscle temperature of mammals: cooling impairs most functional properties.

Our purpose was to study the effect of a decrease in skeletal muscle temperature on single and repeated shortening, isometric, and lengthening contractions of mammalian skeletal muscles. Fast extensor digitorum longus muscles of mice were studied in situ and in vitro at 25 and 35 degrees C. No difference in isometric force was observed, but maximum and sustained powers were reduced by 40 and 62%, respectively. With cooling, maximum power absorption, which is proportional to the external work required to lengthen the muscle, increased significantly at each velocity of lengthening from 0.5 to 4.0 optimum fiber length/s. The 10 degrees C decrease in muscle temperature produced a decrease in power that was primarily a result of the decrease in the velocity of shortening, whereas the increase in power absorption was likely due to an increase in the number of strongly bound cross bridges resulting from a decreased rate of detachment. During voluntary exercise at decreased muscle temperatures, maximum and endurance performances are inevitably impaired by the decreases in maximum and sustained power of individual motor units.