The temperature of a muscle has a significant effect on the velocity and power of the muscle contraction, with performance generally declining with decreasing temperatures and increasing with rising temperatures. The Q coefficient represents the degree of temperature dependence a muscle exhibits as measured by contraction rates. A Q of 1.0 indicates thermal independence of a muscle whereas an increasing Q value indicates increasing thermal dependence. Values less than 1.0 indicate a negative or inverse thermal dependence, i.e., a decrease in muscle performance as temperature increases.
Q values for biological processes vary with temperature. Decreasing muscle temperature results in a substantial decline of muscle performance such that a 10 degree Celsius temperature decrease results in at least a 50% decline in muscle performance. Persons who have fallen into icy water may gradually lose the ability to swim or grasp safety lines due to this effect, although other effects such as atrial fibrillation are a more immediate cause of drowning deaths. At some minimum temperature biological systems do not function at all, but performance increases with rising temperature (Q of 2-4) to a maximum performance level and thermal independence (Q of 1.0-1.5). With continued increase in temperature, performance decreases rapidly (Q of 0.2-0.8) up to a maximum temperature at which all biological function again ceases.
Within vertebrates, different skeletal muscle activity has correspondingly different thermal dependencies. The rate of muscle twitch contractions and relaxations are thermally dependent (Q of 2.0-2.5), whereas maximum contraction, e.g., tetanic contraction, is thermally independent.
Muscles of some ectothermic species. e.g., sharks, show less thermal dependence at lower temperatures than endothermic species