Temperature effects on mammalian muscle contraction.

Effects of temperature (range 37 degrees C to 8 degrees C) on isometric twitch and tetanic tensions of mammalian (rat) fast and slow muscles are re-examined. The twitch tension of a fast muscle increased in cooling it 20 degrees C but decreased on further cooling, whereas that of a slow muscle decreased monotonically with cooling. The cooling-potentiation in the fast muscle was evident within a week after birth but the cooling-depression in the slow muscle was not established until about the fourth week. The maximal tetanic tension in either muscle, and at all ages, decreased 10-20% in cooling to 25 degrees C but decreased more markedly in cooling below 20 degrees C; the tension at 10 degrees C was 40-50% of that at 35 degrees C. Preliminary observations made on fast muscle showed that the apparent stiffness/tension ratio was higher at low temperatures. It is postulated that the cooling depression of tetanic tension in mammalian muscle may be due to a direct effect on cross-bridges, whereas twitch tension variation may represent a net outcome of the same effect and the cooling-increase in the calcium sensitivity of myofibrillar activation.