Effect of temperature on posttetanic potentiation in human dorsiflexor muscles

The effect of temperature on post-tetanic potentiation (PTP) has been exami ned in the muscles of small mammals but not in human skeletal muscle. We ex amined PTP in the ankle dorsiflexor muscles of 10 young men by evoking twit ches before and after a 7-second tetanus at 100 Hz in a control (room air s imilar to 21 degreesC) condition and after immersion of the lower leg in wa rm (45 degreesC) and cold (10 degreesC) water baths for 30 min. Exposure to cold decreased tetanus and pre-tetanus twitch peak torque, but increased r ise time, half-relaxation time, and muscle action potential (M-wave) amplit ude; exposure to warm water had little effect. PTP was smallest in cold exp osure 5 s post-tetanus, but persisted throughout the 12 min test period, wh ereas PTP had subsided by 6 min post-tetanus in control and warm exposures. M-wave amplitude initially decreased after exposure to warm water, recover ed, then decreased again by 11 min post-tetanus. In contrast, exposure to c old had no initial effect but did increase the M-wave amplitude during the last half of the 12 min test period, similar to that seen in the control. T he greatest immediate decrease in rise time and half-relaxation time was ob served in the control; however, by 12 min post-tetanus warm exposure showed the greatest increase in rise time and half-relaxation time above pre-teta nus values. The decrease in the unpotentiated twitch torque with cooling in human dorsiflexors is typical for muscles with a predominance of type I (s low) fibres. The effect of cold on PTP is similar to that seen previously i n mammalian muscles with a predominance of type II (fast) fibres, although the underlying mechanism of the cooling effect appears to differ.