Combined effect of heat stress, dehydration and exercise on neuromuscular function in humans

This study examined the combined effect of exercise induced hyperthermia an d dehydration on neuromuscular function in human subjects. Six trained male runners ran for 40 min on a treadmill at 65% of their maximal aerobic velo city while wearing a tracksuit covered with an impermeable jacket and pants to impair the evaporation of sweat. These stressful experimental running c onditions led the runners to a physiological status close to exhaustion. On average, the 40 min run ended at a heart rate of 196 (SD 8) beats . min(-1 ), a tympanic temperature of 40 (SD 0.3) degreesC and with a loss of body m ass of 2 (SD 0.5)%. Pre- and post-running strength tests included measureme nts of maximal knee extension and flexion torques in both isometric and iso kinetic (at 60 and 240 degrees . s(-1)) conditions. A 20 s endurance test a t 240 degrees . s(-1) was also performed. Surface electromyographic (EMG) a ctivity was recorded from six knee extensor and flexor muscles during the e ntire protocol. The treadmill run led to clear decrements in maximal extens ion torque and EMG activity both in isometric and at the slowest isokinetic velocity (60 degrees . s(-1)). However, no differences in these parameters were observed at 240 degrees . s(-1) Furthermore, the EMG patterns of the major knee extensor and flexor muscles remained remarkably stable during th e treadmill run. These results demonstrate that the exercise-induced hypert hermia and dehydration in the present experiments had only minor effects on the neuromuscular performance. However, it is also suggested that high int ernal body temperature per se could limit the production of high force leve ls.