Exercise heat stress does not reduce central activation to non-exercised human skeletal muscle

In this study we measured the central activation ratio (CAR) of the leg extensors and the elbow flexor muscles before and after exhaustive exercise in the heat to determine whether exercise-induced hyperthermia affects the CNS drive to exercised (leg extensors) and/or non-exercised (forearm flexors) muscle groups. Thirteen subjects exercised at fixed intensities representative of a percentage of peak power output (PPO) for 10 min periods (50 %, 40 %, 60 %, 50 %) and then at 75 % PPO until exhaustion in ambient conditions of 39.3 +/- 0.8 degrees C and 60.0 +/- 0.8 % relative humidity. Before and immediately following exercise subjects performed a series of maximal voluntary contractions (MVCs) with the leg extensors (exercised muscles) and forearm flexors (non-exercised muscles). The degree of voluntary activation during the sustained MVCs was assessed by superimposing electrical stimulation to the femoral nerve and the biceps brachii. Exercise to exhaustion increased the rectal temperature from 37.2 +/- 0.2 to 38.8 +/- 0.2 degrees C (P < 0.0001). The mean heart rate at the end of exercise to exhaustion was 192 +/- 3 beats min(-1). Leg extensor voluntary force was significantly reduced from 595 +/- 143 to 509 +/- 105 N following exercise-induced hyperthermia but forearm flexor force was similar before and after exercise. The CAR of the leg extensors decreased from 94.2 +/- 1.3 % before exercise to 91.7 +/- 1.5 % (P < 0.02) following exercise-induced hyperthermia. However, the CAR for the forearm flexors remained at similar levels before and after exercise. The data suggest that the central nervous system selectively reduces central activation to specific skeletal muscles as a consequence of exercise-induced hyperthermia.