Effect of opioid antagonism on the ability to tolerate maximal anaerobic exercise

The effect of naloxone-induced endogenous opioid antagonism on the ability to tolerate maximal anaerobic exercise was studied in 11 young men. Volume-matched infusions of placebo or naloxone 4 mg were administered in a randomised double-blind crossover trial before a 30-second maximal cycle ergometer test (Wingate test). Mechanical power output was calculated for each 5 seconds of cycling and expressed as peak power (highest output during any 5-second period), average power (average output during the 30 seconds of cycling) and power decline (difference between peak power and the lowest 5-second power output divided by time). Naloxone did not significantly modify (P greater than 0.05) any index of anaerobic power. Similarly, the perception of leg pain during exercise was similar with placebo and naloxone. Therefore, insofar as naloxone may be used to examine the functional role of opioid-mediated mechanisms, these findings are not tenable with the hypothesis that endogenous opioids modulate the ability to tolerate maximal anaerobic exercise.