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J Appl Physiol 93: 2023-2028, 2002. First published August 23, 2002; doi:10.1152/japplphysiol.00521.2002
8750-7587/02 $5.00
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Vol. 93, Issue 6, 2023-2028, December 2002

Effect of naloxone on perceived exertion and exercise capacity during maximal cycle ergometry

Anthony L. Sgherza1, Kenneth Axen1,2,3, Randi Fain3, Robert S. Hoffman4, Christopher C. Dunbar1, and François Haas2,3

1 Department of Physical Education and Exercise Science, Brooklyn College, Brooklyn 11210; 2 Joan and Joel Smilow Cardiac Prevention and Rehabilitation Center, The Rusk Institute of Rehabilitation Medicine, 3 Division of Pulmonary and Critical Care Medicine, Department of Medicine, and 4 Department of Emergency Medicine, New York University School of Medicine, New York, New York 10016

We assessed the effects of naloxone, an opioid antagonist, on exercise capacity in 13 men and 5 women (mean age = 30.1 yr, range = 21-35 yr) during a 25 W/min incremental cycle ergometer test to exhaustion on different days during familiarization trial and then after 30 mg (iv bolus) of naloxone or placebo (Pl) in a double-blind, crossover design. Minute ventilation (VE), O2 consumption (VO2), CO2 production, and heart rate (HR) were monitored. Perceived exertion rating (0-10 scale) and venous samples for lactate were obtained each minute. Lactate and ventilatory thresholds were derived from lactate and gas-exchange data. Blood pressure was obtained before exercise, 5 min postinfusion, at maximum exercise, and 5 min postexercise. There were no control-Pl differences. The naloxone trial demonstrated decreased exercise time (96% Pl; P < 0.01), total cumulative work (96% Pl; P < 0.002), peak VO2 (94% Pl; P < 0.02), and HR (96% Pl; P < 0.01). Other variables were unchanged. HR and VE were the same at the final common workload, but perceived exertion was higher (8.1 ± 0.5 vs. 7.1 ± 0.5) after naloxone than Pl (P < 0.01). The threshold for effort perception amplification occurred at ~60 ± 4% of Pl peak VO2. Thus we conclude that peak work capacity was limited by perceived exertion, which can be attenuated by endogenous opioids rather than by physiological limits.

peak oxygen consumption; lactate threshold; endogenous opioids; physiological fatigue


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