MUSCLE ENERGETICS AND PULMONARY OXYGEN UPTAKE DURING INCREMENTAL ISOMETRIC PLANTAR FLEXION EXERCISE

Abstract

The purpose of this study was to clarify the relationship between muscle energetics and pulmonary oxygen uptake (VO2) during incremental exercise. Five male subjects performed a repetitive intermittent (6-s contraction / 4-s relaxation) isometric plantar flexion exercise. Exercise intensity was raised every 1 min by 10% maximal voluntary contraction (MVC) starting from 10% MVC to the intensity where the subject could no longer maintain the prescribed force. The measurement site was at the medial gastrocnemius muscle. Changes in phosphocreatine (PCr), inorganic phosphate (Pi) and pH in the muscle were measured by 31P-magnetic resonance spectroscopy. The muscle oxygen kinetics was measured by near infrared spectroscopy. Also, the muscle oxygen consumption was determined from the rate of deoxygenation during transient arterial occlusion (Hamaoka et al. J. Appl. Physiol. 1996). The electromyogram (EMG) was also recorded. Pulmonary VO2 was measured by the breath-by-breath gas analysis method. During exercise below 30% MVC, although the EMG amplitude increased and muscle oxygenation gradually decreased, the muscle PCr and oxygen consumption did not change significantly. However, an exercise above 30% MVC muscle PCr decreased and oxygen consumption increased with exercise intensity. Pulmonary VO2 also changed slightly, and no significant difference between exercise intensity was observed below 40% MVC. When exercise intensity was raised above 40% MVC, pulmonary VO2 increased with exercise intensity. There was a significant linear correlation between muscle PCr and muscle oxygen consumption (r = 0.975, p < 0.0001), muscle oxygen consumption and pulmonary VO2 (r = 0.962, p < 0.0001), and muscle PCr and pulmonary VO2 (r = 0.977, p < 0.0001). Furthermore, the decrease in PCr and increase in muscle oxygen consumption preceded the increase in pulmonary VO2. These results suggest that the breakdown of intramuscular PCr is a factor to stimulate muscle oxygen consumption resulting in the increase in pulmonary VO2. The results of this study indicated that muscle PCr, muscle oxygen consumption, and pulmonary VO2 are closely related during an incremental plantar flexion exercise.