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The purpose of the present study was to investigate the relationship between endurance time and fatigue factors at varying intensities of handgrip isometric exercise. Seven subjects performed isometric contractions at 10%, 30% and 50% of maximum voluntary contraction (MVC) sustained to exhaustion. Continuous changes in high-energy phosphates and muscle oxygen content (oxy-Hb/Mb) in the forearm flexor muscle were measured by 31P magnetic resonance spectroscopy (31P-MRS) and near infrared spectroscopy (NIRS), respectively. The endurance time to exhaustion was 963•}236 (mean•}S. D.), 209•}41.9 and 95.3•}13.6s at 10%, 30% and 50%MVC, respectively, From the onset of exercise at each %MVC, both PCr and oxy-Hb/Mb fell and Pi (H2PO4-) rose linearly with time, whereas intracellular pH remained relatively constant at resting values during the early phase. Thereafter, intracellular pH showed a linear decline. The rates of pH decline were 0.03•}0.02, 0.22•}0.14 and 0.51•}0.17 pHunit/min, while the rates of H2PO4increase were 20.9•}19.3, 118 •} 48.9 and 434•}242 (% of resting)/min at 10%, 30% and 50%MVC, respectively. There were correlations between the changes in endurance times to exhaustion and the rates of pH decline (r= -0 .58•`-0.87) and H2PO4increase (r=-0.37•`-0.74, (n=7)) at each intensity of exercise. In particular, significant correlations (p<0.05) between endurance time and the rate of pH decline were found at both 10% and 30%MVC. These results suggest that muscle fatigue (endurance time) at each intensity in handgrip isometric exercise is closely correlated with the rate of pH decline. (Jpn. J. Phys. Fitness Sports Med. 1998, 47 : 549•`560) key words: endurance time, the rate of pH decline, the rate of H2PO4increase

THE RELATIONSHIP BETWEEN ENDURANCE TIME AND FATIGUE FACTORS AT VARYING INTENSITIES IN HANDGRIP ISOMETRIC EXERCISE