For high-intensity cycle ergometer exercise, the relationship between power output ( P) and its tolerable duration ( t) has been well characterized by the hyperbolic relationship: ( P- theta;(F)). t=W', where theta;(F) has been termed the "critical power" or "fatigue threshold". The curvature constant (W') reflects a constant amount of work which can be performed above theta;(F), and it may be regarded as a muscle energy store. The relationship of this energy store to muscle mass is not known. Therefore, the purpose of this study was to determine the relationships among W', accumulated peak oxygen deficit (accumulated peak O(2)-deficit), and muscle cross-sectional area (CSA) of the thigh for high-intensity cycle ergometry in humans. A group of 17 healthy male subjects (aged 21-41 years) participated in this study. The theta;(F) and W' of the P- t hyperbolic relationship and the accumulated peak O(2)-deficit was calculated by standard procedures. The CSA of muscle, fat and bone in the right thigh were measured using ultrasonography. The mean (SD) of theta;(F), W', accumulated peak O(2)-deficit, and muscle CSA of the thigh were 200.0 (17.8) W, 12.60 (2.94) kJ, 2.29 (0.41) l, and 185.3 (22.6) cm(2), respectively. The muscle CSA of the thigh was positively correlated with W' ( r=0.59, P<0.01) and with accumulated peak O(2)-deficit ( r=0.54, P<0.05). The relationship between W' and accumulated peak O(2)-deficit also showed a positive correlation ( r=0.63, P<0.005). Our results indicated that W' derived from the P- t hyperbolic curve as anaerobic working capacity is related to the CSA of muscle.