Relationship between the curvature constant parameter of the power-duration curve and muscle cross-sectional area of the thigh for cycle ergometry in humans

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–θF)·t=W′, where θF has been termed the "critical power" or "fatigue threshold". The curvature constant (W′) reflects a constant amount of work which can be performed above θF, 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 O2-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 θF and W′ of the P-t hyperbolic relationship and the accumulated peak O2-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 θF, W′, accumulated peak O2-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) cm2, respectively. The muscle CSA of the thigh was positively correlated with W′ (r=0.59, P<0.01) and with accumulated peak O2-deficit (r=0.54, P<0.05). The relationship between W′ and accumulated peak O2-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.