The effect of glycogen depletion on the curvature constant parameter of the power-duration curve for cycle ergometry

For high-intensity cycle ergometer exercise, the relation between power (P) and its tolerable duration (t) has been well characterized by the hyperbol ic relationship: (P-theta(F)) t = W', or P = W' (1/t) + theta(F), where the ta(F) may be termed the 'fatigue threshold'. The curvature constant (W') re flects a constant amount of work which is postulated to be equivalent to a finite energy store that relates to the oxygen-deficit: phosphagen pool, an aerobic glycolysis and oxygen stores. Compared to theta(F), the physiologic al nature of W' has received little consideration. The purpose of this stud y was therefore to establish the parameters of the power-duration curve (th eta(F) and Ur) for subjects in normal glycogen (NC) and glycogen depleted ( GD) states. Seven healthy male subjects (aged 22 to 41 years) each performe d four high-intensity square-wave exercise bouts on an electrically braked cycle ergometer under two different muscular glycogen content conditions, i .e. NG and GD states. Subjects performed the following exercise on the even ing before the trial day to induce the GD state. Initially, they performed a 75-min cycling exercise at 60% of (V) over dot O(2)max. After a 5-min res t period, they subsequently repeated a 1-min cycling bout at 115% of (V) ov er dot O(2)max (separated by I-min rest periods) until the subject could no longer maintain the prescribed pedal rate for the full minute. Subjects th en reported to the laboratory after an overnight fast and performed a singl e high-intensity exercise bout. The GD procedure was repeated four times at I-week intervals. In the GD state, the respiratory exchange ratio (RER) (V ) over dot O-2/(V) over dot O-2 value during a recumbent control period pri or to the trial was significantly lower than that in the NG state [GD: 0.84 +/-0.02, NG: 0.94+/-0.04, mean+/-SD]. There was no significant difference f or theta(F); between GD and NG state [NG: 197.1+/-31.9 W, GD: 190.6+/-28.2 W]. W' in contrast was significantly reduced by the GD procedure [NG: 12.83 +/-2.21 kJ, GD: 10.33+/-2.41 kJ]. The present results indicate that the mus cular glycogen store seems to be an important determinant of the curvature constant (W') of the power-duration curve for cycle ergometry.