The Y-intercept of the maximal work-duration relationship and anaerobic capacity in cyclists

The degree to which the y-intercept (Y-int) of the linear regression of maximal work output on exercise duration represented anaerobic capacity was determined in ten well-trained male cyclists [peak oxygen uptake (VO2peak)=69.8 (SD 4.2) ml.kg(-1) min(-1)]. Each cyclist performed three exhausting cycle sessions on separate occasions; the mean exercise durations were 312, 243 and 141 s for the low (approximately 104% VO2peak), medium (approximately 108% VO2peak) and high (approximately 113% VO2peak) intensities respectively, and Y-int (kilojoules; joules per kilogram) was derived from the regression of work output on exercise duration. The muscle anaerobic adenosine 5'-triphosphate (ATP) yield (Sigma ATP) and anaerobic capacity (AC) were estimated from changes in metabolites in the vastus lateralis muscle and blood lactate concentration during the high intensity cycling session. The activities of glycogen phosphorylase, phosphofructokinase and citrate synthase, as well as muscle buffer value (in vitro beta) were also determined. The Y-int (kilojoules) was positively correlated (P less than or equal to 0.05) with AC (r=0.73), Sigma ATP (r=0.70) and in vitro beta (r=0.71); similar correlations (P less than or equal to 0.05) were observed for Y-int (joules per kilogram). The Y-int was not correlated (P>0.05) with any enzyme activity. When the Y-int was transformed into oxygen equivalents [litres of oxygen equivalent (l O-2 Eq)] it was, on average, 0.92 l O-2 Eq lower than AC (P less than or equal to 0.05); however, an alternative method of establishing the work-duration regression yielded a mean Y-int which was only 0.191 O-2 Eq less than AC (P greater than or equal to 0.05). These findings support the validity of Y-int as a work estimate of anaerobic capacity in well-trained cyclists.