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 ( {Mathematical expression} = 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% {Mathematical expression}, medium (approximately 108% {Mathematical expression} and high (approximately 113% {Mathematical expression} 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 (ΣATP) 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 β) were also determined. The Y-int (kilojoules) was positively correlated (P≤0.05) with AC (r=0.73), ΣATP (r=0.70) and in vitro β (r=0.71); similar correlations (P≤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 (1 O₂ Eq)] it was, on average, 0.92 1 O₂ Eq lower than AC (P≤0.05); however, an alternative method of establishing the work-duration regression yielded a mean Y-int which was only 0.19 1 O₂ Eq less than AC (P≥0.05). These findings support the validity of Y-int as a work estimate of anaerobic capacity in well-trained cyclists.