Prolonged cycling reduces power output at the moderate-to-heavy intensity transition

To determine the effect of prolonged exercise on moderate-to-heavy intensity transition power output and heart rate.

Fourteen endurance-trained cyclists and triathletes took part in the present investigation (13 males, 1 female, V·O₂peak 59.9 ± 6.8 mL^.kg^-1.min^-1). Following a characterisation trial, participants undertook a five-stage incremental step test to determine the power output and heart rate at the moderate-to-heavy intensity transition before and after two hours of cycling at 90% of the estimated power output at first ventilatory threshold (VT₁).

Power output at the moderate-to-heavy intensity transition significantly decreased following acute prolonged exercise when determined using expired gases (VT₁, 217 ± 42 W vs. 196 ± 42 W, P < 0.0001) and blood lactate concentrations (LoglogLT, 212 ± 47 W vs. 190 ± 47 W, P = 0.004). This was attributable to loss of efficiency (VT₁, -8 ± 10 W; LoglogLT, - 7 ± 9 W) and rates of metabolic energy expenditure at the transition (VT₁, - 14 ± 11 W; LoglogLT, - 15 ± 22 W). The heart rate associated with the moderate-to-heavy intensity transition increased following acute prolonged exercise (VT_1, 142 ± 9 beats^.min^-1 vs. 151 ± 12 beats^.min^-1, P < 0.001; LoglogLT, 140 ± 13 beats^.min^-1 vs. 150 ± 15 beats^.min^-1, P = 0.006).

These results demonstrate the external work output at the moderate-to-heavy intensity transition decreases during prolonged exercise due to decreased efficiency and rates of metabolic energy expenditure, but the associated heart rate increases. Therefore, individual assessments of athlete 'durability' are warranted.