Influence of duty cycle on the power-duration relationship: Observations and potential mechanisms

The highest sustainable rate of aerobic metabolism [critical power (CP)] and the finite amount of work that can be performed above CP (W' [curvature constant]) were determined under two muscle contraction duty cycles. Eight men completed at least three constant-power handgrip tests to exhaustion to determine CP and W' for 50% and 20% duty cycles, while brachial artery blood flow (Q(BA)) and deoxygenated-[hemoglobin+ myoglobin] (deoxy-[Hb + Mb]) were measured. CF was lower for the 50% duty cycle (3.9 +/- 0.9 W) than the 20% duty cycle (5.1 +/- 0.8W; p < 0.001), while W' was not significantly different (50% duty cycle: 452 +/- 141 J vs. 20% duty cycle: 432 +/- 130 J; p > 0.05). At the same power output, Q(BA) and deoxy-[Hb + Mb] achieved higher end-exercise values for the 20% duty cycle (9.87 +/- 1.73 ml-s(-1); 51.7 +/- 4.7 mu M) than the 50% duty cycle (7.37 +/- 1.76 ml.s(-1), p < 0.001; 44.3 +/- 2.4 mu M, p < 0.03). These findings indicate that blood flow influences CP, but not W'. (C) 2013 Elsevier B.V. All rights reserved.