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

• Critical power (CP) was lower for the 50% duty cycle protocol than the 20% duty cycle protocol.
• W’ (curvature constant) was not different between the two duty cycle protocols.
• Q̇BA was higher for the 20% duty cycle than the 50% duty cycle.
• Deoxy-[Hb + Mb] was higher for the 20% duty cycle than the 50% duty cycle.
• These findings support the notion that CP and W’ are determined by aerobic and ‘anaerobic’ 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 (Q̇BA) and deoxygenated-[hemoglobin + myoglobin] (deoxy-[Hb + Mb]) were measured. CP was lower for the 50% duty cycle (3.9 ± 0.9 W) than the 20% duty cycle (5.1 ± 0.8 W; 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, Q̇BA 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 μM) than the 50% duty cycle (7.37 ± 1.76 ml·s−1, p < 0.001; 44.3 ± 2.4 μM, p < 0.03). These findings indicate that blood flow influences CP, but not W’.