Cardiorespiratory responses to sinusoidal exercise below and across critical power: energy balance and fatigue

Purpose: Sinusoidal exercise was partially investigated, without considering its energetics and the effects of fatigue. This study aimed to compare the cardiorespiratory and metabolic responses, assess the contribution of aerobic and anaerobic lactic metabolism to the energy balance, and determine the fatigue effects, in sinusoidal protocols, across and below critical power (CP). Methods: Eight participants (males, 26.6 ± 6.2 yrs.; 75.6 ± 8.7 kg; maximum oxygen uptake 52.8 ± 7.9 ml•min-1•kg-1; CP 218 ± 13 W) underwent exhausting sinusoidal cycloergometric exercises, with sine midpoint (MP) at CP (CPex) and 50 W below CP (CP-50ex). Sine amplitude (AMP) and period were 50 W and 4 min, respectively. MP, AMP and time-delay (tD) between mechanical and metabolic signals of expiratory ventilation (V’E), oxygen uptake (V’O2) and heart rate (fH) were assessed sine-by-sine. Blood lactate ([La-]) and rate of perceived exertion (RPE) were determined at each sine. Results: V’E, V’O2, and fH MP were higher in CPex than CP-50ex. V’O2 AMP was 304 ± 11 and 488 ± 36 ml•min-1 in CPex and CP- 50ex, respectively. Asymmetries between rising and declining sine phases were observed in CPex, with unchanged tDs. V’O2 MP and RPE increased progressively during CPex. [La-] increased by 2.1 mM in CPex but remained stable during CP-50ex. Anaerobic contribution was larger in CPex than CP-50ex. Conclusions: This study represents the first analysis of the energetic aspects of sinusoidal exercise across and below CP. The reduced aerobic component during CPex than CP-50ex associated with lactate accumulation explained lower V’O2 AMP in CPex. The asymmetries in CPex suggest progressive decline of muscle phosphocreatine con- centration, leading to fatigueas indicated by RPE.