Abstract Purpose Previous studies investigating sinusoidal exercise were not devoted to an analysis of its energetics and of the effects of fatigue. We aimed to determine the contribution of aerobic and anaerobic lactic metabolism to the energy balance and investigate the fatigue effects on the cardiorespiratory and metabolic responses to sinusoidal protocols, across and below critical power (CP). Methods Eight males (26.6 ± 6.2 years; 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 sinusoid midpoint (MP) at CP (CP_ex) and 50 W below CP (CP-50_ex). Sinusoid amplitude (AMP) and period were 50 W and 4 min, respectively. MP, AMP, and time-delay (t_D) between mechanical and metabolic signals of expiratory ventilation (${\dot{V}}_{E}$ V ˙ E ), oxygen uptake (${\dot{V}}_{{{\text{O}}}_{2}}$ V ˙ O 2 ), and heart rate (${f}_{H}$ f H ) were assessed sinusoid-by-sinusoid. Blood lactate ([La⁻]) and rate of perceived exertion (RPE) were determined at each sinusoid. Results ${\dot{V}}_{{{\text{O}}}_{2}}$ V ˙ O 2 AMP was 304 ± 11 and 488 ± 36 ml·min⁻¹ in CP_ex and CP-50_ex, respectively. Asymmetries between rising and declining sinusoid phases occurred in CP_ex (36.1 ± 7.7 vs. 41.4 ± 9.7 s for ${\dot{V}}_{{{\text{O}}}_{2}}$ V ˙ O 2 t_{D up} and t_{D down}, respectively; P < 0.01), with unchanged t_Ds. ${\dot{V}}_{{{\text{O}}}_{2}}$ V ˙ O 2 MP and RPE increased progressively during CP_ex. [La⁻] increased by 2.1 mM in CP_ex but remained stable during CP-50_ex. Anaerobic contribution was larger in CP_ex than CP-50_ex. Conclusion The lower aerobic component during CP_ex than CP-50_ex associated with lactate accumulation explained lower ${\dot{V}}_{{{\text{O}}}_{2}}$ V ˙ O 2 AMP in CP_ex. The asymmetries in CP_ex suggest progressive decline of muscle phosphocreatine concentration, leading to fatigue, as witnessed by RPE.