Prolonged constant load cycling exercise is associated with reduced gross efficiency and increased muscle oxygen uptake

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Abstract

This study investigated the effects of prolonged constant load cycling exercise on cycling efficiency and local muscle oxygen uptake responses. Fourteen well-trained cyclists each completed a 2-h steady-state cycling bout at 60% of their maximal minute power output to assess changes in gross cycling efficiency (GE) and muscle oxygen uptake (mVO₂ ) at time points 5, 30, 60, 90, and 120 min. Near-infrared spatially resolved spectroscopy (NIRS) was used to continually monitor tissue oxygenation of the Vastus Lateralis muscle, with arterial occlusions (OCC) applied to assess mVO₂ . The half-recovery time of oxygenated hemoglobin (HbO₂ ) was also assessed pre and post the 2-h cycling exercise by measuring the hyperemic response following a 5-min OCC. GE significantly declined during the 2-h cycling bout (18.4 ± 1.6 to 17.4 ± 1.4%; P 2 increased, being significantly higher after 90 and 120 min than at min 5 (+0.04 mlO₂ /min/100 g; P = 0.03). The half-recovery time for HbO₂ was increased comparing pre and post the 2-h cycling exercise (+7.1 ± 19s), albeit not significantly (d: 0.48; P = 0.27). This study demonstrates that GE decreases during prolonged constant load cycling exercise and provides evidence of an increased mVO₂ , suggestive of progressive mitochondrial or contractile inefficiency.

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