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

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 (mVO(2)) at time points 5, 30, 60, 90, and 120min. 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 mVO(2). The half-recovery time of oxygenated hemoglobin (HbO(2)) 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<0.01). Conversely, mVO(2) increased, being significantly higher after 90 and 120min than at min 5 (+0.04 mlO(2)/min/100g; P=0.03). The half-recovery time for HbO(2) 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 mVO(2), suggestive of progressive mitochondrial or contractile inefficiency.