Prior heavy exercise increases oxygen cost during moderate exercise without associated change in surface EMG

The aim of this study was to test the hypothesis that prior heavy exercise results in a higher oxygen cost during a subsequent bout of moderate exercise due to changes in muscle activity. Eight male subjects (25 ± 2 yr, ±SE) performed moderate-moderate and moderate-heavy-moderate transitions in work rate (cycling intensity, moderate = 90% LT, heavy = 80% VO₂ peak). The second bout of moderate exercise was performed after 6 min (C) or 30 s (D) of recovery. Pulmonary gas exchange was measured breath-by-breath and surface electromyography was obtained from the vastus lateralis and medialis muscles. Root mean square (RMS) and median power frequency (MdPF) were computed. Prior heavy exercise increased Δ over(V, ̇) O₂ / Δ WR (C: +2.0 ± 0.8 ml min^-1 W^-1, D: +3.4 ± 0.8 ml min^-1 W^-1; P < 0.05) and decreased exercise efficiency (C: -13.3 ± 5.6%, D: -22.2 ± 4.9%; P < 0.05) during the second bout of moderate exercise in the absence of changes in RMS. MdPF was slightly elevated (∼2%) during the second bout of moderate exercise, but MdPF was not correlated with over(V, ̇) O₂ (r = 0.17). These findings suggest that the increased oxygen cost during moderate exercise following heavy exercise is not due to increased muscle activity as assessed by surface electromyography.