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The slow component of O2 uptake (V˙O2sc) represents a progressive decline in work efficiency during strenuous, constant work rate cycling. Although most of this “excess” O2 uptake can be explained by factors intrinsic to the exercising muscles, it has been proposed that respiratory muscle work rate may also contribute to the V˙O2sc response. To date, however, no study has provided a comprehensive analysis of the mechanical power of breathing (Pb) in relation to the V˙O2sc while performing strenuous exercise.The mechanical Pb was measured in 12 recreational cyclists (24 ± 1 yr, 70.9 ± 3.9 kg) during heavy- and severe-intensity cycling. The resistive and elastic components of Pb were quantified using the modified Campbell diagram.Total Pb significantly increased (P < 0.05) over the V˙O2sc phase during both heavy (Δ13 ± 2 J·min−1) and severe work rate transitions (Δ219 ± 56 J·min−1). The magnitude of this increase was relatively greater during severe cycling trials (P < 0.05). The greater “slow component” rise in total Pb during severe work rate transitions was consequent to relatively larger increases in the inspiratory and expiratory resistive Pb and inspiratory elastic Pb (P < 0.05).The findings of this investigation support the thesis that the energetic contribution from respiratory muscles to the V˙O2sc amplitude is disproportionately higher during severe- compared with that during heavy-intensity exercise.