Cardiorespiratory Changes During Prolonged Downhill Versus Uphill Treadmill Exercise

Oxygen uptake (V?O (2) ), heart rate (HR), energy cost (E (C) ) and oxygen pulse are lower during downhill compared to level or uphill locomotion. However, a change in oxygen pulse and E (C) during prolonged grade exercise is not well documented. This study investigated changes in cardiorespiratory responses and E (C) during 45-min grade exercises. Nine male healthy volunteers randomly ran at 75% HR reserve during 45-min exercise in a level (+1%), uphill (+15%) or downhill (-15%) condition. V?O (2) , minute ventilation (V? (E) ) and end-tidal carbon dioxide (P (et) CO (2) ) were recorded continuously with 5-min averaging between the 10 (th) and 15 (th) min (T1) and 40 (th) and 45 (th) min (T2). For a similar HR (157 +/- 3 bpm), V?O (2) , V? (E) , and P (et) CO (2) were lower during downhill compared to level and uphill conditions (p<0.01). V?O (2) and V? (E) decreased similarly from T1 to T2 for all conditions (all p<0.01), while P (et) CO (2) decreased only for the downhill condition (p<0.001). Uphill exercise required greater E (C) compared to level and downhill exercises. E (C) decreased only during the uphill condition between T1 and T2 (p<0.01). The lowest V?O (2) and E (C) during downhill exercise compared to uphill and level exercises suggests the involvement of passive elastic structures in force production during downhill. The lower cardiorespiratory response and the reduction in P (et) CO (2) during downhill running exercise, while E (C) remained constant, suggests an overdrive ventilation pattern likely due to a greater stimulation of efferent neural factors.