The effect of treadmill incline on maximal oxygen uptake, gas exchange and the metabolic response to exercise in the horse

In healthy man, conditions that change muscle O-2 delivery affect the achievable maximum rate Of O-2 uptake ((V) over dot (O2,max)) as well as the metabolic (e.g. lactate threshold, LT) and gas exchange (e.g. gas exchange threshold, Tge) responses to incremental exercise. Inclined (1) compared to level (L) running increases locomotory muscle EMG at a given speed in the horse, indicative of elevated metabolic demand. To our knowledge, the effect of treadmill incline on (V) over dot (O2,max), LT and Tge has not been addressed in the exercising quadruped. We used blood sampling and breath-by-breath expired gas analysis to test the hypothesis that 1 (10% gradient) would increase (V) over dot (O2,max) and the rate of O-2 uptake ((V) over dot (O2)) at LT and Tge in six Thoroughbred horses during incremental running to volitional fatigue. (V) over dot (O2,max) was significantly higher for I (I, 77.8 +/- 4.1; L, 65.5 +/- 5.31 min(-1); P < 0.05), but peak plasma lactate concentration was not (I, 28.0 +/- 3.7; L, 25.9 +/- 3.0 mm). Arterial P-CO2 increased to 62.1 +/- 3.3 and 57.9 +/- 2.7 Torr (I vs. L, P < 0.05), yet despite this relative hypoventilation, a distinct Tge was present. This Tge occurred at a significantly different absolute (I, 49.6 +/- 3.2; L, 42.4 +/- 3.2 I min(-1); P < 0.05), but nearly identical relative (V) over dot (O2) (I, 63.6 +/- 1.2; L, 63.9 +/- 1.6% (V) over dot O-2,O-max) in I and L. Similarly occurred at a significantly greater absolute (V) over dot (O2) (I, 37.3 +/- 2.8; L, 26.9 +/- 2.11 min(-1)), but a relative (V) over dot (O2) that was not different (I, 47.9 +/- 2.1; L, 43.9 +/- 4.5% (V) over dot (O2,max)). In addition, Tge occurred at a significantly higher (P < 0.05) absolute and relative (V)over dot (O2) than LT for both I and L tests. In conclusion, (V) over dot (O2,max) is higher during inclined than level running and both LT and Tge in the horse occur at a similar percentage of (V) over dot (O2,max) irrespective of the absolute level of (V) over dot O-2,O-max. In contrast to humans, LT is a poor analogue of Tge in the horse.