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O2 kinetics in
the horse during moderate and heavy exercise
Departments of Anatomy and Physiology and Kinesiology, Kansas State University, Manhattan, Kansas 66506-5602
Received 6 January 1997; accepted in final form 17 June 1997.
Langsetmo, I., G. E. Weigle, M. R. Fedde, H. H. Erickson, T. J. Barstow, and D. C. Poole.
O2 kinetics in the
horse during moderate and heavy exercise. J. Appl.
Physiol. 83(4): 1235-1241, 1997.
The horse is a
superb athlete, achieving a maximal
O2 uptake (~160
ml · min
1 · kg1)
approaching twice that of the fittest humans. Although equine O2 uptake
(O2) kinetics are
reportedly fast, they have not been precisely characterized, nor has
their exercise intensity dependence been elucidated. To address
these issues, adult male horses underwent incremental treadmill testing
to determine their lactate threshold (Tlac) and peak
O2
(O2 peak),
and kinetic features of their O2 response to
"square-wave" work forcings were resolved using exercise
transitions from 3 m/s to a
below-Tlac speed of 7 m/s or an
above-Tlac speed of 12.3 ± 0.7 m/s (i.e., between Tlac and O2 peak) sustained
for 6 min. O2 and
CO2 output were measured using an
open-flow system: pulmonary artery temperature was monitored, and mixed
venous blood was sampled for plasma lactate.
O2 kinetics at work levels
below Tlac were well fit by a
two-phase exponential model, with a phase
2 time constant
(
1 = 10.0 ± 0.9 s) that
followed a time delay (TD1 = 18.9 ± 1.9 s). TD1 was similar to
that found in humans performing leg cycling exercise, but the time
constant was substantially faster. For speeds above
Tlac,
TD1 was unchanged (20.3 ± 1.2 s); however, the phase 2 time constant
was significantly slower (1 = 20.7 ± 3.4 s, P < 0.05) than for exercise below
Tlac. Furthermore, in four of five
horses, a secondary, delayed increase in
O2 became evident
135.7 ± 28.5 s after the exercise transition. This "slow
component" accounted for ~12% (5.8 ± 2.7 l/min) of the net
increase in exercise O2. We
conclude that, at exercise intensities below and above
Tlac, qualitative features of
O2 kinetics in the horse
are similar to those in humans. However, at speeds below
Tlac the fast component of the
response is more rapid than that reported for humans, likely reflecting
different energetics of O2
utilization within equine muscle fibers.
exercise energetics; lactate threshold; ventilation threshold; slow component of oxygen uptake
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