Rationale: The rate at which oxidative ATP provision can meet the energy demand of exercise is a determinant of contractile dysfunction [1]. Evidence suggests that females experience less contractile dysfunction than males for the same duration and intensity of exercise [2], however whether this is caused by bioenergetic differences or differences in the contractile properties of exercising muscle is unknown. Therefore, this study compared the pulmonary oxygen uptake (V̇O2) kinetics during moderate and heavy intensity exercise in males and females.
Methods: Sixteen healthy adults (8 of each sex, 27 ± 5 years) completed three experimental visits. First, participants completed a submaximal incremental exercise test (+25 W every 5 minutes) to identify lactate threshold (LT), then a maximal incremental exercise test (25 W.min-1) to exhaustion to identify V̇O2peak and power at V̇O2peak (Pmax). Visits two and three involved three six-minute cycling bouts at 80% of LT (moderate intensity), interspersed with six minutes of unloaded pedalling, and one 30-minute bout at a work rate 30% between LT and Pmax (heavy intensity).
Data from the final two visits were filtered and linearly interpolated (1s intervals), then pooled to form a dataset of six moderate and two heavy intensity transitions. The first 20 s of each transition was removed. Thereafter, three minutes of pre-transition data and six (moderate) or two (heavy) minutes of post-transition data were fit with a mono-exponential curve to obtain the parameters of the phase II kinetics. The V̇O2 slow component was also quantified for the heavy intensity bouts.
Results: Absolute V̇O2peak was greater in males (3.47 ± 0.58 vs 2.49 ± 0.44 L.min-1, p=0.002), however relative values were not statistically different (46.2 ± 6.6 vs 40.5 ± 6.7 ml.kg-1.min-1, p=0.111). Males achieved greater power outputs at V̇O2peak and LT (p≤0.023), meaning power outputs for subsequent bouts were 30% greater compared to females.
The primary amplitude of the moderate intensity transition was not different between male and females (24 ± 3 vs 24 ± 5 %V̇O2peak, p=0.949). The time constant was also not different (27.9 ± 7.5 vs 24.8 ± 6.6s, p=0.385). Similarly, in the heavy intensity domain, neither the primary amplitude (43 ± 5 vs 38 ± 7 %V̇O2peak, p=0.179) or time constant (28.8 ± 7.9 vs 27.2 ± 7.1s, p=0.633) were different. Likewise, the amplitude of the V̇O2 slow component was not different between sexes (12 ± 7 vs 11 ± 3 %V̇O2peak).
Conclusion: No sex differences were observed in the V̇O2 response to exercise in the moderate or heavy intensity domains, implying there was no sex difference in the bioenergetic stress experienced. Combined with evidence of no hormonal effect [3] on these parameters, this suggests females should not be excluded from studies of cardiopulmonary responses to exercise. Deoxyhaemoglobin kinetics recorded via near infrared spectroscopy of the vastus lateralis will also be shared.