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Abstract

The locomotor muscle metaboreflex (LMM) is essential in the exercise-mediated cardiovascular response, coordinating blood flow distribution to support skeletal muscle’s metabolic demands. At low altitudes (LA: < 1,500 m), LMM sensitivity is reduced in females, and whether sex-based differences in the LMM persist at high altitudes (HA: > 2,500 m) is unknown. Utilizing the effects of post-exercise circulatory occlusion (PECO), via lower-limb cuff inflation to 220 mmHg, allows for continued stimulation of the LMM while isolating it from concurrent reflexes (i.e., central command, arterial baroreflex), thereby trapping exercise-produced metabolites within the skeletal muscle. We hypothesized that the cardiorespiratory responses to PECO would be attenuated in females compared to males following sub-maximal cycling exercise, at both LA (1,045m; Calgary, Canada) and HA (3,500m; La Paz, Bolivia). Twenty healthy participants (age = 31 ± 7 years), females: n = 10, completed a 2-day protocol that was repeated at LA and HA. First, a maximal cardiopulmonary exercise test to determine participants’ gas exchange threshold (GET) for use in sub-maximal testing. Twenty-four to forty-eight hours later, participants sub-maximally cycled at the workrate (WR) corresponding to GET, followed by a round each of control or PECO, in randomized order. The changes in minute ventilation (∆V̇E), heart rate (∆HR), and mean arterial pressure (∆MAP) served as indicators of LMM sensitivity. Maximal exercise tests were repeated on days 1-3 of acclimatization to 3,500 m, sub-maximal tests were repeated on days 6-10. During occlusion, the absolute difference between control and PECO for ∆V̇E was not different from females to males (4 [6, 15] l/min; P=0.308) and not different at HA (1 [-6, 8] l/min; P=0.698). ∆HR was not different in females compared to males (2 [-9, 14] beats/min: P=0.624), nor HA compared to LA (5 [-4, 14] beats/min; P=0.145). ∆MAP was not different between sexes (3 [-4, 10] mmHg; P=0.249) or altitudes (3 [-3, 8] mmHg; P=0.192). Our results demonstrate there was no difference between sexes in LMM sensitivity at LA or HA. Suggesting that LMM sensitivity at HA may depend on other sex-based cardiopulmonary factors (e.g., cyclic ovarian hormones) or central command.