Active ascent accelerates the onset of acute mountain sickness at 4,300 m

Acute mountain sickness (AMS) usually peaks after the first night at high altitude (HA) and resolves within 2-3 days, but the effect of active ascent on AMS is debated. This study examined the incidence, severity, and time course of AMS following active versus passive ascent from 3,950 to 4,300 m. Thirty-two healthy participants traveled from low altitude (55-399 m) to 3,950 m. Participants either hiked (active group, n = 16) or were driven (passive group, n = 16) to 4,300 m, staying for 4 days (HA1-HA4). AMS was assessed daily using the Environmental Symptoms Questionnaire, with an AMS-cerebral (AMS-C) factor score ≥0.7 indicative of sickness. The active group compared with the passive group expended more energy (1,265 ± 351 vs. 408 ± 208 kcal) and demonstrated lower peripheral oxygen saturation (75 ± 3 vs. 82 ± 3%) during ascent. In the active group, the incidence of AMS peaked on HA1 (81%), remained elevated on HA2 (75%) and HA3 (57%), and decreased on HA4 (30%). In the passive group, the incidence of AMS peaked on HA2 (69%) compared with HA1 (44%), remained elevated on HA3 (33%), and decreased on HA4 (8%). The AMS incidence was higher in the active compared with the passive group on HA1. High-intensity exercise during ascent worsens AMS symptoms on HA1, likely due to greater hypoxemia during ascent. Tasks that need to be performed immediately upon arrival at HA may be negatively affected by strenuous exercise, and personnel should consider appropriate countermeasures to minimize the impact of exercise on AMS in the early hours of HA exposure.NEW & NOTEWORTHY Active ascent to 4,300 m high altitude (HA) accelerated the onset of AMS, confirming previous findings at 3,600 m. Peak AMS symptoms occurred on HA1 in active ascenders and on HA2 in passive ascenders. Unlike at 3,600 m, active ascenders did not recover faster compared with passive ascenders. AMS was likely exaggerated in active ascenders on HA1 due to an ∼7% greater arterial desaturation, increasing hypoxemia during the first 2-3 h at high altitude.