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 four days (HA1-HA4). AMS was assessed daily using the Environmental Symptoms Questionnaire, with AMS-Cerebral (AMS-C) factor score ≥0.7 indicative of sickness. The active compared to the passive group expended more energy (1265±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 to HA1 (44%), remained elevated on HA3 (33%) and decreased on HA4 (8%). The AMS incidence was higher in the active compared to 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.