Time-dependent changes in aerobic capacity of acute sub-high altitude exposure in elite youth endurance athletes
Abstract
Purpose This study investigated the non-linear, time-dependent alterations in cardiorespiratory fitness, running economy (RE), and metabolic kinetics during the first week of acute exposure to a sub-high altitude (1,300 m) in elite youth endurance athletes. Methods Fourteen male national-level middle- and long-distance runners (age: 19.8 ± 0.7 years; sea-level V˙O2max: 67.2 ± 3.8 mL·kg-¹·min-¹) completed field-based incremental exhaustion tests on a standard 400-m outdoor track at sea level (T0) and on day 1 (T1, within 18 h post-arrival), day 4 (T2), and day 6 (T3) after rapid rail-based ascent to 1,300 m. Maximal oxygen uptake ( V˙O2max), peak running speed ( vV˙O2max), and submaximal RE (steady-state V˙O2 at 12, 14, and 16 km·h-¹) were assessed using an automated metabolic cart. Blood lactate concentration ([La-]b) and maximal heart rate (HRmax) were measured. Daily waking morning heart rate (morning HR) and resting arterial oxygen saturation (SpO2) were continuously monitored. Training volume was strictly standardized to 70% of the habitual sea-level load. Results One-way RM ANOVA revealed a significant main effect of time on V˙O2max ( F[3,39]=18.42,p<0.001,ηp2=0.59). Crucially, polynomial contrasts confirmed a highly significant quadratic trend ( Fquadratic[1,13]=34.21,ptrend<0.001,ηp2=0.72), statistically validating a non-linear V-shaped temporal kinetic profile. V˙O2max dropped from T1 to a distinct nadir at T2 ( −7.0%, 95% CI:[60.1,64.7] mL·kg−1·min−1,p<0.001), and partially rebounded at T3. Concurrently, submaximal RE at 14 km/h significantly deteriorated at T2, as evidenced by a 5.3% increase in distance-specific oxygen cost (from 210.5±12.4 at T1 to 221.7±14.2 mL·kg−1·km−1 at T2, p=0.010), which also exhibited a significant quadratic pattern ( ptrend=0.008). Post-exercise blood lactate peaked at T2 ( 11.2±1.4 mmol·L−1,p=0.015), while HRmax remained statistically stable ( F[3,39]=0.21,p=0.885,ηp2=0.015). Conclusion Acute relocation to 1,300 m induces a distinct physiological nadir on Day 4, which may be related to transient autonomic adjustments and respiratory muscle work, rather than hematological adaptations. Early adaptation emerges by Day 6. Coaches may consider closer monitoring and temporary adjustment of training intensity during the early days following ascent, particularly around day 4.