Time-dependent changes in aerobic capacity of acute sub-high altitude exposure in elite youth endurance athletes.

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 ˙ O 2 max : 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 ˙ O 2 max ), peak running speed ( v V ˙ O 2 max ), and submaximal RE (steady-state V ˙ O 2 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 ˙ O 2 max ( F [ 3 , 39 ] = 18.42 , p < 0.001 , η p 2 = 0.59 ). Crucially, polynomial contrasts confirmed a highly significant quadratic trend ( F quadratic [ 1 , 13 ] = 34.21 , p trend < 0.001 , η p 2 = 0.72 ), statistically validating a non-linear V-shaped temporal kinetic profile. V ˙ O 2 max dropped from T 1 to a distinct nadir at T 2 ( - 7.0 % , 95 % CI : [ 60.1 , 64.7 ] mL · kg - 1 · min - 1 , p < 0.001 ), and partially rebounded at T 3 . Concurrently, submaximal RE at 14 km/h significantly deteriorated at T 2 , as evidenced by a 5.3% increase in distance-specific oxygen cost (from 210.5 ± 12.4 at T 1 to 221.7 ± 14.2 mL · kg - 1 · km - 1 at T 2 , p = 0.010 ), which also exhibited a significant quadratic pattern ( p trend = 0.008 ). Post-exercise blood lactate peaked at T 2 ( 11.2 ± 1.4 mmol · L - 1 , p = 0.015 ), while HR max remained statistically stable ( F [ 3 , 39 ] = 0.21 , p = 0.885 , η p 2 = 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.