Association between environmental heat stress and 30:15 intermittent fitness test performance in youth football players: a two-season observational study.

Intermittent field tests in football are commonly conducted outdoors, exposing players to environmental heat stress. This study examined the influence of environmental heat stress on the 30:15 Intermittent Fitness Test (VIFT) performance in highly trained young football players. A total of 642 VIFT assessments were collected from 312 male players (U14-U19) across two seasons. Environmental conditions were obtained from an open-source service. Heat stress was quantified via the wet-bulb globe temperature (WBGT) and categorized into unrestricted (<22 °C), low-risk (22 °C-28 °C) and high-risk (>28 °C) conditions. Linear mixed models were used to analyze the associations between anthropometric, neuromuscular, and environmental factors on VIFT performance. VIFT decreased by ∼0.4 km/h in low-risk and ∼1.5 km/h in high-risk heat conditions (d = 0.12-0.45; P < 0.01) compared to unrestricted. These differences persisted after adjusting for anthropometric characteristics and CMJ performance, except in U19 players, where the VIFT decrement in low-risk conditions disappeared (P = 0.183). The VIFT declined by 0.09-0.15 km/h for every 1 °C increase in WBGT (r = -0.74 to -0.28; P < 0.05). Our findings suggest the VIFT performance might decrease by one running stage (0.5 km/h) for every ∼5 °C increase in WBGT. Practitioners should account for environmental conditions when profiling players and prescribing training based on the VIFT.