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

Abstract

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.