Heat Stress Was Linked to 1.5 km/h Slower Fitness Test Scores in 312 Young Footballers

Across 642 fitness tests in 312 young footballers, heat stress was associated with slower scores on a standard intermittent running assessment. The authors calculate that heat stress of roughly 5 degrees Celsius costs about one running stage.

Heat Stress is rarely recorded when a squad runs its pre-season fitness testing, yet it may be quietly rewriting the results. A study published on September 29, 2026 in Frontiers in Sports and Active Living puts a number on how much.

Vincenzo Rago and colleagues at the University of Kalba in Sharjah, with co-authors at Saarland University, the University of Technology Sydney, the Portuguese Football Federation and United Arab Emirates University, analysed 642 assessments collected from 312 male players in the U14 to U19 age groups.

The testing ran from July 1, 2022 to May 30, 2024, four times per season: pre-season in July, early season in September, mid-season in January and end-season in April. The measure was the 30:15 Intermittent Fitness Test, and the outcome of interest was the final velocity reached, known as VIFT.

Conditions were quantified using wet-bulb globe temperature, or WBGT, which combines air temperature, humidity, radiant heat and wind into a single figure. The team sorted sessions into unrestricted conditions below 22 degrees Celsius, low-risk heat stress between 22 and 28 degrees, and high-risk heat stress above 28 degrees.

What the 30:15 Test Actually Asks of a Player

The protocol is simple to run and punishing to complete. Players perform 30-second shuttle runs separated by 15 seconds of passive recovery, starting at 8 km/h, with the speed rising by 0.5 km/h every 45-second stage until they can no longer keep pace.

Because it alternates effort and rest, the test reflects the demands of football more closely than a continuous run does. That also makes it sensitive to anything that impairs recovery between bouts, which is precisely where heat stress exerts its influence.

The final velocity reached, abbreviated VIFT, is then used directly to prescribe interval speeds for the weeks that follow. That is why the number carries weight well beyond the afternoon on which it was recorded.

How Much Heat Stress Moved the Numbers

In low-risk conditions, final velocity fell by roughly 0.4 km/h compared with unrestricted sessions. In high-risk conditions the decrement reached about 1.5 km/h. Effect sizes spanned d = 0.12 to 0.45, and the differences were statistically significant at P less than 0.01.

The relationship was also continuous rather than stepped. Scores declined by 0.09 to 0.15 km/h for every additional degree Celsius of WBGT, with correlations from r = minus 0.74 to minus 0.28 and P values below 0.05.

The authors translate that gradient into a figure coaches can hold in mind: performance appears to drop by one running stage, or 0.5 km/h, for roughly every 5 degrees Celsius of additional heat stress.

The Effect Survived Adjustment, With One Exception

A reasonable objection is that bigger, more powerful players might simply be tested at different times of year. The team addressed it with linear mixed models that adjusted for anthropometric characteristics and countermovement jump performance.

The differences held after those adjustments in almost every group. The single exception was the U19 squad, where the decrement in low-risk conditions disappeared once adjustment was applied, at P = 0.183. The high-risk effect remained.

That age-group pattern is worth noting rather than over-reading. Older adolescents carry more muscle mass and more training history, and they may also arrive with greater heat acclimatisation, though the study did not measure it.

Why This Matters for Player Profiling

Intermittent field tests are widely used to set training zones, to benchmark players against squad norms and to track progress across a season. All three uses assume the score reflects the athlete.

If a July score is depressed by heat stress and a January score is not, a player can appear to have improved substantially over the autumn while having changed very little. Prescribing intervals from an artificially low July result would also set target speeds below what the player can sustain.

The same logic has surfaced in endurance settings, where researchers have examined how ambient air quality relates to marathon finishing times. Environmental variables that go unrecorded do not disappear from the data; they simply become invisible.

Squad benchmarking carries the same hazard. A player tested in August and a teammate tested in November are not being measured on the same scale, even though both results land in the same column of the same spreadsheet and are read side by side at selection meetings.

How Heat Stress Is Estimated, and Why That Matters

WBGT was not measured on the pitch. It was estimated using the method developed by Liljegren and colleagues, drawing on meteorological data from a weather station 5.6 kilometres from the test site.

The authors are direct about the trade-off. That approach has been validated against direct measurement, but it cannot capture the microclimate a player actually experiences, including local variation in solar radiation, wind and surface heat radiating off the pitch.

The thresholds themselves are borrowed from occupational and sporting heat guidance, where WBGT bands are used to trigger changes to session length, work-to-rest ratios and fluid breaks. Applying the same bands here makes the findings easier to map onto policies many clubs and federations already operate.

What Coaches and Athletes Can Reasonably Take From This

The finding does not say that training in warm conditions is harmful or unproductive, and it does not establish that heat stress causes the performance drop, since conditions and calendar were entangled. What it does support is treating test scores as context-dependent.

General, low-risk practices follow from that framing rather than from any new physiology:

• Record WBGT, or at minimum temperature and humidity, alongside every field test result.
• Compare like with like, setting pre-season scores against previous pre-seasons rather than against mid-season figures.
• Treat a summer score as a floor for prescription, and re-test in cooler conditions before locking in training velocities.
• Allow a gradual build-up when sessions move into warmer conditions, and keep fluid access straightforward.

Age-group context is worth keeping in view too. Work on plyometric training and match running in U20 footballers and on jump and sprint responses in untrained adolescents shows how much variation sits within a single age band, while research on aerobic capacity gains in adolescent team-sport athletes illustrates the size of change a genuine training effect tends to produce.

None of this is individualised guidance. Squads with medical or sports science staff should set heat policy with them, and anyone training young athletes in warm conditions should follow the heat guidance issued by their own governing body.