A FIFA referee widely condemned as the worst at the World Cup was experiencing heat strain levels linked to impaired decision-making, according to newly disclosed biometric data.

Uzbek official Ilgiz Tantashev came under heavy criticism for his officiating during France's last-16 victory over Paraguay, in which the Paraguayan side received no bookings despite committing repeated violent fouls. French newspaper L'Equipe awarded Tantashev a 1/10 rating, while BBC pundit Thomas Hitzlsperger described it as "the worst performance I've seen so far at this tournament."

However, wearable technology company Core, which monitored the referee's thermal data across his three World Cup matches, found that elevated heat strain may have been a contributing factor in his performance.

During the France v Paraguay fixture, Tantashev's core body temperature peaked at 39.3°C, and his Heat Strain Index (HSI) score reached 6.7 out of 10. On the HSI scale, a score of 3 is classified as "high," while 7 or above is considered "dangerous." The 2026 tournament, staged across North America during peak summer, has placed unprecedented thermal demands on everyone on the pitch — players and officials alike.

Academic research has demonstrated that heat can adversely affect perception, attention, and anticipation, and that footballers operating in such conditions make poor on-field decisions more frequently.

"The referee's Heat Strain Index climbed to a level associated in published research with reduced reaction time and impaired decision-making," Core chief commercial officer Ross McGraw told City AM.

"That's not a comment on any individual official, or a criticism of any one match or any one decision. It's a data point that should worry anyone thinking about how football protects the people running the game, and whether the sport's duty of care is actually covering everyone on the pitch," McGraw said.

Tantashev's Use of Core Technology at the World Cup

Tantashev had partnered with Core to optimise his heat training in preparation for the tournament. During matches, he wore a sensor connected to a smart watch, enabling him to monitor his thermal data in real time.

The data also revealed that climatic conditions, rather than the distance covered, were the primary driver of heat strain. The France v Paraguay match was played in 34°C heat at 5pm and yielded a significantly higher HSI score than games in which Tantashev ran up to 900 metres further but in cooler temperatures.

"This goes beyond a sport story," McGraw added. "Harvard and George Washington University researchers analysed 845,000 workplace injuries from 2023 and linked roughly 28,000 of them to heat exposure — across both outdoor and indoor jobs."

"Their lead author was explicit that this isn't only heatstroke: it's hand-eye coordination, attention, memory, and judgment breaking down before anyone calls it a heat injury."

Tantashev's other World Cup assignments included Scotland's 1-0 defeat by Morocco in Boston, a 6pm kick-off played in temperatures ranging from 24°C to 27°C, and the 3-3 draw between Algeria and Austria, which took place at 9pm in Kansas City as temperatures dropped from 29°C to 20°C.

"We all saw the headlines about heat and impact on players. The cooling tech, heat training, hydration breaks, but what about the referees?" McGraw said.

"We have worked with many top athletes and teams to prepare for extreme heat conditions, but even the Core team was blown away by the heat strain the referees faced in the World Cup."

Core's data also indicated that hydration breaks — controversially introduced for all matches at the 2026 tournament — did not significantly reduce Tantashev's heat stress levels, though the absence of control data for comparison makes it difficult to draw definitive conclusions.