Exclusive: Criticised World Cup referee was suffering extreme heat strain, data showsUzbek referee Ilgiz Tantashev's core temperature passed 39C during the France-Paraguay gameA Fifa referee labelled the worst at the World Cup was experiencing a level of heat strain associated with impaired decision making, new data has revealed. It found that Tantashev’s core temperature peaked at 39.3C and his Heat Strain Index (HSI) score was 6.7/10 for the France v Paraguay fixture. During the World Cup he wore a sensor connected to a smart watch to monitor his data in real time. The data also indicated that climatic conditions, rather than distance covered, were the biggest driver of heat strain. “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.”