Thermoregulatory Response During Cross-Country Marathon Mountain Biking Competitions—An Exploratory Multiple-Case Study

Objective: This study investigated core temperature (Tc) responses during elite Cross-Country Marathon (XCM) competitions performed under different environmental conditions and competition loads. Methods: Four elite XCM athletes (three males and one female) were monitored during three official competitions Tc was continuously recorded using a non-invasive sensor. The mean and maximal Tc (Tcmean and Tcmax), the change in Tc (ΔTc), and the time and percentage of race time spent above 38.5 °C were calculated. The competition load included Normalized Power (NP), Intensity Factor (IF), Training Stress Score (TSS), and heart rate (HR). Linear mixed models and athlete-specific z-scores were used to evaluate thermoregulatory responses. Results: Tcmax ranged from 38.5 to 39.5 °C, with inter-individual variability ranging from 3% to 94% of race time spent with a Tc above 38.5 °C. XCM2, which recorded the highest relative humidity (91%), showed the greatest thermal exposure in three of the four athletes. In the race-time, Tcmax was higher during XCM2 than XCM1 (β = −0.54 °C, p = 0.038). In the TSS, Tcmax was higher during XCM2 (β = −0.79 °C, p = 0.008) and XCM3 (β = −0.60 °C, p = 0.027) compared with XCM1. Exposure to Tc > 38.5 °C was also higher during XCM2 than XCM1 (β = −73.1%, p = 0.021). Conclusions: XCM athletes exhibited individual variability in Tcmax and time spent above 38.5 °C. These exploratory findings highlight the value of continuous Tc monitoring for characterizing individual thermoregulatory responses during competition.