Within-athlete association between morning on-track peak heart rate and same-day pre-performance salivary cortisol change under dry-track conditions: a multi-season single-case field study in a professional formula racing driver.

INTRODUCTION: Professional Formula racing often requires multiple high-stakes on-track sessions within the same competition day, creating a need to characterize pre-performance physiological state under field constraints. Salivary cortisol is a widely used marker of hypothalamic-pituitary-adrenal axis activity, but field-ready options for its immediate assessment during competition remain limited. Wearable heart-rate monitoring during on-track driving may provide an immediately available field signal reflecting composite cardiovascular load. This multi-season single-case field study examined the within-athlete association between wearable heart-rate data and same-day pre-performance salivary cortisol change in one professional Formula racing driver. MATERIALS AND METHODS: The primary observational series comprised 23 race-day observations across three competitive seasons. During each morning on-track session, the driver wore a wearable electrocardiogram device, and the peak heart rate (HRpeak) was extracted. Saliva samples were collected approximately 30 and 10 min before each morning and afternoon session and assayed for cortisol. Within-day cortisol change (ΔCortisol) was defined as afternoon minus morning pre-session cortisol, averaged across the two samples. The primary analysis included 18 dry-track observations with complete HRpeak and cortisol data. Five additional observations were examined in an exploratory post hoc analysis. RESULTS: In the primary analytical sample, morning HRpeak was positively associated with ΔCortisol (r = 0.720). The positive direction was retained in sensitivity checks using 5-s and 10-s average heart rates anchored on the HRpeak and addressing serial dependence, season/session characteristics, and -30 min and -10 min sample-specific cortisol changes. Including the single wet-track observation reduced the correlation to r = 0.316. In the post hoc analysis, standardized ΔCortisol residuals were negative for the three planned-timing nap opportunities. No inferential comparison was performed. DISCUSSION: Wearable on-track HRpeak may provide an immediately available field signal of composite cardiovascular load associated with later same-day endocrine variation in the participating driver. This interpretation is limited to dry-track conditions and does not support the use of HRpeak as a stress-specific marker, prediction method, or decision rule; the association does not establish causality. The post hoc observations provide no evidence regarding nap efficacy. Nevertheless, these descriptive findings may inform future prospective studies.