Within-Athlete Associations Between WHOOP Activity Strain, Coach-Prescribed Training Objectives, and Session-RPE in an Elite Junior Swimmer: A Longitudinal Single-Case Study

Background and Objectives: Wearable technologies increasingly monitor internal training load in high-performance sport, but their alignment with coach-prescribed physiological objectives remains unclear. This study examined within-athlete associations between WHOOP Activity Strain, coach-prescribed objectives, athlete-perceived load, and completed swimming exposure in one elite junior swimmer. Methods: An exploratory longitudinal single-case secondary analysis was conducted using routinely collected training and athlete-monitoring data from 4 January to 3 July 2026. WHOOP Activity Strain and Session-RPE were synchronized by session, and generalized estimating equations assessed differences across prescribed objectives and variable associations. Results: Of 161 sessions with exact-plan cards, 134 were classified and 128 had strict WHOOP matches. Conventional robust-covariance GEE suggested an objective-related difference (exchangeable p = 0.013; AR(1) p = 0.014), but this was not statistically detectable with the bias-reduced covariance estimator (p = 0.080 and p = 0.075, respectively) and was not reproduced using the legacy classification. WHOOP Activity Strain was consistently associated with Session-RPE Load, Session-RPE, and actual swimming distance. Conclusions: Within this athlete, WHOOP Activity Strain was associated with perceived load and completed swimming distance. Objective-related differences were post hoc and sensitive to the classification source and covariance estimator; they should therefore be considered exploratory and hypothesis-generating.