Can we just play? Internal validity of assessing physiological state with a semi-standardised kicking drill in professional Australian Football.

Purpose: To examine associations between exercise heart rate (HRex) during a continuous-fixed submaximal fitness test (CF-SMFT) and an intermittent-variable protocol (semi-standardised kicking drill, SSD) in Australian Football (AF) athletes, controlling for external intensities, within-session scheduling, and environmental conditions. Methods: Forty-four professional AF athletes (22.8±8.0 years) were monitored over 10 sessions involving a 3-minute CF-SMFT (12 kmh-1) as the first activity, and a SSD administered 35.7±8.0 minutes after the CF-SMFT. Initial HR (HRi) and HRex were collected, with external intensities measured as average velocity (mmin-1) and average acceleration-deceleration (ms-2). Environmental conditions were sampled. A penalised hierarchical linear mixed model was tuned for a Bayesian Information Criterion (BIC) minima using a 10-fold cross-validation, with out-of-sample of prediction accuracy assessed via Root-Mean-Squared Error (RMSE). Results: SSD average acceleration-deceleration, HRi, temperature and ground hardness were significant moderators in the tuned model. When model covariates were held constant, a 1%-point change in SSD HRex associated with a 0.4%-point change in CF-SMFT HRex (95%CI: 0.3 to 0.5). The tuned model predicted CF-SMFT HRex with an average RMSE of 2.64±0.57 over the 10-fold cross-validation, with 74% and 86% of out-of-sample predictions falling within 2.7 and 3.7%-points respectively, from observed values; representing the lower and upper limits for detecting meaningful changes in HRex according to the documented typical error. Conclusions: Our findings support the use of a SSD to monitor physiological state in AF athletes, despite varied scheduling within-session. Model predictions of CF-SMFT HRex from SSD HRex closely aligned with observed values, considering measurement imprecision.