PlayerLoad™ : reliability, convergent validity, and influence of unit position during treadmill running

Purpose: The study aimed to establish the test-retest reliability and convergent validity of PlayerLoadTM (tri-axial accelerometer data) during a standardized bout of treadmill running. Methods: Forty-four team-sports players performed two standardized incremental treadmill running tests (7-16 km·h-1) 7 d apart. Players’ VO2 (n=20), heart rate (n=44), and tri-axial accelerometer data (PlayerLoadTM; n=44) measured at both the scapulae and at the centre of mass (COM), were recorded. Accelerometer data from the individual-component planes of PlayerLoadTM (anterior-posterior PlayerLoadTM [PLAP], medial-lateral PlayerLoadTM [PLML] and vertical PlayerLoadTM [PLV]) were also examined. Results: Moderate to high test-retest reliability was observed for PlayerLoadTM and its individual planes (ICC: 0.80-0.97; CV: 4.2-14.8%) at both unit locations. PlayerLoadTM was significantly higher at COM versus scapulae (223.4 ± 42.6 vs. 185.5 ± 26.3 au; p = 0.001). The percentage contributions of individual planes to PlayerLoadTM were higher for PLML at the COM (scapulae: 20.4 ± 3.8%; COM: 26.5 ± 4.9%; p = 0.001), but lower for PLV (scapulae: 55.7 ± 5.3%; COM: 49.5 ± 6.9%; p = 0.001). Between-subject correlations between PlayerLoadTM and VO2, and between PlayerLoadTM and heart rate, were trivial to moderate (r = -0.43 to 0.33), whereas within-subject correlations were nearly perfect (r = 0.92 to 0.98). Conclusions: PlayerLoadTM had a moderate to high degree of test-retest reliability and demonstrated convergent validity with measures of exercise intensity on an individual basis. However, caution should be applied in making between-athlete contrasts in loading, and when using recordings from the scapulae to identify lower-limb movement patterns.