Objectives: The purpose of this investigation was to evaluate the intra- and intersession reliability of a new inertial measurement unit (ZEPP Sensor (ZS)) for bat swing mechanical analysis. Design and Methods: This investigation included 16 male collegiate baseball players (89.53 ± 12.5 kg, 180.61 ± 6.5 cm, 20.12 ± 0.8 years). Following a whole body dynamic warm-up and 5 dry swings, 5 swings were recorded where athletes hit balls off of a tee with bats instrumented with a ZS. Data collection took place on 3 days. Intrasession reliability analyzed trial to trial data, while intersession reliability analyzed data from each of the 3 sessions. Both relative and absolute measures of reliability were calculated with intraclass correlation coefficients (ICC), coefficients of variation (CV), and limits of agreement (LOA). Results: Within and between session acceleration/time related variables produced excellent relative reliability measurements (ICCs 0.882-0.988) as well as acceptable absolute measures of reliability (CVs 1.9%-9.2%). Variables derived from the gyro scope did not display the same consistency (intrasession vertical angle ICC = 0.492, intersession attack angle CV = 108.6%). Conclusions: Considering acceleration/time data, the ZS appears to be a reliable method of monitoring bat swing mechanics, but not for the angular position variables. Changes in body position have been demonstrated with fatigue in jumping performance. Similarly, bat position changes may occur with fatigue and should be monitored, but coaches and sport scientists should use caution when selecting variables to monitor and only select those that are reliable.
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