In this study, we aimed to assess the countermovement jump (CMJ) using a developed instrument encompassing an off-the-shelf Inertial Measurement Unit (IMU) in order to analyze performance during the contraction phase, as well as to determine the jump height and the modified reactive strength index (RSImod), using force plate (FP) data as reference. Eight athletes (six males and two females) performed CMJs with the IMU placed on their fifth lumbar vertebra. Accuracy was measured through mean error (standard deviation), correlation, and comparison tests. The results indicated high accuracy, high correlation (r), and no statistical differences between the IMU and the FP for contraction time (r = 0.902; ρ < 0.001), negative impulse phase time (r = 0.773; ρ < 0.001), flight time (r = 0.737; ρ < 0.001), jump time (r = 0.708; ρ < 0.001), RSImod (r = 0.725; ρ < 0.001), nor minimum force (r = 0.758; ρ < 0.001). However, the values related to the positive impulse phase did not have the expected accuracy, as we used different devices and positions. Our results demonstrated that our developed instrument could be utilized to identify the contraction phase, jump height, RSImod, and minimum force in the negative impulse phase with high accuracy, obtaining a signal similar to that of an FP. This information can help coaches and athletes with training monitoring and control, as the device has simpler applicability making it more systematic.

This study aims to verify the validity of the Push Band 2.0 (PB2.0) device on the reactive strength index (RSI) measurement, using a force plate (FP) and an optical sensor device, OptoJump (OPT), as a reference. Twenty trained athletes performed 60 drop jump trials with a height box of 30 cm. A randomized repeated measures study was conducted during a single session using the PB2.0, the OPT, and the plate force manually synchronized to obtain RSI data for each jump. Validity was analyzed by contrasting three measures: the intra-class correlation coefficient (ICC), the Bland–Altman test, and R2 coefficient of determination. Bland–Altman analysis showed that RSI and FP for PB2.0 (media = −0.047; IC 93.34%) of all data were within the confidence interval, indicating a statistically reliable result. The RSI measured by the OPT and PB2.0 also provided similar values (media = −0.047). These data are identical to other validity measures (ICC and linear correlation) but differ in the R2 values. The explained variation of PB2.0 measures attained only 29.3% of the FP (R2 = 0.293) and 29.5% (R2 = 0.295) of the OPT assessment, showing a very low determination coefficient. The results of this study point to caution in the use of PB2.0 when measuring RSI in scientific research.

The aims were: (1) to determine the accuracy and time-consuming of the automatic digitalization feature of 3D motion analysis systems; and (2) to determine the reliability of the 3D position when a football player performed a hard kick with spin. Four high-speed cameras (Casio ZR200 (512x384)) with a 240 Hz field rate were used. Automatic scanning of the Kinovea® and APAS® software was used. The accuracy was determined through mean absolute error, maximum absolute error, mean standard deviation of absolute error, and the variability values. The reliability was calculated with the Intra-Class Correlation. The mean absolute error (3.62 to 3.78 mm), maximum absolute error (9.09 to 11.61 mm), mean standard deviation of absolute error (0.01 to 0.59 mm), variability values (<1 mm), and the auto-digitalization time (16 to 31 minutes) were calculated and were within the values obtained in the literature. The reliability was determined with the Intra-Class Correlation, and the results were higher than 0.90, warrantied a high consistency for chosen software. The accuracy and the reliability intra-operator were determined, and the results indicated that it is possible to apply the free 3D camera system, with consistency in free-kick analysis.

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