# Concurrent Validity of the Inertial Measurement Unit _Vmaxpro_ in Vertical Jump Estimation
## 4 Discussion
These data coincide with those obtained with other accelerometers where overestimation error is observed when compared to a force platform. These results agree with other studies that use _Push Band_[55; 56] or other accelerometers [57]. For example, [PERSON] et al. [19], which places the IMU _Catapult System_ on the torso, found differences of 9.4 cm using take-off velocity as a variable for height estimation. This error is larger than others found, although the authors indicated that the location of the IMU on the torso is a factor that can increase the systematic error. However, in this same study, the bias is reduced by half, 4.3 cm, when they used flight time as the calculation variable. Similarly, [PERSON] et al. [58] found differences of 4.1 cm when comparing a MoCAP system versus the IMU _Vert_ for maximum jumps in competition. Therefore, the reference instrument used may affect the magnitude of the systematic error found and affect the validity of its accuracy. On the other hand, when analyzing the jump values obtained using flight time, double integration, and take-off speed versus MoCAP, it is appreciated that the greatest difference is found for the calculation of VJH from take-off velocity. [PERSON] et al. [52] found that while for double integration and flight time, the difference was around 1.3 cm, it was around 5.7 cm for the take-off velocity method, values closer to those obtained in this study. Consequently, the differences between the two instruments could be explained as a function of the calculation variables used to estimate the jump. In the case of _Vmaxpro_, VJH has been estimated from the take-off velocity, taken as the maximum speed recorded, which is not entirely true since the maximum speed was reached about 27 ms after take-off [37]. This delay implies a variation in take-off velocity detection of 0.162 m/s, which, translated to jumping, may imply increments of 1 cm to 4 cm [59; 60]. Such increments may explain the overestimation of measurements when compared to force platforms but not in our case, as significantly lower values are obtained. The sampling frequency may also influence the ability of the IMU to detect the precise instant of take-off [10] or, in the case of _Vmaxpro_, the exact moment at which the maximum velocity is reached. However, the 1000 Hz sampling rate advertised by the manufacturer seems sufficient to minimize the error, as it would imply a 0.2 cm error in the jump but fail to explain the nearly 10 cm error between the two instruments. Even so, the Bluetooth technology of the device transmits at a significantly lower frequency of 65 Hz, and depending on whether the data are transmitted continuously to the software or temporarily stored and then sent to the software in data packets, the error will be greater or lesser in magnitude. In addition, the ability of the device to detect the exact moment of take-off may also be affected by variability due to the way the device is positioned on the body segment. In this study, the IMU was attached above the hip using an elastic device supplied by the manufacturer, which in turn was anchored to a belt. This fixation system can generate vibrations in the instrument given its elastic characteristics and cause an error in the IMU measurement [11; 61]. In this sense, fixing the accelerometer with adhesive tape on the same body segment (above the hip) showed lower absolute error values (4.0 cm), although the correlations were somewhat lower compared to a force platform (\(r\) = 0.74) [62]. Consequently, the criterion instrument against which the IMU is compared, the body segment on which the measuring instrument is placed, the resolution of the instrument, the attachment to the body segment, and the way height is estimated may affect the validity of the _Vmaxpro_ IMU [18] when compared with a MoCAP system. As a result, these factors mainly affect the appearance of a systematic error that generates an underestimation of the measurement.