# Concurrent Validity of the Inertial Measurement Unit _Vmaxpro_ in Vertical Jump Estimation
## 4 Discussion
Accordingly, the SEE values are also in agreement with those observed in other studies. At the same time, in addition to the intercept and \(C_{b}\), the existence of a high systematic bias can be confirmed through the study of the difference in the means and the analysis of the Bland-Altman plots. The simple observation of the mean of the differences is already an indicator of this error, which is further consolidated by the existence of statistically significant differences between the two methods of 10.54 cm and a moderate ES in the Wilcoxon paired sample test. The systematic error detected in the Bland-Altman analysis (10.58 cm) shows similar numbers to those obtained in the mean difference and regression results (8.46 cm), implying that the _Vmaxpro_ device performs measurements that underestimate the results by approximately 10 cm when compared to a MoCAP-based gold standard. This difference is three times greater than those found by [PERSON] et al. [18] in their analysis of the differences between a MoCAP system and the accelerometer _Catapult system_ in real conditions, where they found differences for maximum jumps of \(-\)3.70 cm. Surprisingly, better results were obtained in this study in terms of validity even though the jumps were less controlled, as they were part of a fitness circuit, which suggests greater variability than in our study. In addition, the accelerometer was placed on the upper back, which indicates that the error would be greater. However, since this study was not carried out in a laboratory, this increase in variability should mainly affect the magnitude of the random error and, to a lesser extent, the systematic error. In any case, the difference between _Catapult_ and MoCAP results is still significant in the same way as in our study. Similar results were found by [PERSON] et al. [10] who observed systematically lower measurements (\(-\)8.89 cm) when studying the IMU _MMS_ (Motus Design Group) and comparing it to a 3D MoCAP system. These authors placed the accelerometer between L4 and L5, close to the center of mass, which is similar to the experimental procedure used in our study where the IMU was placed above the hip and showed lower measurements in absolute terms. In contrast, [PERSON] et al. [52] found differences of 5.7 cm for the _MicroStrain_ IMU (Inertia-Link-3 DM-GX2) placed at the hip in the CMJ. In this case, unlike our study, the IMU tends to overestimate the measurement. Similarly, [PERSON] et al. [37] found slight differences due to the overestimation of the _Push band 2.0_ IMU when analyzing peak velocities. These results translate into an overestimation of the jump (0.32 cm) and, in addition, a proportional error. Such heteroscedasticity is not present in the case of _Vmaxpro_, as reflected in the scatter plot (Figure 2b), showing no clustering trend in the data. The latter is reinforced by the Cusum test for which no significant differences are seen with the linearity condition. On the other hand, the IMU position represents the position concerning a reference system and not the body center of mass position which depends on the postures adopted by the body [10; 52], which may lead to measurement errors. When comparing the systematic error found in this work with other studies that use force platforms as a criterion instrument, it is observed that the differences are smaller than those found for MoCAP systems and are even below one cm [47; 48; 50; 53]. However, when analyzing the validity of _Push Band 2.0_, which represents an IMU with very similar characteristics to _Vmaxpro_, [PERSON], [PERSON], and [PERSON] et al. [9] found systematic overestimation errors of 4.4 cm in the CMJ. These values, although lower in absolute value than those found for Vmaxpro, are closer to the latter but opposite in sign, as was the case when compared to a motion capture system. Similarly, [PERSON] et al. [54] found even higher overestimation values for _Push Band 2.0_ (14.4 cm).