# Can IMU Provide an Accurate Vertical Jump Height Estimate?
## 4 Discussion  
This study aimed to validate the current sensor device and positioning in relation to VJ height estimation for the two most common jump modalities (CMJ and SQJ) used in testing. The results indicate an excellent level of agreement between the devices for measurement of VJ height using the FT method as well as a high level of reliability of the VJ height estimate for both jump modalities. Based on the aforementioned, it can be argued that the presented IMU-based system can be used to provide coaches a lightweight, portable alternative to photoelectric cells and contact mats for the purposes of in-field VJ height measurement as well as an alternative to the previously used IMU placement on the athletes back. Overall JH of 30.29 \(\pm\) 3.41 and 27.36 \(\pm\) 2.67 cm was determined for the CMJ and SQJ, respectively, using an FP. The calculated JH from IMU data was 30.11 \(\pm\) 3.25 and 27.54 \(\pm\) 3.43 cm for the respective jump modalities (Table 1). Regarding the validity of IMU for measurement of VJ height, the results of a paired sample \(t\)-test have shown no statistically significant mean difference between the devices for both CMJ (t = 0.897, \(p\) = 0.379) and SQJ (t = \(-\)0.564, \(p\) = 0.578), with negligible effect size (d = 0.176 and d = 0.111, respectively) [30] (Table 2). The high ICC values of 0.975 and 0.921 for the respective jump modalities indicate an excellent [34; 37] level of agreement between the devices for the measurement of VJ height using the FT method. This is further supported by the low McV values of 1.90% for the CMJ and 3.56% for the SQJ task. The Bland-Altman systematic bias value of \(-\)0.18 cm for CMJ and 0.18 cm for SQJ (Table 2) indicate that the difference of the calculated JH between the devices has no practical significance. The lower SQJ height results have yielded a higher value of the standard deviation of the differences, thus providing a wider LOA (\(-\)3.14 and 3.5 cm) when compared to CMJ (\(-\)2.26 and 1.9 cm, for upper and lower LOA, respectively) (Table 2). Figures 4 and 5 show the agreement of the devices for the respective jump modalities. The presented results show a somewhat better agreement when compared to the ones by [17] who determined a Bland-Altman mean bias of 0.59 cm with the 95% LOA of \(-\)0.35; 1.53 cm between an IMU and an FP when examining drop jumps from three different heights using an IMU placed on the athletes' ankle. A more recent study by [26] used a Polar V800 and a stride sensor to determine the JH for CMJ and SQJ tasks and compare validate them versus an FP. The results of [26] show a bias \(\pm\) random error of \(-\)0.30 \(\pm\) 2.36 and \(-\)0.45 \(\pm\) 1.85 cm for the SQJ and CMJ modalities, respectively. The same study determined an ICC value of 0.93 and 0.97 for the respective jump modalities. These results are in line with the ones obtained in the present study. A study by [14] has reported a Bland-Altman bias of 6 cm, with a wide LOA of \(-\)2.14 cm. Present findings suggest that JH may be accurately estimated using the proposed positioning. This may help coaches and practitioners in terms of a more rapid assessment of VJ height when working with large groups of athletes. Regarding the test-retest reliability of IMU for VJ height estimation, a low McV of 4.12 and 5.93% was determined for the CMJ and SQJ, respectively. An ICC value of 0.888 was determined for the CMJ, while for the SQJ the value of ICC was 0.872 (Table 3). The results indicate a high level of reliability [34; 37] of the VJ height estimate for both jump modalities. As a whole, the presented results support a high level of validity and reliability of an IMU for VJ height estimation using the FT method. The presented results are slightly lower when compared to the results of [26] who determined the ICC values of 0.9 for both CMJ and SJ using Polar V800 and a stride sensor. The same study has determined the McV value of 4.83% and 4.66% for the SQJ and CMJ which are in line with the ones determined in the present study.