## 4 Discussion  
The objective of this study was to clarify the relationship between the acceleration from the IMU and the VGRF during single-leg drop landing after ACLR and to provide basic information for the use of IMU as an alternative method to estimate VGRF in the clinical setting. The present study showed that the LSI of the VGRF was linearly associated with the LSI of the resultant acceleration at the shank, thigh, and lumbar sensors. In addition, on the uninvolved limb, the resultant acceleration at the shank, thigh, and lumbar sensors was linearly associated with the VGRF, whereas on the involved limb, the resultant acceleration at the thigh was the only predictor of the VGRF. Furthermore, the resultant acceleration at the thigh could detect interlimb asymmetry in the VGRF with high sensitivity, specificity, positive predictive value, and negative predictive value. These results partially support our hypothesis, but caution should be taken when using IMUs to predict VGRF asymmetry, as the coefficients of determination for predicting VGRF is lower in the involved than in the uninvolved limbs. A significant linear relationship was found between the resultant acceleration and VGRF in the involved and uninvolved limbs. A recent scoping review on wearable technology after ACLR concluded that such technology can assess external workload outcomes, including kinematics and kinetics, after ACLR [38]. This finding is consistent with the results of the present study. A previous study on healthy women has found a strong correlation between the VGRF and resultant acceleration at the knee (tibial tuberosity) and upper trunk (the first/second thoracic spine) [14]. One reason why the correlation coefficient in this study was lower than that in previous studies may be the differences in the positions of the wearable IMUs. In this study, wearable IMUs were attached to the participants' shank, thigh, and lumbar region, whereas in a previous study, IMUs were attached to the tibial tuberosity and the first/second thoracic vertebrae [14]. The mounting position used in this study was one of the most frequently used positions reported in the scoping review of biomechanical analysis using IMUs after ACLR [21]. The resultant acceleration indicates the overall movement of the mounting position of the sensor, without considering the axial components. Therefore, the difference in the position of the wearable inertial sensor may affect the resultant acceleration. A recent scoping review showed that the position of wearable IMUs varies depending on the study [21]. Future studies should examine which position of the wearable IMUs is most closely related to VGRF during landing. The coefficient of determination between the resultant acceleration and the VGRF was lower in the involved limb than in the uninvolved limb. A typical feature of movement after ACLR is the compensation of knee joint loads by other joints such as the hip and ankle joints, which is known as intralimb compensation [39]. Intralimb compensation is compensation by the adjacent joint within the same limb to reduce the knee extension moment during movement [39]. In fact, intralimb compensation has been reported to reduce knee extension moment by increasing trunk flexion during single-leg drop landing after ACLR [22]. Such compensatory movements remain in patients 3 years after ACLR [40] and may contribute to poor quadriceps strength in the long postoperative period [41,42]. However, it is difficult to objectively identify such compensatory movements in a clinical setting. The differences in the coefficient of determination between the uninvolved and involved limbs in this study may suggest the existence of intralimb compensation; however, further study is needed. The relationship between the LSI of the VGRF and that of the resultant acceleration showed a significant linear relationship.