Lower-limb sagittal joint moments during gait are important variables for evaluating the risk of disease progression (Chehab et al., 2014, Tateuchi et al., 2017, Teng et al., 2015). For example, a higher first peak knee extension moment during the stance phase is a risk factor for the progression of knee osteoarthritis (Chehab et al., 2014), and a higher knee extension moment during the second half of the stance phase is another risk factor for the progression of osteoarthritis of the patellofemoral joint (Teng et al., 2015). Furthermore, according to a previous study, an increase in areas of hip flexion and extension moments (i.e., the hip joint moment impulse in the sagittal plane) is a risk factor for the progression of hip osteoarthritis (Tateuchi et al., 2017). Based on this, the quantification of lower-limb sagittal joint moments during daily walking is important to continuously evaluate the progression risks of these orthopedic diseases.

Generally, the gold standard method to calculate lower-limb sagittal joint moments during gait is to use a motion capture system that includes several cameras and force plates (Chehab et al., 2017). To use this equipment, the kinematics for each lower-limb segment (e.g., the acceleration of the center of mass) and external force (i.e., the ground reaction force [GRF]) should be established. Therefore, lower-limb sagittal joint moments during daily walking (e.g., traveling and shopping) cannot be calculated using the gold standard method because setting a motion capture system and force plates for daily walking using this method is difficult.

To address this issue, our idea is to estimate lower-limb sagittal joint moments during gait using the vertical GRF (vGRF) as it can be measured using a wearable device, such as an insole device. Lower-limb sagittal joint moments during gait are generally calculated through inverse dynamics using GRFs. Therefore, we consider it reasonable to estimate lower-limb sagittal joint moments using vGRF. However, no study has estimated the hip, knee, and ankle sagittal joint moments during gait using the vGRF (only one study has proposed a method to estimate the ankle sagittal joint moment using the vGRF (Hullfish and Baxter, 2020)).

Therefore, this study aimed to estimate lower-limb sagittal joint moments (including those of the hip, knee, and ankle joints) during gait using the vGRF and confirm the accuracy of the proposed method.