## 4 Discussion
On the other hand, VGRF biofeedback offers several practical advantages: it is easier to implement, relies on readily available force sensors, and is generally more intuitive for real-time interpretation. In the present study, participant #15 (the 'green dot participant') was identified as an outlier for knee flexion angle during baseline and recall 1 walking (Figure 6A). Including or excluding this participant did not affect knee flexion angle, so their data were included in all other analyses. Despite this deviation, statistical outcomes remained consistent across all variables except for knee extension torque during recall 2, where results varied depending on the inclusion or exclusion of this participant. When the outlier was excluded, a significant difference in knee extension torque was observed during recall 2 compared to baseline. This participant exhibited a distinct walking pattern compared to the rest of the group, characterized by walking with more extended (straighter) knees. As a result, they generated the lowest knee extension torque during walking. In a healthy gait, the support moment is achieved by coordinated contributions from the ankle, knee, and hip joints [8]. With aging, older adults may increase their hip joint torque to compensate for lower ankle and knee torque output [9]. While dominant contribution from any individual joint is likely not desirable, an approach that allows individuals to modulate relative joint contributions may offer an opportunity for improving gait efficiency or safety. In the present study, biofeedback was used to promote increases in the first peak of VGRF. Our participants did not increase the hip flexion angle or hip extension torque. Rather, they increased the maximum knee flexion angle and produced higher knee extension torques, consistent with previous findings in patients with anterior cruciate ligament reconstruction [27; 28]. These results suggest that VGRF biofeedback can increase vertical support through enhanced knee extensor torque, without changes in hip extension torque, making it a potential therapeutic tool for older adults with low VGRF and knee extension torque. With aging, there is a general reduction in the lower extremity's range of motion. Older adults often exhibit a reduced knee range of motion and lower peak knee flexion angle during loading response compared to younger adults [34]. Typically, a decrease in the peak knee flexion angle might suggest that the GRF torque arm was closer to the knee joint center, resulting in a lower knee extension torque. Interestingly, in this study we found that older adults increased knee flexion angle and knee extension torque during acquisition and recall trials. Additionally, the knee flexion angle during the swing phase increased, indicating that older adults bent their knees more during the swing phase with increased VGRF, leading to more flexed knees during the stance phase. Despite this, they exhibited reduced knee flexion excursion during biofeedback trials. This finding contrasts with a previous study in younger adults, where an increase in VGRF via biofeedback resulted in greater knee flexion excursions during biofeedback compared to baseline walking [27]. A potential explanation is that older adults may have stiffened up their legs to manage the increased loading force during biofeedback. Exercise programs using biofeedback to increase VGRF should consider the potential implications of reduced knee flexion excursion and evaluate its relevance to their specific target population. Our study found that the VL and GMAX muscle EMG profiles during the first half of the stance phase were similar to previous studies, showing peaks in activities for both muscles during the loading response [35,36]. Both muscles' activities increased during biofeedback and recall trials compared to baseline walking (Figure 8), indicating enhanced neuromuscular recruitment of hip and knee extensor muscles in response to the increase in the first peak of VGRF.