## 4 Discussion  
In this study, we investigated the inter-joint coordination of the amputated and non-amputated limbs for eight unilateral transfemoral amputees and compared them with the inter-joint coordination for eight able-bodied participants. The added value of this study was to target the inter-joint coordination, whereas previous studies often focused on the kinematics or muscle electromyography (EMG) of the transfemoral gait. The inter-joint coordination during level walking was quantified by CRP and DI. Our results indicated that there were differences in the inter-joint coordination for both the amputated and non-amputated limb groups compared with the able-bodied controls. This was mainly observed by changing the relative phase angles and adapting to a higher degree of decomposition movement between the hip-knee, knee-ankle, and hip-ankle joint couples. The results supported our hypothesis that the prosthetic mechanism contributed to the accommodated inter-joint coordination, and the non-amputated limbs adjust the control strategy to fulfill the requirements of a prosthetic gait  
Figure 5: Comparison of the deviation phase (DP): (a) hip–knee, (b) knee–ankle, (c) hip–ankle. * indicates a significant difference relative to the control group (\(p<0.05\)). Figure 6: Comparison of the decomposition index (DI): (a) hip–knee, (b) knee–ankle, (c) hip–ankle. * indicates a significant difference compared to the control group. via a compensatory mechanism. This finding is significant because it elucidates the motor control strategy for transfemoral amputees and provides a more in-depth view of their insufficient athletic ability. Thus, this study can facilitate improvements in rehabilitation and prosthetic designs. The inter-joint coordination between the amputated limbs and the controls was significantly different (with regard to the CRP) for the knee-ankle and hip-ankle joints. The CRPs were only weakly or moderately correlated. The findings reflected the attenuation of the control strategy upon limb salvage. Because the prosthetic knee joint did not resist flexion in the normal range, either the amputees or the prosthetic adjusted the knee joints to a higher level of extension or hyper-extension to withstand the body support during the single support phase. Moreover, the kinematics of the prosthetic ankle joint were different from those of the human ankle [33]. These factors contributed to the differences in the joint angles and angular velocities and thus the differences in coordination between the amputated limb group and the able-bodied control group. Despite the fact that the non-amputated limb group exhibited an intact system without limb salvage, the knee-ankle CRP of the non-amputated limb group was significantly different from that of the able-bodied control group during swing, with an RMS difference of 18.6% (\(p=0.01\)). The swing phase of the non-amputated limbs indicated that the amputated limbs were in a single support phase. [PERSON] et al. [2] investigated the gait asymmetry of unilateral transfemoral amputees and reported that the amputated limbs had a poor body support ability in the single support phase. Based on our findings, we anticipated that the amputees adjusted the movement of the distal segments (foot and shank) of the non-amputated limb as a form of compensation to maintain a dynamic balance during walking. Our findings revealed significant differences in the DI for the majority of the comparisons, particularly for the proximal joints (hip joint). The hip joint of the non-amputated limb group nearly came to a stop during the last 10% of the gait cycle, which contributed to the large DI. This phenomenon may facilitate early preparation for the next gait cycle to accommodate the weakened single support of the amputated limbs, which was evidenced by the shortened swing phase in our study. The hip joint of the amputated limb was the only limb part without salvage and produced an active power to drive the residual limb and the suspended prosthesis.