## 4 Discussion  
The current study aimed to quantify the inter-limb sharing of the total leg stiffness, and the bilateral symmetry in leg stiffness, while supporting the body against collapse during the stance phase of gait in children with diplegic CP before and after TRS. Before TRS, the children with diplegic CP walked with a decreased share of total leg stiffness during weight-acceptance and increased bilateral leg stiffness asymmetry during single-limb support and weight-transfer during gait when compared to healthy Controls. The asymmetry in bilateral leg stiffness was significantly reduced in the CP group, especially in the terminal stance phase, with inter-limb sharing of total leg stiffness becoming similar to that of Controls. The results supported the hypothesis that the CP group would walk with improved, close-to-normal bilateral symmetry and inter-limb sharing of leg stiffness after tendon release surgery. Double-limb support (DLS) is the key feature of human walking as opposed to running, enabling the body-weight transfer between the lower limbs for the subsequent single-limb support and swing of the contralateral limb for continuous progression of the body. Smooth and stable transfer of the body weight during DLS relies on the highly coordinated kinematic and kinetic interactions of the lower limbs, the quality of which could be used to evaluate the roles of the lower limbs in the control of the whole-body balance during gait. During loading response (weight-acceptance), the CP group showed a significantly decreased share of the total leg stiffness without a significant difference in the bilateral leg stiffness symmetry before TRS when compared to the Controls. The healthy Controls were expected to sustain greater stiffness and share of the total leg stiffness during weight-acceptance than the CP group was, which could be beneficial for preventing collapse when facing a sudden load during this phase of gait. However, the tightness of the musculendonious units in the CP group led them to walk with more flexed lower limbs, and with an increased muscular effort (i.e., net internal joint moments) [15], which appeared to contribute to the decrease in the total leg stiffness sharing in the weight-acceptance limb. TRS increased the total leg stiffness sharing during loading response, which may be related to previously reported benefits of TRS, including improvement of dynamic lower limb alignment, impact absorption, smoothness of body-weight transfer with reduced energy expenditure and improved endurance [22; 23]. During pre-swing (weight-release), the task is to transfer the body weight from the support limb to the contralateral limb. Before TRS, the CP group showed a significantly increased share of the total leg stiffness and bilateral stiffness asymmetry when compared to the Controls. These results suggest that the patients kept the body weight more on the trailing support limb during pre-swing, showing a more cautious but less smooth pattern of weight transfer to the contralateral limb. After TRS, the stiffness-sharing was decreased and the transfer of the body weight to the leading stance limb became easier. The reduced asymmetry also suggests improved muscle and joint conditions between limbs, as well as overall gait improvement after TRS. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Group** & **Load Response (\%)** & **Mid-Stance (\%)** & **Terminal Stance (\%)** & **Pre-Swing (\%)** \\ \hline Con & 66.7 (44.8) & 16.2 (9.5) & 17.9 (8.5) & 27.3 (23.7) \\ Pre-OP & 38.9 (38.7) & 49.0 (37.7) & 76.2 (41.1) & 75.9 (54.7) \\ Post-OP & 52.6 (45.3) & 36.4 (30.9) & 32.2 (20.1) & 37.3 (26.3) \\ \(p\) (Con vs. Pre-OP) & 0.15 & 0.02 * & \textless{}0.01 * & 0.02 * \\ \(p\) (Con vs. Post-OP) & 0.49 & 0.06 & 0.05 * & 0.38 \\ \(p\) (Pre-OP vs. Post-OP) & 0.48 & 0.42 & 0.01 * & 0.09 \\ \hline \hline \end{tabular}
\end{table}