# The Effects of Tai Chi Chuan Exercise Training on the Lower Extremities of Middle-Aged and Elderly
## 4 Discussion
### Effects of Age Factor on Kinematics and Dynamics of Lower Limbs  
The results showed that the jump height of the elderly group was 26.67% lower than that of the middle-aged group. Performance in the countermovement jump decreased due to the age factor. Previous studies also showed that a decline in lower-limb muscle capacity due to the age factor reduces performance in the countermovement jump [25].  
\begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{3}{c}{**Middle-Aged**} & \multicolumn{2}{c}{**Elderly**} \\ \cline{2-5}  & **TCG** & **HG** & **TCG** & **HG** \\ \hline Peak hip moment (N-m/kg) & \(-\)1.06 \(\pm\) 0.24 & \(-\)0.81 \(\pm\) 0.35 & \(-\)1.04 \(\pm\) 0.25 & \(-\)1.02 \(\pm\) 0.38 \\ Peak knee moment (N-m/kg) \(\bullet\) & 1.25 \(\pm\) 0.22 & 1.08 \(\pm\) 0.32 & 1.16 \(\pm\) 0.26 & 0.94 \(\pm\) 0.22 \\ Peak ankle moment (N-m/kg) \(\bullet\) & \(-\)1.37 \(\pm\) 0.14 & \(-\)1.27 \(\pm\) 0.20 & \(-\)1.31 \(\pm\) 0.13 & \(-\)1.21 \(\pm\) 0.17 \\ Peak hip power (W/kg) \(\bullet\) & 3.57 \(\pm\) 0.72 & 2.44 \(\pm\) 1.18 & 3.05 \(\pm\) 0.66 & 2.70 \(\pm\) 0.69 \\ Peak knee power (W/kg) \(\bullet\) & 5.52 \(\pm\) 1.91 & 4.25 \(\pm\) 1.81 & 4.47 \(\pm\) 1.11 & 3.37 \(\pm\) 1.12 \\ Peak ankle power (W/kg) \(\bullet\) & 9.80 \(\pm\) 1.25 & 8.64 \(\pm\) 1.18 & 8.35 \(\pm\) 1.30 & 6.88 \(\pm\) 1.19 \\ \hline \end{tabular} Note: hip flexion was (+), hip extension was (\(-\)), knee flexion was (\(-\)), knee extension was (\(+\)), ankle dorsiflexion was (\(+\)), and ankle plantar flexion was (\(-\)). \(\bullet\) indicates a significant difference between age factor (\(p\) < 0.05); \(\bullet\) indicates a significant difference between exercise type factor (\(p\) < 0.05).  
\end{table}
Table 2: Joint dynamic during take-off phase.  
Regarding age factor, this study showed that compared with the middle-aged group, the elderly group had a lower peak vertical velocity of 6.47% and the instantaneous velocity\({}_{\text{takeoff}}\) declined by 11.86%. This result showed that the peak vertical velocity and the instantaneous velocity\({}_{\text{takeoff}}\) may have been lower due to age increase. The peak vertical velocity and instantaneous velocity\({}_{\text{takeoff}}\) were related to the sum of the angular velocities of the hip, knee and ankle during the countermovement jump. The peak knee and ankle angular velocities of the elderly group in this experiment were 8.06% and 8.25% lower than those of the middle-aged group, respectively. [PERSON] and [PERSON] (2019) pointed out that younger individuals exhibit more knee and ankle joint activity during walking. In comparison, the middle-aged group in this study had relatively high peak angular velocities of the knee and ankle joints, indicating that elderly people may have a lower angular velocity due to age increase.  
[PERSON] et al. (2019) pointed out that lower-limb power also decreases with age, and this result was similar to the age factor results in this study. The peak knee and ankle power values of the elderly group were 19.84% and 17.35% lower than those of the middle-aged group, respectively (Figure 1). During the take-off phase, the knee joint stretches and the ankle joint plantar flexes. Therefore, take-off requires extension power of the knee joint and plantar flexes. Therefore, take-off requires extension power of the knee joint and plantar flexion power of the ankle joint. Many studies suggested that the plantar flexion power of the ankle joint decreases with age ([PERSON] et al., 2019; [PERSON] et al., 2020; [PERSON] et al., 2020). [PERSON] et al. (2020) also pointed out that the power of the lower limbs decreases sharply with age. Therefore, in the elderly group, the peak knee and ankle joint power may have declined due to age increase. This may also be the main reason why the elderly group had a lower jump height than the middle-aged group.  
Figure 1: Peak hip, knee and ankle power. Note: * indicates a significant difference between age factor. # indicates a significant difference between exercise type factor.