## 4 Discussion
### Trunk Muscle Group  
Regarding the activation timing (Figure 2), both the rectus abdominis and external oblique abdominis muscles were activated in advance in the catch stage when standing. Table 1 indicates that the rectus abdominis was first activated at 56.91% and the external oblique abdominis initiated muscle firing at 54.92% of the cycle time. By contrast, in the kneeling position, the two muscles started to be activated only during the pull period (60.73% in the rectus abdominis and 62.25% in the external oblique muscle). We inferred that, in a relatively unstable stance, the core must be stabilized in advance to facilitate the limb strength transmission. According to the study conducted by [PERSON] et al. (2013), when a paddling action was performed on an unstable water surface, core muscles were first activated early in the pull phase to provide posture stability and standing balance, and then thigh and shoulder muscles continued to be activated late in the pull phase to provide the driving force for propulsion [15]. The core muscle group was located at the body's center of gravity (COG), affected the functional efficiency of the kinetic chain and enhanced the transmission and control of the limb strength [18]. By contrast, erector spine muscles were activated earlier in the kneeling position than in the standing position (53.42% vs. 58.42%, \(p<0.05\)) and were more likely to be activated in the catch stage (Figure 2). We inferred that the catch action can be performed using more torso rotation in the standing position whereas in the kneeling position the paddle can only be inserted into the water by arching the back and extending the arm, necessitating the recruitment of the erector spinae during the catch phase to facilitate this action.  
As shown in Table 3, the difference in the maximal activation values for the external oblique abdominis muscle in the kneeling and standing positions was significant (2.53 vs. 5.04 \(\mu\)V, \(p<0.05\)), and a larger external oblique abdominis muscle firing was observed in the standing position. This echoes the finding that the catch action can be accomplished using more torso rotation in the standing position. As a result, during the pull period, torso rotation provided by the external oblique abdominis muscle can be fully used to enhance the paddling force.  
\begin{table}
\begin{tabular}{l l c c c} \hline  & & **Kneeling** & **Standing** & \(p\)**-Value** \\ \hline \multirow{6}{*}{Shoulder and upper limb muscles} & Pectoralis major & 4.26 \(\pm\) 6.30 & 4.20 \(\pm\) 2.94 & 0.97 \\  & Lower trapezius & 17.32 \(\pm\) 10.95 & 26.43 \(\pm\) 22.52 & 0.19 \\  & Latissimus dorsi & 14.53 \(\pm\) 14.35 & 18.21 \(\pm\) 16.34 & 0.40 \\  & Biceps brachii & 59.05 \(\pm\) 45.98 & 28.44 \(\pm\) 24.70 & 0.02 \\  & Triceps brachii & 10.61 \(\pm\) 9.02 & 25.70 \(\pm\) 17.57 & 0.02 \\  & Wrist flexor & 37.76 \(\pm\) 33.73 & 21.38 \(\pm\) 12.70 & 0.12 \\  & Wrist extensor & 52.89 \(\pm\) 55.29 & 35.46 \(\pm\) 15.22 & 0.24 \\  & Rectus abdominis & 8.08 \(\pm\) 10.37 & 10.69 \(\pm\) 10.21 & 0.31 \\  & External oblique & 2.53 \(\pm\) 1.47 & 5.04 \(\pm\) 2.47 & \textless{}0.01 \\  & Encoder spinae & 20.93 \(\pm\) 28.18 & 14.52 \(\pm\) 15.55 & 0.33 \\  & Multifilidus & 9.59 \(\pm\) 7.25 & 8.14 \(\pm\) 5.71 & 0.33 \\  & Gluteus maximus & 2.71 \(\pm\) 1.45 & 2.34 \(\pm\) 1.22 & 0.13 \\  & Rectus femoris & 5.78 \(\pm\) 3.11 & 9.52 \(\pm\) 6.97 & 0.10 \\  & Biceps femoris & 7.01 \(\pm\) 8.14 & 10.45 \(\pm\) 5.12 & 0.15 \\  & Tibialis anterior & 22.92 \(\pm\) 15.33 & 24.15 \(\pm\) 17.35 & 0.81 \\  & Gastrocnemius & 4.50 \(\pm\) 3.80 & 8.90 \(\pm\) 5.68 & 0.03 \\ \hline \end{tabular}
\end{table}
Table 3: Maximum activation of each muscle in different postures (\(\mu\)V).