-check knee/hip angle. Are they not too tight?
-check the upper body movements. Does the rider have some flexibilities on their upper body movements?
Knee/hip angle
In order to achieve the maximum efficiency on pedalling, especially for people with difficulty controlling their body movements, it’s important to produce a best posture for pedalling. Thus hip/knee angle is also important.
upper body movements
Another thing we often forget is, we actually unconsciously use our pelvis, spine, and even upper body, when producing pedalling movements, in order for smooth control. Too much posture support can cause jerking movements.