請問運動力學的問題。 下表有沒有大大可以幫我解釋一下，這分為四個階段，其中包括初始衝擊峰值，二次沖擊峰值，重量接受度 發球drive-ofF。 drive-off 階段是甚麼意思? stance是姿態還是立場?? 第三期重量接受度，我覺得翻得很奇怪。 為什麼第三四期，這麼平穩?

Sports Mechanics is not my specialty, but I would like to give my view on this problem.

Stance I : Initial Impact Peak
This happens when the incoming ball is first caught by the racket .

Phase II: Secondary Impact Peak
When the incoming ball first collides with the racket, it will rebound. The player then swings the racket to catch up with the ball again. This is the second impact.

Phase III: Weight Acceptance
As far as the player swings the racket, the ball is being pushed forward rapidly, causing it to accelerate At such moment, the force mainly comes from the swinging arm of the player. Hence, the curve on vGRF (vertical ground reaction force) varies not much.

Phase IV: Drive-off
Here, the racket provides a final strike onto the ball and makes it flying away from the racket .