A posteriorly directed GRF does what to forward progression of the center of mass?

A posteriorly directed GRF does what to forward progression of the center of mass?

• A. Has no effect
• B. Moves it medially
• C. Accelerates it forward
• D. Slows it

Answer: (D)

The key idea is that the horizontal component of the ground reaction force controls forward movement of the body. If the ground pushes the body backward (posteriorly directed), that backward force creates backward acceleration or reduces forward acceleration. In other words, it slows the forward progression of the center of mass. The vertical component mainly supports body weight and doesn’t drive forward movement, so a posterior-directed GRF tends to slow rather than speed up forward progression.

The key idea is that the horizontal component of the ground reaction force controls forward movement of the body. If the ground pushes the body backward (posteriorly directed), that backward force creates backward acceleration or reduces forward acceleration. In other words, it slows the forward progression of the center of mass. The vertical component mainly supports body weight and doesn’t drive forward movement, so a posterior-directed GRF tends to slow rather than speed up forward progression.