The second vertical GRF peak primarily reflects which muscular action?

The second vertical GRF peak primarily reflects which muscular action?

• A. Foot slap
• B. Knee buckling
• C. Push from the plantar flexors to reverse downward movement
• D. Hip flexor deceleration only

Answer: (B)

The second vertical GRF peak occurs during the push-off phase, when the ankle plantarflexors contract to propel the body upward and forward. This concentric plantarflexion against the ground provides the impulse that reverses the downward movement of the body’s center of mass from the preceding stance, creating the second peak in the vertical ground reaction force. Foot slap happens at initial contact, not during push-off; knee buckling would indicate instability rather than a propulsive peak; hip flexor deceleration alone doesn’t account for the main upward propulsion at this stage.

The second vertical GRF peak occurs during the push-off phase, when the ankle plantarflexors contract to propel the body upward and forward. This concentric plantarflexion against the ground provides the impulse that reverses the downward movement of the body’s center of mass from the preceding stance, creating the second peak in the vertical ground reaction force. Foot slap happens at initial contact, not during push-off; knee buckling would indicate instability rather than a propulsive peak; hip flexor deceleration alone doesn’t account for the main upward propulsion at this stage.