During midstance, vertical GRF is lower than body weight because of what?

During midstance, vertical GRF is lower than body weight because of what?

• A. Complete loss of ground contact
• B. Plantarflexor push-off
• C. Excessive medial-lateral force
• D. Relative unweighting from upward momentum

Answer: (D)

During midstance, the body's center of mass is moving over the stance leg and its vertical motion is slowing as it passes the highest point of its vertical trajectory. This upward movement creates a momentary state where the leg doesn’t have to support the full body weight, because the inertia of the body provides some of the load support. In equations terms, the vertical ground reaction force equals mass times vertical acceleration plus weight; with a downward vertical acceleration (due to the decelerating upward motion), the vertical reaction force ends up slightly less than body weight. This is described as relative unweighting from upward momentum. The other ideas don’t fit because there’s still ground contact, push-off happens later, and lateral forces don’t explain the vertical load drop.

During midstance, the body's center of mass is moving over the stance leg and its vertical motion is slowing as it passes the highest point of its vertical trajectory. This upward movement creates a momentary state where the leg doesn’t have to support the full body weight, because the inertia of the body provides some of the load support. In equations terms, the vertical ground reaction force equals mass times vertical acceleration plus weight; with a downward vertical acceleration (due to the decelerating upward motion), the vertical reaction force ends up slightly less than body weight. This is described as relative unweighting from upward momentum. The other ideas don’t fit because there’s still ground contact, push-off happens later, and lateral forces don’t explain the vertical load drop.