Vertical ground reaction force is slightly less than body weight during which period?

Vertical ground reaction force is slightly less than body weight during which period?

• A. Initial contact only
• B. Midstance
• C. Loading response
• D. Terminal stance

Answer: (A)

At heel contact, the body is still decelerating its downward motion as the foot just begins to bear weight. The vertical acceleration of the center of mass is downward, so the ground reaction must be slightly less than body weight (GRF = weight + m·a_y, and a_y is negative). That brief moment is when the vertical ground reaction force sits just below body weight. As loading response and the rest of the gait cycle continue, the leg actively decelerates the body and then later raises it, producing forces that are at or above body weight. Hence this slight dip below body weight occurs during the initial contact phase.

At heel contact, the body is still decelerating its downward motion as the foot just begins to bear weight. The vertical acceleration of the center of mass is downward, so the ground reaction must be slightly less than body weight (GRF = weight + m·a_y, and a_y is negative). That brief moment is when the vertical ground reaction force sits just below body weight. As loading response and the rest of the gait cycle continue, the leg actively decelerates the body and then later raises it, producing forces that are at or above body weight. Hence this slight dip below body weight occurs during the initial contact phase.