External torques during gait are generated by what?

External torques during gait are generated by what?

• A. Air resistance
• B. Only passive ligaments
• C. Only voluntary muscle contraction
• D. Ground reaction forces applied under the foot

Answer: (D)

External torques come from forces that act outside the body, with the ground reaction force being the key player during gait. When the foot is in contact with the ground, the ground pushes back with a reaction force at the point of contact. This force, acting at some distance from each joint’s center, creates a moment (torque) about the joints as the body moves. The line of action and point of application of this ground reaction force change during the stance phase, changing the direction and magnitude of these external torques that help drive or resist joint motion. Air resistance is typically small at walking speeds, ligaments provide internal passive resistance rather than external torques, and muscle contractions produce internal moments, not external ones. Therefore, forces from the ground under the foot are the source of the external torques in gait.

External torques come from forces that act outside the body, with the ground reaction force being the key player during gait. When the foot is in contact with the ground, the ground pushes back with a reaction force at the point of contact. This force, acting at some distance from each joint’s center, creates a moment (torque) about the joints as the body moves. The line of action and point of application of this ground reaction force change during the stance phase, changing the direction and magnitude of these external torques that help drive or resist joint motion. Air resistance is typically small at walking speeds, ligaments provide internal passive resistance rather than external torques, and muscle contractions produce internal moments, not external ones. Therefore, forces from the ground under the foot are the source of the external torques in gait.