Understanding where the GRF falls relative to a joint axis determines what?

Understanding where the GRF falls relative to a joint axis determines what?

• A. The external torque acting on the limb
• B. The patient's height
• C. The exact oxygen cost only
• D. The amount of arm swing only

Answer: (A)

Where the ground reaction force falls relative to a joint axis tells you the external torque acting on that limb. The GRF is an external force, and its line of action creates a moment around the joint equal to the force times the perpendicular distance from the axis (the moment arm). If the force line passes in front of the axis, it produces a torque in one direction; if it passes behind, it produces the opposite torque; if it goes through the axis, the moment arm is zero and no external torque arises. This external torque is what muscles must counteract to control movement. Other options don’t reflect how the GRF’s position relative to a joint axis determines the rotational effect on the limb.

Where the ground reaction force falls relative to a joint axis tells you the external torque acting on that limb. The GRF is an external force, and its line of action creates a moment around the joint equal to the force times the perpendicular distance from the axis (the moment arm). If the force line passes in front of the axis, it produces a torque in one direction; if it passes behind, it produces the opposite torque; if it goes through the axis, the moment arm is zero and no external torque arises. This external torque is what muscles must counteract to control movement. Other options don’t reflect how the GRF’s position relative to a joint axis determines the rotational effect on the limb.