Torque, also called the moment of force, is the tendency of a force to rotate an object about an axis. It equals the force multiplied by the moment arm, which is the perpendicular distance from the axis to the force's line of action. Torque is measured in newton-meters.
In the body, joints act as axes. Muscles create internal torques by pulling on bones, while gravity, carried loads, ground reaction forces, and contact with equipment create external torques. Whether a joint rotates, stays still, or is slowed down depends on the balance between these torques.
Key Ideas
- The same force creates more torque when it acts farther from the axis
- A force whose line of action passes through the axis creates no torque about it
- The net joint moment represents the combined effect of all muscles and other tissues crossing a joint
- Net torque produces angular acceleration, in the same way net force produces linear acceleration
In laboratory biomechanics, net joint moments are typically calculated through inverse dynamics, which combines motion capture, force plate data, and estimates of segment mass and inertia. Isokinetic dynamometers measure torque at a single joint directly.
Observing It in Video
Video cannot measure torque, but it can show the geometry that influences it. In a side view of a squat or deadlift, drawing a vertical line through the bar shows how far the load sits from the hip or knee, which gives a visual indication of where the external torque demand is greater. Comparing body positions across repetitions can show how trunk angle or bar path changes that geometry.
Limits and Pitfalls
- Torque values require force information that standard video does not provide
- Estimating joint centers from the skin surface introduces error
- A 2D view captures only the moment arm in that plane; rotation out of plane can distort it
- Net joint moments do not reveal how load is shared between individual muscles, ligaments, and joint surfaces
Torque is a useful concept for explaining why small changes in body position can noticeably change how hard a movement feels.