Moment Arm

When discussing levers, it was shown that the action line of the force (both of the effort force and resistance force) was applied at 90° to the segment. The distance from the line of force and the axis was called the lever arm.¹

lever 3

However, most forces are applied at angles other than 90° in the human body.

The moment arm is always the shortest distance between the action line and the joint axis and is found by measuring the length of a line drawn perpendicular to the force vector, intersecting the joint axis.¹

momentarm

When the action line of a force is 90° to the lever, the moment arm and the lever arm are equivalent. When the force is not applied at 90 degrees to the lever, the lever arm is no longer the shortest distance between the action line of the force and the joint axis.

The concept of a moment arm is important because it determines the degree of effectiveness of influence of a force to produce or prevent the rotation of an object around an axis.²