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  • I’m trying to understand the lever system in the body. While studying the MS chapter in the O’Sullivan review book, it says, ‘…most muscles in the human body are third class levers (elbow flexion)…’. Why is elbow flexion a third class lever?

A third class lever is described as one having its load and fulcrum on either side of the effort. This can be represented symbolically by using the alphabets L E F to mark the beginning, mid-point and terminal points on a line that represents the lever. By way of illustration (see Fig. 1), the dynamics of elbow flexion also follows this linear arrangement of a 3rd class lever.

Elbow flexion as a 3rd class lever.

Fig. 1. Elbow flexion as a 3rd class lever. Hand (L), radial tuberosity (E) and elbow joint (F).

For example, a patient is trying to execute a biceps curl using a 1 kg dumbbell. The load (L) is the 1 kg dumbbell in the hands at the distal end of the lever. The effort (E) is the pull of the Biceps Brachii’s muscle tendon (as well as the pull of other elbow flexors) on the radial tuberosity of the radius. Finally, the fulcrum (F) is the movable elbow joint itself (the point of articulation between the humerus and the radius bones). Therefore, the action of elbow flexion in overcoming a load can correctly be classified as a third class lever.

Image reference

in Musculoskeletal