Quick questions on Lever systems: WACE Physical Education Studies Unit 3

Every lever has three parts. The fulcrum is the fixed pivot, which in the body is a joint. The effort is the force applied to move the lever, which is the pull of the muscle at its insertion. The load, or resistance, is the weight being moved, which is the body part plus anything it carries.

In a first class lever the fulcrum lies between the effort and the load, like a seesaw. In the body, extension of the neck is a first class lever: the joint between the skull and the spine is the fulcrum, the neck extensor muscles at the back provide the effort, and the weight of the head in front is the load. First class levers can favour either force or speed depending on where the fulcrum sits.

In a second class lever the load lies between the fulcrum and the effort, like a wheelbarrow. The classic body example is rising onto the toes (plantar flexion): the ball of the foot is the fulcrum, the body weight through the ankle is the load in the middle, and the calf muscles pulling on the heel provide the effort. Second class levers always give a mechanical advantage, because the effort arm is longer than the load arm, so a small muscle force moves a large load.

In a third class lever the effort lies between the fulcrum and the load, and this is the most common arrangement in the body. The biceps curl is the standard example: the elbow is the fulcrum, the biceps insertion just below the joint provides the effort, and the weight in the hand at the far end is the load. Because the effort arm is short and the load arm is long, third class levers are at a mechanical disadvantage for force, so the muscle must produce a large force. In return, a small muscle shortening moves the hand a long way very quickly.