All exercises involve levers created by the body limbs or body itself. The potential force of the levers is created by the length of the limbs and/or body and where the resistance and muscular force is applied. There are three major classes of levers in the body: first, second and third.
The first class lever is similar to the see-saw in that it has its fulcrum or balance point between the force and the resistance. When you sit on one end you apply a downward resistance on one side of the fulcrum. To raise yourself, a downward force must be applied on the other side.
For example, if someone else sits on the opposite side another downward resistance is brought into play. If the weights are equal and the distance from the fulcrum is equal, you will be in balance with no movement occurring. In order to get movement, you will have to make your body “heavier” by creating more force or some momentum to place the see-saw into motion.
The most common exercise that uses the first class lever is the heel raise. The axis is in the middle of the ankle joint while the force is the pull of the Achilles tendon behind it and the resistance is against the ball of the foot in front. In this case the first class lever does not produce a great amount of force. It is best suited for a maximum range of motion and speed of movement.
The second class lever is one in which the weight (resistance) is distributed between the axis of rotation and the application of force. It is most suited for a gain in force and is easily visualized by imagining a wheelbarrow. The axis is the wheel, the weight is in the bucket located in the middle and the force is your arms pulling the handles upward.
The best weight training exercise to describe this lever is the push-up. In this case the axis is the balls of the feet in contact with the floor, the weight is the center of gravity of your mass, and the force is in the arms, pushing you upward.
In the third class lever the force is applied between the axis and the resistance. This is the most common type of lever found in the body. For example, in the biceps curl, the biceps inserts approximately one inch below the elbow joint. The point of attachment is known as the point of application of force. The elbow is the axis of rotation, the resistance is the forearm, and the weights held in the hand.
Thus the distance from the point of application of force to the axis is very short (the force arm) and mechanically inefficient. The key reason for this is that the resistance arm, the distance from the axis to where the weight is located, is quite long. This places the weight far from the application of force.
A short force arm and a long resistance arm is most advantageous for speed, not for the production of force. In the case of speed, a short contraction of the muscle can move the end of the limb (hand) a great distance even though there is very little movement at the actual insertion of the muscle on the bone. This relationship is also advantageous for ROM.
The speed advantage of the third class lever system is most important in sports, not when lifting weights. Individuals with short limbs have an advantage in lifting heavier weights because of their shorter resistance arms. There are, however, exceptions to this; in the deadlift for example, longer arms allow you to raise the weight less distance.