In this animation we discuss how the body converts the linear forces created by the muscles into rotational movement. We use a model of the elbow joint to establish that the muscles use the bones of the body as levers with the joints being the fulcrums (or rotational centers) of these systems. We then go on to point out two main design features of these joints. The first of these is that when a joint is straight then it produces very little torque (or rotational force). This means that a second set of muscles called a synergistic muscle group has to surround these joints to provide both support to the joint and different angles through which the initial bending of the joint can be achieved.
The second design issue is that through altering the position of the connection between the muscle and the bone you can alter both the amount of torque produced and speed with which the joint rotates. This pay off between the speed and power of the joints is usually biased towards speed by using what is called a third class lever system. However, when you raise yourself on the ball of your foot this gives a mechanical advantage to the muscle by using a second class lever. The layout of these three lever class systems is illustrated at the end of the animation.
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Mechanics and structure of muscle