It is due to muscles acting upon a system of levers; the skeleton that movement of some part of the body occurs. There are three types of lever but all levers have three parts:
The Fulcrum: the point of movement or pivot, generally at the centre of a joint.
A Load: the body's weight or some external object.
An Effort: a muscular force to move the load.
There are three classes of lever and each is classified depending upon where in relation to each other the fulcrum, the load and the effort are.
The fulcrum is between the effort and the load.
Both effort and load are in the same direction.
An example of this is the head where the head pivots on the atlas (fulcrum).
The load is the weight of the head going down.
The effort is the muscles at the back of the neck pulling down.
The fulcrum is at one end of the lever.
The load is in the middle of the lever.
The effort is at the opposite end of the lever to the fulcrum withthe direction of effort opposite the load.
An example of this is stepping up onto your toes. The fulcrum is at the toes. The load is that of the body going through the middle of the foot and the effort is in the calf muscles pulling the body upon to the toes.
This is the most effective lever as a relatively small force can move a largeweight.
Like second order levers the fulcrum and load are at opposite ends of the lever but the effort is off-centre of the lever towards the fulcrum. This is not as efficient as second order levers but small muscle movement creates long lever movement.
An example of this is a biceps curl. The load is in the hand, the fulcrum is at the elbow and the biceps make the effort.