## Mechanical Advantage and Velocity Ratio

Many parts of the body can be treated as parts of a lever if they involve a pivot at a joint. In general muscles apply forces - the effort - to the parts of a joint. This will result in a turning around the pivot and another force - the load - being supported or balanced. We can define a ratio, called the 'mechanical advantage'.
$Mechanical \: Advantage = \frac{Load}{Effort}$

Limb structures in the body tend to have a mechanical advantage less than 1 - a mechanical disadvantage.
There is however, a reason for having a mechanical advantage less than 1. The movement advantage is measured by a quantity called the 'velocity ratio' or 'movement ratio' for the system, equal to the distance moved by the load divided by the distance moved by the effort. The velocity ratio, like the mechanical advantage, is dimensionless - does not have units. A mechanical  advantage less than one implies a velocity advantage greater than one and vice versa, and if one quantity is greater than one, then the other is less than one.
For a perfectly efficient joint, Work done by joint equals energy supplied to joint. Hence
$Effort \times d_{EFFORT}= Load \times d_{LOAD} \rightarrow \frac{Load}{Effort}=\frac{d_{EFFORT}}{d_{LOAD}}$

This means that a small movement in the upper arm muscle can produce a large movement at the hand.