On the other hand, if the activity requires a relatively large force when for example, we're lifting some type of heavy object, many motor neurons are actually used. Now, typically small muscles like the muscles found in our fingers require few motor units while large muscles like the muscles in our legs require a larger number of motor units. So let's take a look at the following diagram to see what we mean by a motor unit. So let's suppose we have the biceps muscle, the triceps muscle and this is the central nervous system, our spine as well as our brain. So we have one motor neuron, the second motor neuron and the third motor neuron. Now every one of these motor units or every one of these motor neurons begins in the central nervous system.