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A motor unit can be described as the motor neuron and all of the muscle fibres it serves. An easier way to look at it may be to imagine the motor neuron as an electrical wire and the muscle fibres as light bulbs. When electricity is applied through the wire the bulbs light up, likewise when the motor neuron is activated the muscle fibres contract. And what happens when muscle fibres contract? We get movement.

Where it becomes complicated is when we look at the amount of motor units within a muscle, and the size of the motor units within each muscle. This will be different in each muscle, and different within each individual person. To make it even more complicated they do not stay the same, they change as we get older, again the level of change is different within individuals. The evidence suggests that as we get older, we get fewer motor units as some ‘die’, and those that remain become larger.

It can be different every day. If I’m testing people then I would be in the lab, depending on the particular muscle being tested then the volunteer would be hooked up to some kind of force transducer so we can see how strong they are. As they make their muscles contract we will detect the electrical activity with electrodes, some go on top of the skin and some are inserted into the muscle. We also take images of the muscle with our MRI and ultrasound scanners.