What is difference between single motor unit and multi motor unit?

Answer 1

There is no 'multi motor unit' to be different from a single motor unit!

A single motor unit is all the muscle fibers controlled by a single motor neuron.

Multiple motor unit summation is when a smooth and steady increase in muscle tension is produced by increasing the number of active single motor units.

Notice that what's being described is not something having to do with a some non-existent thing called a 'multi motor unit', but rather is a summation of many active singlemotor units.

Answer 2

A single motor unit consists of a single motor neuron and the muscle fibers it innervates, while a multi motor unit consists of multiple motor neurons innervating a group of muscle fibers. Single motor units are important for fine motor control, while multi motor units are involved in generating larger forces.