How do motor units differ?
The muscle fibers are functionally organized as motor units.
A motor unit consists of a single motor neuron and all of the muscle fibers it innervates .
Motor units in different muscles do not always contain the same number of muscle fibers.
Muscles performing delicate and precise movements have motor units with a small number of muscle fibers, whereas muscles performing more powerful but less precise contractions have motor units with many muscle fibers.
Those that move the eye, the number of muscle fibers per motor unit can be less than 10, whereas, in the large muscles of the thigh, the number can be several hundred.
A whole muscle contracts with either a small force or a large force, depending on the number of motor units stimulated to contract. This relationship is called multiple motor unit summation .
And yes, those last two are called tetanus. In the disease, the muscles never relax.