Motor unit summation is the build-up of the myoelectrical signals of the active motor unit during continuous, sustained muscle contraction. Sustained contraction causes the motor unit to fire continuously, creating myoelectrical signal fusion that produces larger forces than a single contraction.
A motor unit consists of an individual motor neuron and the muscle fibers that the neuron innervates. Muscle fibers depolarize in response to the action potential from the neuron, which creates an electric field around the muscle fibers, which is the signal known as the muscle fiber action potential. The sum of all muscle fiber action potentials is the myoelectrical signal of the motor unit.