Wave summation is an increase in muscle contraction strength based on how rapidly a muscle is stimulated. Wave summation occurs because muscles that are rapidly stimulated are not able to relax between repeated stimulations.
When muscles are not able to relax during stimulation, the sum of contractile forces builds up over time. The more rapidly and consistently a muscle is stimulated, the higher the wave summation overall. Muscles that are stimulated rapidly without rest form a smooth, continuous contraction referred to as tetanus.
High wave summation increases muscle contraction strength as a result of calcium buildup within the muscles. During rapid muscle stimulation, calcium remains in the sarcoplasm rather than being removed. With multiple muscle stimulations, the calcium in the sarcoplasm actually increases. Higher calcium levels in muscle tissue correspond with more powerful contractions. Calcium increases the power of muscle contractions by making the cross-bridges in the muscle more active.
Muscle contraction is also influenced by motor unit summation. The motor unit consists of a neuron and every muscle fiber connected to it. A nerve impulse moving through the axon of a neuron causes every muscle cell attached to the motor unit to contract. In motor unit summation, stronger contractions are achieved by stimulating large motor units of muscle.