When a greater number of motor units are present in a muscle and are able to be simultaneously stimulated causing an increase in muscle force (Sherwood, and Kell 271-272).
simply: Wave summation occurs when a second stimulus is applied before relaxation occurs completely. In depth: In order for a muscle to contract electrical shock or nerve impulse must cause...
Wave summation releases more calcium and increases the force of contraction by increasing the firing rate of motor neurons (The second contraction takes place before the first is completely at rest)...