A graded muscle response occurs when a muscle contracts with different degrees of force based on certain circumstances, according to Dr. Gary Ritchison from the Department of Biological Sciences at Eastern Kentucky University. Two types of graded muscle responses are motor unit summations and wave summations.
A motor unit summation relies on the number of motor units stimulated within a skeletal muscle to cause contractions. When more motor units are stimulated, the greater the contraction within the muscle, notes Dr. Ritchison. A motor unit consists of a single motor neuron and all of the muscle fibers attached to it. A wave summation that increases the frequency of muscle stimulation also increases muscle strength. Rapid stimulation prevents calcium levels in muscles from decreasing, therefore a stronger contraction occurs because calcium creates more active neuron-muscle fiber interactions.
A motor unit summation increases the electrical stimulation of motor unit cells, states lecturer Jim Swan from the Department of Biology at University of New Mexico. When more electricity is applied, more muscles contract. This is also called quantal summation. A wave summation can lead to tetanization, a process that sustains a contraction after the muscle stimuli reach a high enough frequency. When wave after wave of muscle contractions reach a maximum point, the muscle remains contracted rather than relaxing. A treppe is similar to a motor unit summation, only this time the muscle gradually increases stimuli as the fibers warm up from a previously relaxed position. The strength of muscles remain the same in a treppe, whereas muscle contractions increase during a quantal summation.