Motor unit summation, or recruitment, controls the force of contraction regarding skeletal muscle tissue. Before the skeletal muscle is stimulated, there is a natural skeletal muscle tone that is present. This skeletal muscle tone keeps the muscles firm, stabilize joints, maintains posture, and readies the body to respond to stimuli. This contraction type can be likened to isometric contractions. Stimuli that will not produce an observable change in contraction strength beyond this tone is deemed sub-threshold stimuli. A stimulus labeled as a threshold stimulus is a stimulus that produces the first observable contraction. A maximal stimulus is the end point of skeletal muscle contraction where all motor units are recruited. The recruitment of the motor units follows a predictable sequence. Henneman’s size principle dictates that the smallest muscle fibers will be activated first since they are controlled by the smallest, most excitable motor neurons. As necessary, larger and less excitable motor units are recruited to aid in more powerful contractions.
The force of muscle contraction is dependent on the number of myosin cross bridges that are attached to actin. Four separate factors determine the force of contraction. One is the number of muscle fibers recruited. More accurately, the greater the stimulus for maximal contraction, the more motor units are recruited. Second is the size of the muscle fibers. Skeletal muscle hypertrophy results in greater cross-sectional area and subsequent increased tension potential. Other factors are attached to greater muscle girth including larger glycogen storage capability that aid in contraction capacity. Third is the frequency of the stimulation. The “summation”, or summary, of contractions is much like adding contractions upon one another. The higher the frequency of contractions, the greater the force the muscle exerts. Since a lighter contraction begins with the endurance adapted fibers type I then the muscle has a greater ability to efficiently use glycogen storage. As the frequency of contractions increases, the glycogen in the type 1 fibers can diminish. If the glycogen within the type I fibers is not enough then the type IIa and type IIb fibers are recruited. Since these type II fibers are associated more with power, it’s apparent why the muscle contractions grow in strength. Once these type II muscle fibers, which are generally anaerobic, are fatigued then the muscle no longer can produce contractions despite a stimulus for contraction still being present. This is skeletal muscle fatigue in essence, excluding the ionic imbalances that occur when potassium, sodium, calcium, and other ions exchange between intracellular and extracellular compartments. The fourth determinant of skeletal muscle contraction is based on the length-tension relationship. An optimal length of muscle stretch would be when the muscle is slightly stretched and the thin and thick filaments are overlapped to permit sliding over the entire length of the thin filaments. The joints of the body prevent degrees of stretch that would result in lack of ability to generate tension, which can happen when the muscle is over-stretched. The stretch-reflex prevents this type of stretch from happening in the majority of cases. The stretch reflex senses muscle stretching and then activates the motor neurons associated with contraction. This would shorten the muscle, by definition.