Nervous Stimulation and Muscular Strength The number of muscle fibers employed in a contraction is determined by the intensity of the nervous stimulation. The strength of the stimulus which elicits a voluntary contraction depends upon the motivation. Emotional states influence the strength of the stimulus by reinforcing or depressing the discharge of the ventral horn cells. Clenching the fists or contracting muscles in parts of the body not involved in the movement may increase the nervous discharge to the muscles involved in the movement by irradiation of nerve impulses in the central nervous system. Excitement and the cheering of spectators may intensify the nervous discharge to the muscles and liberate adrenalin which increases the strength of muscle contractions and allays fatigue. Changes in the irritability threshold of muscle, fibers may also influence the number of fibers brought into activity with a given intensity of nervous discharge and hence influence the force exerted. The irritability threshold is lowered by an increase in muscle temperature and by the accumulation of small amounts of metabolic waste products. The irritability threshold is elevated when the muscle is cold or fatigued. In general, the strength of contraction of a muscle may be adjusted in two ways: (1) by varying the number on the constituent motor units which takes part in the contraction and (2) by varying the degree of tetanus and hence the tension exerted by each muscle fiber. A fast, powerful contraction is the result of a short burst of impulses from a large number of motor neurones. A slower and less powerful contraction results from a more prolonged discharge at a slower frequency and from fewer motor neurones. The terrific rate of impulse discharge required for a fast, powerful contraction cannot be maintained for more than a brief period. It is believed that an actual blockage in the transmission of impulses across the neuromuscular junction occurs under these conditions, rather than a complete exhaustion of the muscle fibers involved. The heaviest weights lifted by weight lifting champions are raised by rapid movements known as the "snap," the "clean" and the "jerk" and the lifters can support the heavy weights for only a very few seconds once they are lifted. Lighter loads can be lifted slowly and the lifts can be repeated a larger number of items. When a light load is moved by a muscle, fewer fibers need be brought into play. The remaining fibers are at rest and stand ready to act if they are needed in succeeding contractions. As the working fibers become fatigued during light work, the threshold of irritability is raise and these fatigued fibers fail to respond to the stimuli. The stimuli pass into fresh fibers whose irritability threshold is low and the burden of the work is thus shifted from the fatigued to the fresh fibers in the muscle.		Home Components of Physical Fitness Clinical Examination Differences Between Fit and Unfit Men Physiological Elements of Fitness The Step Test The Effects of Training on the Physiological Systems Work Output Physical Condition of Muscle Application of Muscular Force Maintaining Optimum Strength Tests of Muscular Strength Strength and appearance Strength as a measure of physical fitness Strength as affected by organic problems Strength as prophylaxis Role of inactivity in production of disease Lack of Sociability in Work Lactic Acid System Muscular Form Capillary Growth Heart Size Heart Rate Training produces physicochemical changes Muscular Strength Nervous Stimulation and Muscular Strength Quality of Champions Factors Which Limit Skill Kinesthesis Balance Reaction Time Muscular Tension Visual Aim