Muscle Tone, Muscle Fatigue, Cause of Fatigue

Muscles in the body are normally firm to the touch. This is due to the continuous slight contraction of a small fraction of the component fibers producing what is known as muscle tone. This tone disappears when the nerve supply to the muscle is destroyed, as occurs in poliomyelitis, so that it is clearly due to the constant arrival of low frequency, asynchronous impulses from the spinal cord. This concept of the nature of muscle tone has been questioned because no electrical potentials could be recorded from electrodes thrust into resting muscles.

Muscle Fatigue

When an excised muscle is stimulated repeatedly at a frequency of about once per second, the height of each contraction eventually begins to decrease. Not only is the amount of shortening diminished, but also the relaxation becomes slower and incomplete (contracture). Finally the muscle fails to respond even to the strongest stimulation, that is, its irritability is completely lost. This diminished capacity for response which results from previous activity is called fatigue.

Cause of Fatigue

If a fatigue excised muscle is cut across and the cut surface tested with litmus paper, it is found that the interior of the muscle is acid. Since the normal muscle gives an alkaline reaction with litmus, it is apparent that fatigue is associated with an accumulation of acid. Chemical analysis reveals that the amount of glycogen (energy-yielding carbohydrate) is less in the fatigued muscle than in the normal muscle. These experiments suggest that fatigue may be due to the accumulation of acid waste products which decrease the irritability of the muscle or to exhaustion of stored fuel supplies. The accumulation of acid waste products (largely lactic acid) in the excised muscle is due in large part to the absence of a normal circulation of blood. As a result the amount of oxygen supplied to the muscle is not sufficient to oxidize the lactic acid nor can it be removed from the muscle by diffusion into the circulating blood. Conditions, are, of course different in the case of muscles in the body. Here the fuel is constantly being replenished by way of the blood; the oxygen supply is adequate to oxidize most, if not all, of the lactic acid produced and much of that which is not oxidized nor reconverted to glycogen diffuses into the blood and is carried away from the muscle. As a result, muscles in the body can perform large amounts of work before their capacity for response is abolished by fatigue. In fact, it is doubtful whether the muscle fatigue of this degree ever occurs in normal exercise.

Site of Fatigue

If the motor nerve of a muscle-nerve preparation is stimulated repeatedly, the muscle eventually fails to contract. If the stimulating electrode is now placed directly on the muscle, contractions of almost normal height are obtained showing that the muscle itself is not fatigued. Since nerve fibers have been shown to be practically nonfatiguable, the fatigue in this case must be in the junction between the nerve and the muscle, the neuromuscular junction or motor end plate. This junction is also the site of action of numerous drugs and chemical agents which may influence muscular work in the body.

If a muscle in the body is voluntarily fatigued by repeated contraction and the motor nerve then stimulated electrically through the skin, strong contractions of the "fatigued" muscle may be obtained. In this case the fatigue cannot have been in the muscle, its nerve or the neuromuscular junction, but rather in the brain or in the spinal cord from which the motor nerve arises.