Vigorous muscle contractions heighten the electrical activity between the muscle and the spinal cord. Impulses traveling from the spinal cord through the gamma fibers to the intrafusal fibers cause the muscle to contract. Contraction stretches an organ called the muscle spindle. Stretching of the muscle spindle results in reflex contractions of the muscle fibers. The ischemia causes pain.
Pain brings about reflex muscle activity and a vicious cycle called muscle spasm is born.
Heating of the gamma fibers slows their transmission rate, which reduces the stimulation of the muscle spindle, allowing the muscle to relax. Heating the muscle tissue also causes an increase in circulation, thus breaking the cycle. Experimental evidence indicates that ultrasound can increase the rate of protein synthesis in fibroblasts, which are responsible for the repair of injured tissue. Studies of tendon repair, says Porter, indicate that the stage of the healing process in which ultrasound is applied is very important. Ultrasound applications begun too early in the repair process could actually retard healing.