These new nerve sprouts grow to reach muscle fibers which have lost their nerve supply. Surviving nerve cells thus are connected to more muscle fibers and there is an increased number of muscle fibers per nerve cell. This means that surviving nerve cells do two to three times as much work on an individual basis during muscle contraction. The resultant increased metabolic work may predispose these nerve cells to overfatigue and early degeneration later in life. After 25 or 30 years of high intensity muscle work, many of the new nerve sprouts begin to wear out and degenerate.
Motor units begin to shrink back to their original number of muscle fibers per nerve cell.
For the polio survivor this means a shrinking of muscle size and decreasing strength. Progressive post-polio muscular atrophy occurs most commonly in polio survivors who have consistently used their residual muscles at high intensity for many years. It is believed to result from chronic overuse damage of the partially re-innervated muscles. Additionally, as people become older they often become less active and this leads to further disuse atrophy. If a person becomes severely ill from other health problems, such as a heart condition, pneumonia, arthritis or a fracture, they also may become weaker from disuse atrophy.