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Update on the Significance of Overuse
Jacquelin Perry, MD, ScD (hon)

Thank you for the invitation to contribute a few words in recognition of the 18^th anniversary of your Post-polio support group. I am honored and welcome this opportunity to briefly review recent research findings, which clarify the significance of initial pathology, exercise, and time on the life style of post-polio survivors.

The investigators^1,2 chose a rat model for their research. Their lesion was acute partial paralysis created by detaching either of two motor nerves from the spinal cord. This impaired the four major muscles controlling the ankle. Motor loss was severe (>80%) in two (gastrocnemius and tibialis anterior), which left less than 20% intact fibers. The loss by the other muscles (soleus and plantaris) was moderate (>50%).

The severe lesions caused the greater changes. At one month electrophysiological studies showed a fourfold increase in the size of the few remaining (20%) motor units and a corresponding increase in force. The new terminal sprouts allowed the active neurons to adopt adjacent muscle fibers, which had lost their nerve supply. Consequently, each surviving neuron cell body now supported four times the normal number of muscle fibers. At 12 months the size of the enlarged motor units was significantly reduced. Intense exercise performed during their eleventh month, by another group of similar prepared animals, increased the loss of motor units.

The moderately damaged (>50%) muscles exhibited a lower response to the neural injury. At one month there was a two-fold increase in motor unit size. Sprouting was infrequent unless the motor units count was less than 40%. At twelve months soleus change was small but still displayed a significant time-related reduction. Response to exercise was inconsistent and might not be evident. Time challenges were slower.

The clinical message from this research is Be aware that an episode of acute poliomyelitis can leave a "scar" which has increased the sensitivity of muscles to overuse. The basic rules to avoid losing function are:

Fatigue is a sign of overuse.
Stop! Don’t get tired
Be active but
Don’t push through fatigue
Braces can protect weak muscles from overuse

It is most appropriate that Tam’s research on motor unit durability focused on the ankle plantar flexor muscles. These are the muscles most significant for weight bearing stability as well as progression^3,4

References

1. Tam SL, Archibald V.Tyreman N, Gordon T. Increased neuromuscular activity reduces sprouting in partially denervated muscles. J Neuroscience. 2001: 21(2)654-667)

2. Tam SL, Archibald V.Tyreman N, Gordon T. Effect of Exercise on stability of chronically enlarged motor units. Muscle Nerve. 2002; 359-369.

3. Perry J, Barnes G, Gronley JK. The postpolio syndrome. Clinical Orthopaedics 1988. 233:145-162.

4.Perry J, Mulroy SJ, Renwick E. The relationship of lower extremity strength and gait in patients with post-polio syndrome. Arch Phys Med Rehabil 1993. 74 (165-169)