Sarcopenia redux: running preserves “motor units”

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I posted a few thoughts last week about an article on sarcopenia — the gradual loss of muscle with age — and new attempts to find drugs that will slow it down. I’d be remiss if I didn’t point out some of the research on the obvious drug-free ways to avoid withering away to nothingness. For instance, I just noticed an article in the current issue of Medicine & Science in Sports & Exercise that compares the number of “motor units” in the muscles of masters runners (average age 65) with age-matched controls, and with a younger group (average age 25). The news is good.

First off, a motor unit is “a single alpha-motor neuron and all of the corresponding muscle fibers it innervates.” Losing motor units is one of the several mechanisms that combine to produce the muscle loss grouped as sarcopenia. In this case, it’s not the muscle fibres themselves that die; rather, it’s the motor neurons that control them. When you’re young, the orphaned neurons often sprout new axons that connect them to other motor neurons — so the number of motor units decreases, but the amount of muscle you can use stays the same. This can hide the problem until your 60s or 70s, at which point you’re no longer able to reinnervate orphaned fibres as well, and motor unit loss becomes a serious issue.

Anyway, the study itself was quite simple. Testing the tibialis anterior (shin) muscle, they found that the masters runners had 140 motor units on average, compared with 150 for the young group but just 91 for the old non-runner group. So there it is: consistent training preserves muscle — not the muscle fibres, in this case, but the motor neurons that control them. As the researchers put it:

The significance of the (…) findings centers on providing an improved understanding of the neuromuscular system through ‘‘elite aging’’ and provides support into the favorable value of long-term physical activity and exercise for protecting neural function.