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Wednesday, July 25, 2007

American Heart Association gets on band wagon for pumping weights

Even since I heard the term "sarcopenia" I began to worry about this muscle loss that accelerates as we get older. I began to counsel patients about the changes described when the sex hormones begin to fall and that their weight could stay the same but the muscles mass decreases and the subtle increase in body fat replaces the muscle weight and little net weight gain occurs .

I became convinced that we all-regardless of gender-should add some resistance exercise to the aerobic programs that we all advocate for general health and sometimes do. Now the American Heart Association has seen the wisdom of this approach.

The story of what goes on mechanistically as our muscles wither away is of course still being worked out but some interesting aspects have been investigated.

There is disconcerting evidence that at least one factor in the diminution of muscle mass as we age is loss of motor neurons in the spinal cord. Until I read this review I had not considered that the muscle atrophy of old age could be neurotrophic but there is considerable evidence suggesting just that. Details can be found in Dr. Lexell's review linked above.

At least two lines of evidence is presented. First, there have been a fairly limited number of morphological studies involving the human lumbosacral cord showing loss of motor neurons after age sixty. One such study demonstrated an alarming 50% decrement in motor neurons in some subjects while the average was only 25% and another study reported a more reassuring loss of only 5%. The small numbers of subjects in these studies limits the accuracy of percentages that are quoted and we have to wonder how much individual variation in this process occurs. Secondly, the histological pattern of muscle loss is of the type seen when nerve fibers or nerve cells fail. As motor units drop out a process of compensatory reinnvervation kicks in and interestingly there is a tendency to replace some of the fast twitch fibers units with slow twitch units, a process referred to a motor unit remodeling. The muscle loss with aging is both in number and size of muscle cell and the fast twitch fibers are disproportionately involved. The remodeled motor units are less efficient, slower and generate less force.

Data also point to decreased numbers of fast twitch muscle fibers even in regular joggers and swimmers at least in the thigh muscles which are those usually tested and biopsied. This is not surprising as the the quadriceps do little more than contract on foot strike to stabilize the patella when we run on level ground and swimming is more a shoulder activity that a quad workout. So jogging or swimming will not immunize against sarcopenia.

Decreased muscle protein synthesis and various hormonal decrements also characterize aspects of the aging sarcopenic process. Replacement of growth hormone-which declines with aging-has not improved muscle strength and function. Muscle protein synthesis does increase with regular resistance training even in the very elderly and muscle function improves along with muscle strength and fiber size.

In short, the only thing to date that can mitigate sarcopenia is resistance training (RT). There are no data to indicate that we can stop motor neuron death but RT is all we've got.

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