Sarcopenia is the name given to the characteristic age-related loss of muscle mass and strength that manifests in all older individuals. A sizable fraction of this decline is self-inflicted, as demonstrated by the gains that can be obtained via resistance training in older individuals. Nonethless, there are inexorable processes of decline, such as the loss of stem cell function in muscle tissue. Researchers have suggested that when it comes to loss of strength, damage and decline in neuromuscular junctions may be to blame, the point of integration between nervous system and muscle tissue. Researchers here suggest that contributing factors could emerge anywhere in the nervous system, including the brain, however.
A recently published study reports findings of a study in which researchers compared how much muscle strength older people could muster voluntarily with how much force their muscles put out when stimulated electrically. The results of this research suggest that physical weakness in aging may be due, at least in part, to impairments in brain and nerve function, rather than changes in the muscles themselves.
The study looked at a group of 66 older adults (average age in their 70s), who were first categorized as severely weak, modestly weak, or strong based on their measured performance on a standardized physical test. In the study, the subjects were asked to push against resistance with their leg extensor muscles, using as much strength as they could generate. When they reached their self-perceived limit, the muscle they were using was then stimulated electrically. If this caused the muscle to put out more force, it was a sign that the strength limitation the person experienced came from somewhere other than the muscle itself.
When the added force that came from electrical stimulation was expressed as a percentage increment, it showed that the weaker the test subjects, the larger a boost their muscles got. The subjects in the "severely weak" group (who were on average older) got an increase of 14.2 percent - twice the 7.1 percent increase shown by those in the "strong" group. When the conventional scientific wisdom linked such weakness mainly to loss of muscle mass, many drug companies looked for medications that acted directly on the muscles, but few proved effective. The new study provides further evidence that the nervous system plays a significant role in the problem.