[Martin Taylor 941014 15:00]
I have some information now about the growth and decline of muscle strength,
from our local specialist. Bottom line: it is a control process in our
sense of the word, though the actual signal pathways are not fully understood.
In fact there are two separate processes, a neural one and a molecular one.
The neural process seems not to be a control process. It is more of
an adaptive process, as far as I can see. Untrained people cannot
bring to bear on a muscle as much "neural current" as can trained people.
By "neural current" here, I mean both the firing rate on a single nerve
fibre that activates a muscle fibre, and the number of nerve fibres brought
into play on the fibres within a muscle. When you train to use maximum
muscle tension (weight training, etc.), you learn to "recruit" more nerve
fibres and to use each one at higher pulse rates. I don't know whether
the ability to use high neural currents declines with disuse.
The molecular process is a control process, it seems. At least my expert
said so. A muscle is constructed of a bundle of nerve fibres, each of which
consists of a bundle of muscle fibrils. A fibril consists of myosin fibres
and actin fibres (almost molecular level of size) that slide against one
another when electrically activated. The fibril gets thicker and shorter.
When one gets stronger, the standard opinion is that the number of fibres
in a muscle doesn't change; in fact it is fixed by about 6 months of age.
Nor, as I understand it, does the number of fibrils in a fibre. But the
number of myosin and actin fibres in a fibril DOES change. More of these
molecular sized fibres are built when the body's hormonal balances indicate
that it has been muscularly fatigued in that area, and some disintegrate
when the balances indicate that the level of fatigue has been low over some
period of time. So there is a reference level for the chemical products
of muscular fatigue, and the myosin and actin fibres are built or destroyed
so as to maintain this level.
Also the expert said that the rate of change in myosin and actin depends on
how far one is from the reference level; a person recuperating from a long
period of being bedridden, for example, will gain a lot faster than will a
normally fit person who begins to train, and a superstrong person will
lose more quickly during inactivity than will a normally fit person.
At least that is the mechanism told to me, so far as I understood it. I'm
sure there are physiologists on CSG-L who can correct or expand this picture.
Gary, I hope this helps. And Bill P., it seems as if your hunch was correct
in principle, if not in detail.
Martin