Muscle Strength

 

There are unavoidable changes associated with aging; degenerative joints or loss of skin elasticity, for example.  We can preserve muscle strength and size.  Our muscles grow when we are developing from childhood to adulthood.  Once we reach adulthood, any further “growth” is referred to as hypertrophy.  We exercise or exert ourselves and any muscular work done against a challenging load leads to increases in muscle mass and cross-sectional area.  This is referred to as muscle hypertrophy.  The increase in dimension is due to an increase in the size (not length) of individual muscle fibers.  Both cardiac (heart) and skeletal muscle adapt to regular, increasing work loads that exceed the preexisting capacity of the muscle fiber. With cardiac muscle, the heart becomes more effective at squeezing blood out of its chambers, whereas skeletal muscle becomes more efficient at transmitting forces through tendinous attachments to bones.

      When we begin a new regimen of exercise the muscles “learn” the new movements and accommodate the new weight loads we impose.  This is a function of neural inputs and physiologically is known as “neural learning”.  For approximately two weeks neural learning serves as the main mechanism for strength building and muscle training in any new exercise routine.  With continued exercise, the muscles’ synthetic contractile protein mechanism becomes upregulated, through stimulation of the family of immediate-early genes, including c-fos, c-jun and myc. These genes appear to dictate the contractile protein gene response and through this response the muscles gain strength (how this occurs is still scientifically poorly defined).    Finally additional contractile proteins become incorporated into the myofibrils resulting in increased muscle fiber size.   The muscle fibers sustain mild trauma from the overload of exercise and this trauma stimulates a component of the muscle fiber, their “satellite” cells, to proliferate.  These cells are located on the outer surfaces of the muscle fibers and are usually dormant..  When these satellite cells proliferate in response to injury, their daughter cells are drawn to the damaged muscle site They then fuse to the existing muscle fiber, donating their nuclei to the fiber, which helps to regenerate the muscle fiber. It is important to emphasize the point that this process is not creating more skeletal muscle fibers (in humans), but increasing the size and number of contractile proteins (actin and myosin) within the muscle fiber.   This injury regeneration process continues for up to 48 hours.  By exercising repeatedly every other day or so, one can keep the process ongoing, maximizing muscle hypertrophy.  But there is a limit to how massive each myofibril will grow.    There are numerous growth factors that have been identified that play some role in muscle hypertrophy including insulin-like growth factor, fibroblast growth factor, hepatocyte growth factor, growth hormone and testosterone.  To date though there are no successful ways to use any of these factors to safely control human muscle hypertrophy.