How Muscles Adapt To Make Us Stronger

try 2 

[if you don’t know how muscles work yet, you might first want to read the previous blog entry].

There are three basic ways to increase the force of a muscle… 1) rate coding—  increase the frequency at which motor units fire… 2) recruitment— increase the number of motor units activated, or…  3) bigger muscles

When someone first starts lifting weights, the initial increase in strength comes via the first two methods, both of which are forms of neural adaptation.   Basically, the brain learns how to generate more force from a given amount of contractile tissue.

Our authors tell us that “there is evidence to show that anaerobic training [like weightlifting] can enhance the firing rates of recruited motor units.”  This “ability to increase firing rate appears to be essential” to develop maximal power.

As we struggle to move loads at the edge of the weight we can handle, the body naturally adapts, increasing not only the firing rate of the motor units, but also recruiting more motor units.   “Maximal force production requires not only the recruitment of a maximal percentage of available motor units, including high-threshold motor units, but also the recruitment of these motor units at very high firing rates.”

“Before a high-threshold motor unit is recruited, all of the motor units below it are recruited sequentially,” write our authors.  “Motor units high in the recruitment order are used primarily for high force, speed, or power production.”  One of the payoffs of exercise is that, “once a motor unit is recruited, less activation is needed in order for it to be recruited” again.

Besides a higher firing rate and greater motor unit recruitment, the size of muscles will also adapt.  “With heavy resistance training,” the authors state, “all muscle fibers get larger,” and thus, the muscle can “produce higher levels of force.”

 Interestingly, it seems that “smaller muscles rely more on an increased firing rate to enhance force production, whereas larger muscles rely more on recruitment.”

When it comes to muscle strength, the length or volume of the muscle has little effect.  It is the cross section of the muscle which determines muscle strength.  In other words, if a short person’s muscle is the same width as a tall person’s, they will be same strength– even if the tall person’s muscle is twice as long.

Muscles grow by increasing the number of myofibrils within the muscle fiber, and also by increasing the number of contractile proteins, aka myofilaments (actin and myosin), within each myofibril.  [if you haven’t looked at that previous post yet, you might want to now; I talk about actin and myosin and myofibrils there].  The addition of actin and myosin to the outside of the myofibril “create the cumulative effect of enlarging the fiber.”  And all the enlarging fibers in turn enlarge the entire muscle.

In fact, the effects of resistance training are so intense and so deep– that, our authors tell us, “resistance training has the potential to alter the activity of nearly 70 genes.”

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There are five steps to enlarging muscle…

1) first, through a stress event like weightlifting, the muscle fibers must actually be damaged

2) then the body’s immune system kicks in, and an inflammatory response ensues;  this is under the control of the endocrine system

3) next, the hormones for remodeling muscle do their part

4) the synthesis of new proteins occurs

5) and lastly, the new proteins are incorporated into the existing muscle fibers; more actin and myosin filaments are constructed, as well as non-contractile proteins used to maintain structural integrity and the proper orientation of contractiles (these are actually laid first)

Besides making new proteins, the body will also slow down normal protein degradation.  Also, our authors point-out that “the more muscle fibers recruited for the performance of the exercise, the greater the extent of potential remodeling in the whole muscle.”

Generally speaking, the magnitude of protein synthesis depends on the following…

…carb and protein intake, amino acid availability, timing of nutrient intake, the mechanical stress of the workout, the hydration levels of the muscle cells, and the responses of the anabolic (protein-building) hormone system– including the number and activity of hormone receptors located at the cell.

Although many people know the importance of anabolic hormones (such as testosterone) in building muscle, they often underrate the importance the hormone receptors, without which the hormones would not be accepted into the cell to do their work.  Studies have shown that “as few as one or two heavy resistance exercise sessions can increase the number of androgen receptors, the receptor for testosterone, in the muscle.”

A couple of interesting side effects of increasing muscle mass…   With the increasing size of the muscle cell (aka, muscle fiber), the mitochondria to cell-size ratio is reduced.  In the same way, the capillary density in a muscle decreases as a muscle grows larger (although there will be a small increase in the number of capillaries).  Both of these phenomena (lower mitochondrial and capillary densities) might lead one to suspect that a muscle-bound man’s ability to do aerobic exercise would be reduced… however, our authors assure us that this is not necessarily the case (although I got the impression no one’s exactly sure why).

Something to remember before we leave off today’s post… only the parts of the muscle worked will get stronger.  If you only repeat a few certain exercises, then only the specific sets of muscle fibers stressed by those exercises will be pressured to augment force.  For you to achieve maximal strength gains, you should do a variety of exercises which not only use different muscles, but also have nice big ranges of motion so that more of the muscle is used.  Furthermore, keep in mind what the authors told us about muscle fiber recruitment– the muscle only recruits more fibers if it needs them… therefore, you must keep pushing yourself beyond the comfort zone in which the currently activated fibers can handle the load.  To restate what was said above, “the more muscle fibers recruited for the performance of the exercise, the greater the extent of potential remodeling in the whole muscle.”

As you can see from the above paragraphs on building muscle, getting stronger requires a full court press from the body– everything from proper nutrition to healthy immune system response… or, as I like to put it more crudely (remembering where most of a man’s testosterone comes from)… weightlifting requires a healthy body “from nuts to neurons.”

 

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