What makes muscle grow?

Bodybuilding & Muscle Building Techniques



(Examining the Muscle Damage Theory and the Substrate Accumulation Theory)

You know, it’s amazing when you think about it, that we can send a space-probe to Mars, yet we still don’t know why muscles grow. You might say, “Wait a minute! I know what makes muscles grow, weight training! Right?” Well that’s right but we still don’t know why weight training stimulates muscle growth. We haven’t identified the signals or the substances that turn on muscle growth. It’s not like we haven’t made progress though. In fact there are two well researched theories that attempt to explain the process of muscle hypertrophy. The first is called the Muscle Damage Theory. This theory states that muscles are damaged during exercise and this damage turns on several signaling cascades that result in muscle hypertrophy. The second theory is called the Substrate Accumulation Theory. The basis of this theory is that several substances accumulate during exercise that may turn on muscle hypertrophy directly or indirectly by stimulating the release of anabolic hormones.
We will examine both theories and give examples of some commonly used training techniques that take advantage of one or both theories.

The Muscle Damage Theory
It’s pretty obvious that muscles can be damaged by exercise. Think about the last time you really thrashed your legs with a good squat workout. They were probably sore for days right? Years ago we thought that muscle soreness was due to lactic acid. We now know that this is not true. In fact the lactic acid that is formed during exercise is rapidly removed and metabolized by your liver, your heart and by slow twitch muscles. The soreness is actually due to muscle damage. When we examine muscles under an electron microscope following heavy resistance exercise we see that the normally orderly arrangement of the myofibers is completely disrupted (1). This damage is referred to as “micro injury” and typically occurs in a small percentage of fibers in a muscle. This muscle damage is common following exercises that have a strong force component (like weight training) and especially following eccentric contractions (2)
Muscle damage proceeds in two phases, the autogenic phase and the phagocytic phase. The autogenic phase begins immediately following the exercise. Mechanical tension during contractions (especially lengthening contractions) opens stretch activated Calcium channels. Calcium then floods the interior of the muscle cell. The increased Calcium concentration activates lysosomes (lysosomes are specialized structures that can digest proteins). These lysosomes may then begin destroying structural components of the muscle. Calcium also activates Phospholipase A2. Phospholipase A2 then begins to punch holes in the muscle cell membrane by converting phospholipids to arachidonic acid. There is also an increase in free radicals. In other words, entry of Calcium (and the subsequent inability to remove the calcium) into the muscle cell initiates a self destruct mechanism in muscle. Approximately 3-4 hours following the damage phagocytes begin to invade the area to clear away the remaining debris. These phagocytes can then release heparin and other inflammatory chemicals and induce swelling (and as a result more soreness). This is the phagocytic phase of muscle damage. In fact the phenomena referred to as the “second day lag” (the increase in soreness two days post-workout) is due to greater phagocyte activity 48 hours post workout leading to more swelling.
“How does this relate to muscle growth? You ask. Good question. When the cell membrane is disrupted (has holes punched in it) several substances that are normally inside of the cell can leak out. Among these are growth factors and prostaglandins. These growth factors may affect protein synthesis in neighboring muscle cells and may induce the migration, differentiation, and proliferation of satellite cells. A satellite cell is an undifferentiated muscle cell that is basically just a nucleus. Remember that the nucleus of a cell is like a protein synthesis factory. Growth factors can cause the satellite cell to fuse with a damaged muscle cell and beef up its protein synthesis ability. Alternatively, satellite cells can fuse together and create a new muscle cell. This fusion of satellite cells to create a new muscle cell is referred to as hyperplasia and has been demonstrated in an animal model. In summary, the muscle cell is damaged by exercise, growth factors are released, satellite cells fuse with existing muscle fibers and increase protein synthesis capabilities or fuse with one another and create new muscle cells.
Although the Muscle Damage Theory is well supported it is easy to poke holes in this theory. For example, if all that is necessary to produce muscle growth is muscle damage than you should be able to get huge arms by having your partner punch you repeatedly in the arm. Also, if muscle damage is “the” signal for muscle growth than any treatment that reduces exercise induced muscle damage should reduce muscle hypertrophy. Recently it has been shown that supplementation with HMB reduces exercise induced muscle damage but increases muscle hypertrophy. This directly contradicts the Muscle Damage Theory. Before we throw out the theory altogether however, we must concede that muscle damage does induce the release of growth factors and the activation of satellite cells. These processes must play a role in hypertrophy (and possibly hyperplasia) and the recovery of a muscle after exercise. It is unlikely however, that muscle damage alone is the only factor necessary for hypertrophy. There must be other factors involved.

Substrate Accumulation Theory
Perhaps a closer look at the substrate accumulation theory will clarify some of these other factors. Heavy exercise (like weight training) consumes a tremendous amount of energy. The body responds by revving up the metabolic processes that produce energy for contracting muscles. This increase in metabolic activity inevitably leads to the accumulation of several of the by products of metabolism. Most notable among these is lactic acid. Lactic acid production has recently been shown to stimulate the production of testosterone and possibly plays a role in growth hormone release as well. Although exercise may not increase resting testosterone levels, it is possible that there are significant effects from exercise induced testosterone (Lamb, Androgens and exercise) increases. In addition, intense muscular effort stimulates the central nervous system and induces a release of the adrenaline hormones, epinephrine and norepinephrine. These hormones are Beta-agonists. Beta-agonists can be potent anabolic agents. The substrate accumulation theory proposes that intense muscular effort leads to the accumulation of metabolic by-products which lead to the release of various hormones. These hormones set up the perfect anabolic cocktail to stimulate muscle hypertrophy.
This theory alone probably doesn’t explain muscle hypertrophy either. You can take loads of anabolic drugs but without the stimulus of exercise you will see very little hypertrophy. The bottom line is that you probably need both some degree of muscle damage and an anabolic hormone response in order to achieve any significant degree of muscle hypertrophy. Luckily for us our bodies can do this automatically (without drugs) in response to exercise. In fact many of the popular training techniques take advantage of either the muscle damage theory or the substrate accumulation theory or both to maximize the stimulus for muscle hypertrophy.
Eccentric Contractions (Muscle Damage Theory) – An eccentric contraction involves the lengthening of a muscle against a resistance. An example would be slowly lowering the weight during a bench press or slowly lowering the weight during a curl. Because of the way muscle filaments (actin ad myosin) interact, an eccentric (lengthening) contraction is much more damaging than a concentric (lengthening) contraction. Let me illustrate with an analogy. Suppose you park a truck halfway up a steep hill and then load it with heavy bricks. You hop in and put it in drive and hit the gas. However, because the load is so heavy, the truck rolls backwards. You could imagine that this would be very damaging to the engine and transmission because these systems are designed to go forward when the transmission is in drive. The same is true for your muscles. They are designed to produce force while shortening, not lengthening.
Plyometrics (Muscle Damage Theory) – Plyometrics involves rapidly contracting your muscles following a rapid stretch. Theoretically you can recruit more muscle fibers immediately following a stretch. This occurs because of the stretch reflex. This reflex causes a muscle to contract when it stretches too rapidly. If we add a voluntary contraction to this reflex what we get is increased force production (more force than could be produced by a voluntary contraction alone).
Rest-Pause (Muscle Damage Theory and Substrate Accumulation Theory) – Rest-pause was developed by Mike Mentzer as way to do a set of 8-10 repetitions with a weight that is very close to your one rep maximum. You perform one rep with very near your one rep maximum. Then you rest 15 seconds or so and complete another rep. You continue in this fashion until you have completed 8-10 repetitions. This is really an ingenious way to up the force production during a set. Normally you would only be able to perform 1-2 reps with the weight (not an optimal number of reps for size and strength), however using rest-pause you would perform 8-10 reps with a much heavier weight than a normal 8-10 rep set.
Compensatory Acceleration (Muscle Damage Theory) – Compensatory Acceleration involves accelerating the weight as fast as possible from the beginning of the movement to the end (exploding the bar off of your chest during a bench press). This was a favorite technique of Franco Columbu (for you younger folks out there he was Arnold’s training partner). It requires more force to accelerate a weight with great speed than to lift it slowly.
Drop Sets (Substrate Accumulation) – Drop sets are performed by simply “dropping” to a lighter weight following the completion of a set and immediately proceeding with another set. You may do several “drops” or a single “drop”.
Supersets (Substrate Accumulation) – Supersets stack several exercises for the same bodypart which are done in rapid succession. It is a particularly grueling way to train and can really shorten the duration of your workout since you can complete several sets in a short period of time.

(1) Armstrong, RB, Ogilvie, RW, and Schwane, JA, Eccentric exercise-induced injury to rat skeletal muscle. J. Appl. Physiol. 54: 80-93, 1983.

(2) Armstrong, RB, Mechanisms of exercise induced delayed onset muscular soreness: a brief review. Med. Sci. Sports Exerc. 16:529-538, 1984.

posted by Frank Mori # What makes muscle grow?


2 thoughts on “What makes muscle grow?”

  1. Thats what I am talking about.

    What makes muscles grow? Obviously exercise, but what kind of exercise and for what kind of muscle? What is optimum for my needs?

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