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Muscle Growth Part II: Why, And How, Does A Muscle Grow And Get Stronger?

by Casey Butt, Ph.D.

Rational and Irrational Hypertrophy

In Part I of this series it was explained that sarcoplasmic hypertrophy produces only moderate increases in muscle size. It was also mentioned that there are were other important reasons why such adaptations are desirable. In this section we'll take a look at those reasons.

Metabolic processes within the cell require ATP to "fuel" them (ATP is the body's primary fuel source for all of its energy). If enough ATP isn't present then a host of cellular processes slow down (including protein synthesis), resulting in the operations of the cell being compromised. This means, among other things, slower removal of waste products, slower recovery from training and slower or less protein synthesis. Research done in the former Soviet Union by Zalessky and Burkhanov has shown that if the contractile components of the cell continue to grow (sarcomere hypertrophy) without a concurrent increase in the energy supplying systems of the cell (i.e. mitochondria, etc. - sarcoplasmic hypertrophy) then such a situation will develop. Essentially, the contractile machinery of the cell has grown too large for the energy systems to support it. In addition, fellow Soviet researchers, Nikituk and Samoilov have demonstrated that such a condition can be brought about through poorly planned resistance training.

Once such a situation is created, the full potential strength of the muscle cannot be exerted because the cell cannot produce and utilize enough momentary ATP to cycle actin-myosin cross-bridges sufficiently. Likewise, when hypertrophy and strengthening is stimulated, growth cannot be supported because the cell lacks the energy systems necessary to support the synthesis and maintenance of new proteins (muscle protein is constantly being broken down and rebuilt - a process of 'maintenance'). In Bodybuilder's terms, you hit a plateau. Because such a condition is unproductive from an adaptative standpoint, it is called irrational hypertrophy. The defining characteristic of this kind of growth is cells that contain significantly larger mitochondria than in the untrained state, but fewer of them per myofibril. The net result is an ATP shortage in the cell.

On the other hand, if training results in proportionate vascular improvements within the cell (mitochondrial density increases - the total number of mitochondria also increases as the existing mitochondria get bigger), such a plateau will not be encountered and training-invoked hypertrophy can proceed. This is called rational hypertrophy, for obvious reasons.

As this article isn't intended to get into the details of training procedures, I'm going to leave this subject by saying that for continued progress with regard to increased muscle mass and/or strength-endurance, sarcoplasmic hypertrophy is, indeed, necessary and must be trained for. How to achieve rational hypertrophy, while avoiding the irrational kind, will be dealt with in other articles on this website.

"But Why Aren't Olympic Lifters Bigger Than Bodybuilders?"

It wouldn't be right not to address the fact, though, that training with weights ~90% of your 1RM and above seems to favor the development of strength more so than muscular size. But, in light of the information presented in Part I of this series, how is that possible? It is theorized that when using loads of ~90% of 1RM and above muscular failure may occur because of signaling problems at the neuromuscular junction, and that this occurs before a significant growth stimulus has been delivered to the cells. In addition, the total time that the muscle fibers are required to produce force is shorter in low-rep sets than in higher-rep sets and this may result in exhaustion of fewer muscle fibers and a lesser growth stimulus. Simply put, a hard set of 8 reps may deliver more growth stimulus to the muscle cells than a hard set of 3 reps because in a 3-rep set (or any low number of reps) failure may occur before a significant growth stimulus has been achieved.

Additionally, when higher reps are performed substrates such as phosphate and hydrogen ions build up in the muscles - some researchers theorize that the presence of these substrates may further facilitate the muscle growth process (though this has not been confirmed). It is also widely believed that lifting heavy weights (~90% of 1RM and above) effectively stimulates the nervous system to 'improve' its recruitment pattern, frequency and efficiency to produce limit strength, making you stronger without actually increasing muscle size.

These reasons are why bodybuilders, as a group, have bigger muscles than Olympic lifters - they typically train with longer-duration, higher-rep sets ...which is an effective method of producing hypertrophy. Olympic Lifters, on the other hand, typically train with short-duration, low-rep sets ...which is an effective method of producing strength gains due to neural adaptations, but produces little in the way of hypertrophy. Accordingly, Olympic lifters, as a group, are much stonger than bodybuilders, but not as heavily muscled.

It also needs to be pointed out that any type of repetitive weight training (regardless of rep range) will result in the type IIB fibers having endurance-type adaptations. This occurs most quickly and profoundly at lighter loads (8-15 rep maximums) because, with these loads, the type IIBs do not twitch with maximum frequency and, therefore, adapt to twitch at lower frequencies for longer periods. This adaptation improves the IIB fibers ability to produce tension for longer periods of time, thus allowing them to be trained in a fashion that produces substantial muscular damage and greater growth stimulation. This gives the Bodybuilder's muscle more potential for growth.

Training in the 8-15 rep range (roughly speaking) also constitutes endurance training for IIB fibers, causing them to adapt so that they have better endurance characteristics (i.e. higher mitochondrial densities and greater abilities to sustain enzyme concentrations). In other words, sarcoplasmic hypertrophy. This increases the IIB fibers' energy production capabilities, allowing for further stimulation of sarcomeric hypertrophy and the development and maintenance of muscle proteins.

Don't do as others have, and use these observations to argue that bigger muscles are not stronger muscles. As was eluded to above, muscles adapt very specifically to specific tasks. If you train using three rep sets then they get good at doing three rep sets. If you train using 8 rep sets then they get good at doing 8 rep sets. Moderate-reps sets, however (such as 8-12 reps), stimulate more muscle growth than low-rep sets (assuming of course, you are training with sufficient intensity). Make no mistake about it though, your legs will be bigger when you're squatting 405 for 8 than they were when you were squatting 275 for 8. For the case of 3 rep sets, you may not be much bigger when you're cleaning 315 for 3 than you were when you were cleaning 185 for 3, but you will have a much more efficient nervous system for the task.

Take Home Lessons!

If you want to grow bigger muscles you must train your muscles against a resistance great enough to stimulate hypertrophy, but not so great that you cannot continue the set long enough to stimulate growth. Practically, that means you must select weights that allow you to complete 6-12 reps (smaller muscle groups may respond better to even higher reps). Most people will use between 70% and 85% of their 1-rep maximums to achieve this. Training in this range produces micro-trauma to the muscle fibers that results in muscle growth (if proper rest and nutrition is supplied).

If your primary concern is increasing limit strength, then you should train with weights over 85% of your 1-rep maximum, and the sets (by necessity) will be of 5 or less reps. If you want to avoid hypertrophy as much as possible, while increasing strength as much as possible, then sets of 1-3 reps using weights of over 90% of 1-rep max. are indicated. Training in this range produces little micro-trauma, thus stimulating little growth, but results in nervous system firing pattern refinements that increase limit strength.

In all cases, if you want to get stronger OR bigger, you MUST train for strength. Getting stronger in the rep range that you're using is the most fundamental sign of progress - it is the rep range that determines whether the training effect will be strength or muscle mass increases. If you are not getting stronger in your training rep range, then your training is not working. This fact cannot be ignored, it cannot be argued around, and it cannot be refuted - it is as fundamental, and as simple, as that.

Muscle Growth Part I: Why, and How, Does A Muscle Grow And Get Stronger?

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