What’s the difference between training for strength, and training for size?

Slow (Type I) And Fast (Type II) Twitch Muscle Fibers

Let’s discuss the differences between slow and fast twitch muscles. These are two separate types of muscle fibers in the body, each matched to motor neurons in your nervous system. When you perform exercises that require a great deal of strength, fast-twitching Type II muscle fibers are called into action by your nervous system to carry the brunt of the force.

These are mainly glycolytic, meaning they suck up glucose from the bloodstream and rely on converting it to energy more than they rely on oxygen. This is why these muscle fibers are drawn white, rather than red. Their slow-twitching Type I counterparts are red, because they enjoy a much larger amount of blood. Although these react differently (slower) to stress, they’re capable of withstanding force for a much longer period of time, and they’re primarily relied on by your nervous system when you’re doing things that require endurance rather than strength – running, swimming, etc.

To rephrase, when you’re lifting weights your body considers heavy – heavy enough that more than a dozen repetitions is practically impossible – your nervous system will immediately call your Type IIa muscle fibers into action. When the going gets even tougher, they call in the Type IIb muscles – and finally, when the threshold of your fibers are exhausted, you go into muscle failure. If you can, however, lift past a certain number of repetitions, then you’re calling Type I muscle fibers into action and are no longer relying mostly on fast twitch muscles. For strength training and bodybuilding, this is key to why reps need to be kept low.

Training For Strength, And Training For Size.

When forced to work at a weight/resistance range that limits you to 2-6 repetitions, your body induces mostly myofibrillated hypertrophy in your fast twitch muscles (which, unlike slow twitch muscles, have to grow in size to keep up with demand whereas slow twitch muscles receive a constant supply of oxygen and are thus less likely to undergo growth) – this causes an increase in strength, which is what most powerlifters and sprinters look for. With enough training, it allows them to modify their nervous system’s reaction in a way that they can fire off all their motor neurons connected to their fast twitch muscle fibers at once, which gives them freakish strength.

But for growth, the golden zone is between 6-12 repetitions. Although the nervous system begins to call in some slow twitch fibers at this point, fast twitch fibers still dominate, and mostly sarcoplasmic hypertrophy ensues in which fast twitch muscle fibers grow to absorb more glucose rather than myofibrillated hypertrophy, which is simply the enlargement of muscle fibers to increase strength. Sarcoplasmic hypertrophy creates larger muscles, instead of focusing simply on strength, and the science behind creating efficient training and nutrition methods to maximize sarcoplasmic hypertrophy has become a multi-million dollar industry. 


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