How muscles grow

How muscles grow

In this lesson, you will learn not only how muscles grow and how long it takes them to start growing, but also how, before muscles start to grow, your nervous system adapts to get the most out of the muscles that you already have.

What is neuromuscular adaptation?

As explained in How muscles work, your muscles are connected to your nervous system. Muscle fibers are activated by motor neurons, which may activate between one and several hundred muscle fibers. The motor neuron and its associated muscle fibers are called a motor unit (Figure 1). The greater the number of motor units involved in a lift, the greater the force of contraction.

Motor unit
Figure 1. Motor unit.

When you first start weight training, the nerve pathways that serve your motor units undergo changes to become more efficient, thus improving your ability to recruit muscle fibers. This is known as neuromuscular adaptation, and is responsible for most of the gains in strength that you experience during the first six to eight weeks of training. The gains in strength occur within just days and are due solely to this process of your body “learning” to use your existing muscles more efficiently. Only then does your body invest in building extra muscle.

What is hypertrophy?

As long as you stick to an effective training program, your muscles will start to grow after the initial neuromuscular adaptation phase (six to eight weeks). The scientific name for the increase in size of your muscles is hypertrophy. Hypertrophy occurs due to an increase in the diameter of your muscle fibers; it does not occur due to an increase in the number of your muscle fibers.

Note that Type IIa and Type IIb muscle fibers increase in size more readily and at a faster rate than Type I fibers. Therefore, the increase in muscle mass and strength is mainly due to the increase in size of Type II fibers, which, as explained in How muscles work, are recruited only when you lift heavy weights.

How does hypertrophy occur?

Muscle fibers are composed of long, cylindrical strands called myofibrils. Myofibrils are in turn composed of filaments of the contractile proteins actin and myosin, which repeat in units along myofibrils called sarcomeres.

When you train, the contractile protein filaments sustain microtears. Damage also occurs to connective tissues. It’s this microdamage that causes post-workout pain and soreness.

While you rest, the connective tissues and contractile filaments are repaired with new proteins. Extra filaments are added to prevent future damage, which increases the diameter of the muscle fibers and therefore increases the diameter and strength of the muscle. The increase in muscle fiber diameter is accompanied by an increase in other cellular properties, such as sarcoplasm.

Sarcoplasm is the cytoplasm of the muscle fiber—that is, the gel-like substance within the muscle fiber, along with its organelles. It differs from the cytoplasm of other types of body cells by containing unusually large amounts of myoglobin and glycogen granules (glycosomes). Myoglobin is a protein that carries oxygen, and glycosomes are the muscle fiber’s primary source of energy. The increase in the amount of sarcoplasm therefore makes sense: larger muscle fibers need more oxygen and energy.

Sarcoplasmic hypertrophy vs myofibrillar hypertrophy

Many authorities divide hypertrophy into two types:

  1. Sarcoplasmic
  2. Myofibrillar

Myofibrillar hypertrophy refers specifically to an increase in the number and diameter of myofibrils, which occurs due to an increase in the size and number of contractile proteins. Sarcoplasmic hypertrophy, on the other hand, refers specifically to an increase in the volume of sarcoplasm.

Due to the increase in size and number of contractile proteins, myofibrillar hypertrophy is accompanied by an increase in strength and a small increase in muscular size, whereas sarcoplasmic hypertrophy is accompanied by an increase in size and a small increase in strength.

These two types of hypertrophy do not occur completely independently of one another: depending on how you train, you can experience either a large increase in sarcoplasm with a slight increase in contractile proteins, a large increase in contractile proteins with a small increase in sarcoplasm, or a relatively balanced increase in both. Sarcoplasmic hypertrophy is more dominant in the muscles of bodybuilders, who train for muscle size, whereas myofibrillar hypertrophy is more dominant in the muscles of powerlifters and Olympic weightlifters, who train for strength.

Different types of training affect how muscles grow. You will learn how to train for each type of hypertrophy and thus either develop muscular size, muscular strength, or both in How many sets and reps should you do?

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