Muscles are made up of long cells known as muscle fibers, innervated by a nerve cell (neuron) which stimulates the muscle to contract. Stimulating the right type of muscle fiber can impact your physique!

One nerve cell can stimulate many muscle fibers to contract at once, known as a motor unit. A small motor unit is a neuron which innervates just a few muscle fibers, while a large motor unit innervates many muscle fibers. Muscle fibers themselves differ as well in that some are more durable but less powerful (slow twitch) while others are more powerful but les durable (fast twitch).

There are 3 types of muscle fibers:

1. Slow twitch fibers (type 1): are red and used primarily for contractions requiring endurance. They are easy to stimulate and are activated first when doing light weight exercise or walking/jogging, stimulated by small motor units for precise movements but low power. Slow twitch muscle fibers are smaller and do not grow as much when stimulated.

2. Fast twitch fibers (type 2): are white and are only activated if enough force is required (f.ex explosive training or heavy weight training), stimulated by large motor units for high power but low precision. These fast twitch fibers are larger in size and grow more if stimulated.

Thus you should aim to stimulate them to gain muscle size. Different muscles do, however, have different amount of each fiber type.

Your calves, for, example, have very few fast twitch fibers and more slow twitch fibers. Therefore, doing high rep exercise for the calves will stimulate them to grow more than heavy low rep.

Fast twitch fibers can be stimulated in 3 ways:

1. Using heavy weight (80-85% of 1RM): depleting fast twitch fibers at failure.
2. Fast explosive movements with light weight: more injury risk
3. Working till failure: will activate fast twitch fibers, but only on the last reps.

This is why, in theory, it is good to train to failure every now and again. We recommend you go to failure on workouts around once every 4 weeks.

Sources:

1. Herbison GJ. Muscle fiber types. Arch Phys Med Rehabil. 1982 May;63(5):227-30.
2. Jacques D. Training adaptations in the behavior of human motor units. Journal of Applied Physiology Published 1 December 2006 Vol. 101 no. 6, 1766-1775.

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