How Muscle Is Built

How Muscle Is Built
How Muscle Is Built

This article provides a fairly simple overview of how muscle is built at the biological level. There are some technical terms in it as it’s hard to explain without them but I’ve tried to keep it as simple and brief as possible.

The Workout

The ATP-PC System

The energy system that powers your workouts is called the ATP-PC system.

A small amount of ATP (adenosine triphosphate) is stored in your muscle cells. This powers the first few reps you perform and is depleted after two or three seconds.

When the ATP in your muscle cells has depleted, PC (phosphocreatine) is then broken down to restock your ATP levels. This provdes energy for a further eight to 12 seconds or so.

When this second batch of ATP has been used up, your muscles turn to their reserves of glycogen to make additional ATP (and lactic acid as a byproduct). This ATP is supplied at a slower rate and lasts for a further 60-90 seconds.

Beyond this you’re getting into the aerobic cycle, which involves a whole bunch of chemical reactions to keep supplying ATP to the muscles at an even slower rate.

It takes two to three minutes to restore the ATP systems, so the takeaway from this bit is that two to three minutes’ rest between working sets is what you need in order to be at your best for the next set.


Hypertrophy is the process by which muscles grow. There are two sorts of hypertrophy: sarcoplasmic and myofibrillar.

There is a fair amount of overlap between the two different sorts of hypertrophy but working in the two to six rep range with weights at 80-90% of your one-rep maximum mainly invokes myofibrillar hypertrophy. This sort of hypertrophy is mainly associated with strength gains and invokes mainly Type IIb fast-twitch muscle fibers.

Working in the eight to 12 rep range mainly invokes sarcoplasmic hypertrophy, which is usually associated with gains in muscle size. This sort of hypertrophy is said to invoke mainly Type I slow-twitch muscle fibers and Type IIa fast-twitch muscle fibers.

The takeaway from this section is that two to six rep sets are better for strength, eight to 12 rep sets are better for muscle size and 13+ rep sets are better for endurance, but – I stress again – there is a lot of overlap.


Pushing your muscles hard during a workout causes them to undergo microtrauma, which is essentially damage to the muscle tissue. The body responds by repairing the damage and adding a little more muscle tissue to try and protect against future muscle damage. This is how your muscles grow.

This microtrauma is thought to be the cause of the delayed onset muscle soreness (DOMS) you feel after a hard workout.


The workout is where you break down the muscles; the recuperation period is where they’re rebuilt.

Protein Synthesis

Protein synthesis is the process of rebuilding the muscles. This takes place in the 24 to 72 hours after a workout and is part of the over all process of hypertrophy. The process is greatly assisted by eating a relatively high-protein diet, although what constitutes “high-protein” is hotly debated.

Some studies have shown that anything over 0.85g protein per lb of body weight is wasted, whereas anecdotal evidence from bodybuilders and powerlifters seems to suggest that anything from 1g to 2.2g of protein per lb of bodyweight is benefical.

I can only give a personal opinion here by saying that I try to get at least 1g of protein per lb of body weight. I have tried eating as much as 1.75g/lb before but haven’t noticed any appreciable difference in progress over 1g/lb.


Adequate rest is required to allow the muscles to rebuild. Sleep is an important part of the process but so is training frequency and one should try to allow at least 48 hours for a muscle group to repair itself after a workout.

Inadequate rest and/or insufficient protein will hinder the muscle building process.

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