The Big ol’ Muscle Training Primer, Part 1

Howdy all!

Welcome to our series called “The Big ol’ Muscle Training Primer”. Today in Part 1 we’ll be spending time talking about how your muscles work, what factors influence your ability to develop strength/endurance, and a little bit about genetics too. Future installments will go into more detail on training adaptations and specific ways to work towards your goals.

Here we go!


Your muscles are pretty awesome, as you know if you’ve ever wanted/needed to move, breathe, live, etc. We’ve covered the different muscle types in previous articles, but here’s a quick refresher:

Smooth muscle surrounds your blood vessels and internal organs in thin layers. The tissue resembles slightly wrinkled bed sheets. Smooth muscle is autonomic, meaning you don’t have to actively think about contracting them; your nervous system does it for you.

Cardiac muscle is fibrous tissue, resembling red cables that are connected to one another. Cardiac muscle is found only in the heart. This allows them to send signals to one another, so the heart can beat in the most efficient way possible.

Skeletal muscle is the topic of this article. These are the muscles which allow you to move, that look good in the mirror when you workout, and they are the ones you can move voluntarily. All skeletal muscles, in their natural state, maintain a slight tonality to them (i.e. they are always a little contracted and ready to go). Inside your body they resemble red bundles of separate fibers which work together but are not connected to one another. We’ll break down skeletal muscle in the next section.

The thing that all muscles have in common is that they are about 80% water. Most of the remaining muscle is protein, with some carbohydrate, fat, and salt present to help the muscle function. Muscle makes up about 40-45% of a males body on average, and 22-27% of a woman’s body on average. In comparison, bone usually makes up 10-12% of a person’s body.

Slow and Fast Twitch Fibers

Your skeletal muscles are a little complicated because their function is a little complicated. To understand we’ll be talking about a concepts.

All or Nothing

The operation of the tissue is simple: either it’s contracted or it’s not. When you move, either a given muscle fiber is activated, providing maximal available effort, or it’s not involved at all. This means that if you do a Barbell Curl with 5lbs, you’re using fewer muscle fibers firing at max capacity, whereas with 50lbs you’re using more muscle fibers firing at max capacity.

Size Matters

When you move, your body automatically activates the smallest muscles it has at its disposal that can complete the movement. Usually, these are the Type I (Slow Twitch) fibers. Once your muscular needs exceeds about 25% of your capacity, the body activates your Type IIa (Fast Twitch A) fibers. If that’s still not enough, and at around 40% of your capacity, the body then activates the Type IIx (Fast Twitch X) fibers. This happens faster than you can think about it, but it’s an important principle to understand as we go into detail about the fiber types themselves.

Muscle Fiber Types

We all have three major muscle types: Type I, Type IIa, and Type IIx. Each fiber type has different aspects that make it better or worse at certain types of movement.

Type I – also called “Slow Twitch” fibers – are used for endurance activities and tasks that don’t require maximal effort. An example would be sustaining a jog/run over 10 minutes or more. These are usually the smallest  fibers, usually activated first in movement, and usually remain relatively small throughout a person’s lifetime.

  • Strength: Can most easily maintain constant effort over longer time periods
  • Weakness: Not able to contract as forcefully as Type II fibers (i.e. they can’t move as much weight)

Type IIx – also called “Fast Twitch X” fibers – are used for maximal effort movements that require little to no sustained endurance. An example would be a heavy, one repetition maximum Deadlift or Bench Press. These muscle fibers tend to be the largest, and have the greatest capacity to grow larger and improve physical aesthetics.

  • Strength: The most forceful muscle fiber, able to contract the hardest overall
  • Weakness: No endurance capacity past the initial few seconds of contraction

Type IIa – also called “Fast Twitch A” fibers – are used for near maximal effort movements that also require a bit of short term endurance. These are an interesting blend between Type I and Type IIa, and would be brought into play with things like five repetition maximums on heavy lifting, or sprinting for longer distances. In the most well trained athletes these fibers can sustain near maximal capacities for up to 3 continuous minutes.

  • Strength: Much stronger than Type I, better endurance than Type IIx
  • Weakness: Not quite as strong as Type IIa, not as much endurance as Type I

An important note is that the terms “slow” versus “fast” don’t apply to the speed of the movement being completed, but rather to the strength and endurance of the fiber itself. The easiest way to learn about muscle fibers is by splitting them into types as we’ve done, but really the capabilities of your muscles are a continuum of factors. A muscle fiber which is “slower” will be slow to fatigue because it draws its overall strength out over time, while a fiber that is “faster” will tire more quickly because it uses its full capacity very quickly.

The Genetics Factor

We should preface the training section with a talk about genetics, because in reality the way you’re born plays a huge role in how your muscles develop. Your genes determine three important factors when it comes to muscular development:

1. Fiber Count

When you’re born, you’ve already got all the muscle fibers that you’re ever going to have. What changes over time is the length, size, and strength of those muscle fibers. The natural process of aging accounts for most of a persons’ regular muscle growth, but training plays a big part once you hit puberty. It’s generally believed that you cannot create more muscle fibers in your body, but failing to exercise them often enough can cause them to atrophy (get weaker and possibly die).

2. Fiber Distribution

As far as we know, the Type I fibers cannot ever be transformed into Type II fibers and the reverse is also true. This means that someone with a predisposition to endurance activities will have more Type I fibers, such as marathon runner Joan Benoit Samuelson who is reported to have about 80% Type I muscle fibers. In contrast, more Type II fibers predisposes you to being better at strength/sprint activities, like Olympic sprinter Carl Lewis who’s thought to have about 70% Type II fibers.

Even when someone is more dominant with one type or the other, certain muscles in all people are predisposed to one fiber type or the other. For instance, your postural muscles are responsible for keeping constant, long term contraction to keep you from falling over. Even in sprinters, these muscles tend to be more Type I fiber. In contrast, non-postural muscles like the biceps or triceps which are used more often for high intensity activities will tend to be higher proportions of Type II fibers.

Conversion between Type II Fibers

While the body cannot seem to transform fibers between Type I and Type II, it can make adaptations between Type II muscle fibers. With training, your Type IIx fibers can be refactored into Type IIa fibers, to provide more strength for heavier movements. This transformation can happen in as little as 4 weeks by lifting heavy weights twice per week.

3. Developed Shape

The chances are good that if your parents look like Greek Demigods of the Gym then you’re probably predisposed to looking like one too. Unfortunately, the opposite is also true. Concentrated weight training and bodybuilding can make a huge difference in how your developed muscles eventually look, but remember that your appearance is genetics first, barbell second.


And that’s it for today!

In Part 2 we’ll go into more details about training adaptations and a little bit of good ol’ physiology.

Happy lifting!