What's the difference between fast & slow twitch muscles? Does it matter for muscle building?

As you may have heard, fast-twitch fibers do have the ability to grow larger (and more quickly) than slow-twitch. 

Our relative proportion of fast vs slow twitch muscle fibers is partly due to our genetics. Some people inherit more fast-twitch muscle fibers from their parents - making them better sprinters, jumpers, etc. Some people inherit more slow-twitch muscle fibers - making them better endurance athletes.

Thankfully, your genetically determined fiber make-up does not determine your muscle building fate. Even if you have a predisposition for more slow-twitch fibers, you can still make great muscle building progress. In fact, a research study took muscle biopsies from professional bodybuilders and found that they had a relatively even split between fast-twitch and slow-twitch muscle fibers.

Slow-twitch muscles can still hypertrophy and grow. They simply don't do so as quickly and easily as fast-twitch muscle fibers.

Many guys don't know that there is a class of intermediate fibers (called FOGs) - that are not strictly fast or slow twitch; these intermediate fibers change and adapt to the type of training we subject them to. The more we lift weights, the more those intermediate fibers act like fast-twitch fibers leading to more noticeable muscle growth. The more we do endurance cardio, the more these intermediate fibers act like slow-twitch muscles.

Some muscles naturally are more slow-twitch than others (like your abdominals and parts of your calf musculature). Their slow-twitch nature is by design, as these muscles need to work constantly, all-day-long, to help stabilize our spines and help us walk.

When we know a muscle is mostly slow-twitch (the soleus in the calf & ab muscles being primary examples), we can intentionally work them in a higher rep range consistent with their fiber make-up.

This  does not mean we use light-weight for endless reps for muscles that are more "slow twitch." It does mean that we may do sets of 20-30 reps with as heavy of a weight as we can handle.

To wrap up the discussion returning to fast-twitch fibers, these fibers respond to the  speed of muscular contraction. That's why it's a very good idea to execute the concentric (shortening) portion of your reps explosively to engage those fast-twitch fibers.

We built these proper rep ranges into the Old School Muscle Program to ensure you're training all fiber types properly.

-Dr. Anthony Balduzzi
Founder, The Fit Father Project
Creator, Old School Muscle & FF30X