Muscle Overview

The role that our muscles play in our health and our lives cannot be overstated. Your body has 650 muscles. About 45 percent of your body is made up of pure muscle. While a lot of that muscle is just water, there’s no denying that your muscles are a significant part of you and your body.

There are 3 different types of muscle:

Cardiac Muscle

You probably recognize the word cardiac as relating to the heart, but technically speaking, "cardiac" muscle exists outside the heart. It's any type of muscle that is made up of fibers that are attached to each other and can send messages across the fibers.

Smooth Muscles

Smooth muscles surround your blood vessels and organs.

Skeletal Muscles

In the simplest terms, these are the muscle you came here to learn about. Skeletal muscles are the ones you flex in the mirror. Sometimes referred to as "voluntary" muscles because, unlike cardiac or smooth muscles, you can move them at will, skeletal muscles are made up of fibers that look something like steel cables.

For the purposes of this site, we will focus exclusively on skeletal muscles, because those are the muscles you can make larger and more toned through exercise. And that's really why you're here, right? Slow Twitch vs. Fast Twitch Fibers But before we can talk about how to grow and firm your muscles, we need a brief explanation of how they work, and that begins with the fibers that make up your muscles.

Slow Twitch vs. Fast Twitch Fibers

But before we can talk about how to grow and firm your muscles, we need a brief explanation of how they work, and that begins with the fibers that make up your muscles.

• Slow Twitch Fibers are used for endurance activities - think marathon running versus sprinting.
• Fast Twitch Fibers, are activated when you are doing any activity that requires more than 25 percent of the maximum strength for that muscle - like power lifting, bench presses or leg squats.

To illustrate the roles that slow twitch and fast twitch fibers play in muscle building, consider your calves. Every wonder why so many guys have trouble building big calves? It's because the calves primary role in your body is to keep you from falling over all day long. That requires a ton of endurance, which means lots of slow twitch fibers, and muscles made up of slow twitch fibers don't grow as big.

Once you understand which muscles have the most potential for growth - those muscles that are made up mostly of Fast Twitch Fibers - you can target those muscles to get bigger and stronger.