Muscle Types & Muscle Actions

The human body has around 650 muscles which divide into the following three different muscle types:

• Skeletal muscle gives your body its shape, creates movement, generates heat, and is generally under voluntary control.
• Smooth muscle is in the walls of hollow organs such as intestines, stomach, bladder, and eye.
• Cardiac muscle exists only in the heart.

The information contained here focuses on skeletal muscle. Therefore you can assume that all uses of the word muscle in this resource refer to skeletal muscle.

MUSCLES CAN ONLY PULL

In terms of muscle actions, skeletal muscles can only pull (contract) in one direction and control a release back in the opposite direction. They can’t push back. For this reason, they are grouped in pairs so that an opposing muscle can pull back in the opposite direction when the direction of load changes.

A simplified example of this is bending your arm at the elbow. When the biceps muscle contracts (agonist) and the triceps muscle relaxes (antagonist), it flexes. When the direction of force reverses, the triceps muscle contracts (agonist) and the biceps muscle relaxes (antagonist). This extension straightens your arm.

The more complicated reality is that when you bend your elbow, there are many more muscle interactions going on. This complexity is where the interactions of the nervous system come into play.

THE DIFFERENT ROLES A MUSCLE CAN PLAY

There are several different roles that muscles can perform during movement; agonist, antagonist, synergist, fixator, and they can alternate or crossover between these different roles.

For general discussion, muscles fall into two categories, prime movers and stabilizers. Prime movers are typically the larger muscles that create movement such as quads, hamstrings, biceps, and triceps, etc. Stabilizers provide balance and support for your body.

THE PROBLEM WITH MOST PROGRAMS

Exercise programs tend to either overlook the stabilizer muscles because they have little bearing on the appearance of your body. Or they can focus on them to the point of overkill. Given the functional importance of stabilizer muscles, they warrant some attention, but not to the detriment of the rest of your body. It is probably of a higher priority to focus on any muscular imbalances and deficiencies to obtain favorable body composition with muscle balance.

MOVEMENT FOCUS

Over time I have found myself giving more priority to movements over muscle groups. Get your movement patterns right, and you will be working all of the muscles your body was built to use. I generally only focus on a particular muscle group to address a weakness. Exercises that require balance are challenging for both stabilizer and prime mover muscles. Good examples are squats, lunges, single leg deadlifts, step-ups, and plyometrics.

Proper technique, relaxed posture, and adequate rest are essential for the stabilizers not to be overworked.