Tonic and Phasic – Why you can’t treat your muscles the same.

The basics are the same.  Our nervous system sends a signal. Groups of muscle fibers respond and contract.  Muscle groups work in concert with each other to create motion in our skeleton.  The action of this muscle contraction is across the hinge of any variety of joint.  Using this joint as a fulcrum, the two lengths opposite the joint either increase their angle or decrease their angle.

Got all that?  Ok good.  When our muscles contract and result in an increased or “opening” of this angle we call it extension. When we decrease or “close” this angle we call it flexion.  There are plenty of other actions that can occur. Rotation, supination, pronation… but we are going to focus on extension and flexion.

Image result for flexion

With the concept of out basic motions in our minds, the next thing we are going to talk about is a concept of tonic and phasic muscles.

Tonic muscles are muscles are muscles that can act without or with only limited stimulus to work.  Sometimes this can be referred to as background muscles.  One example are the sphincter muscles in our digestive tract.  We don’t consciously think about controlling them, but they continue to work even without out imput.  Tonic muscles are muscles generally higher in slow twitch fibers, which means that they are fairly resistant to fatigue and are capable of doing a lot of work.

Phasic are muscles that only act when we tell them too. These muscles are higher in fast twitch muscle fibers. These are some of the more powerful movers in our body, but they generally fatigue faster than tonic muscles.

Image result for muscle groups

Most of the time when we refer to Tonic and phasic muscles we are talking about smooth muscle fiber that is used to manage our bodily functions and not specifically our large muscle groups.  The distinction in this case comes from the effects of loading these muscles. This is especially true in the case of faulty loading, either by over loading, unstable loading or excessive volume.  When any or all of these occur to a muscle group, the results are not always equal.

The major tonic muscles in our body include


Hip Flexors

Erector Spinae (Low Back)



The interesting this about these is that when it comes to skeletal muscles, the majority of our tonic muscles are also flexors. These muscles are responsible for creating flexion, commonly we see these movements in exercises such as Leg Curls, Back Extensions, Bench Press, Chest Flys, and Curls.  We described tonic muscles as muscles that can act in the background. At first it doesn’t sounds accurate because we can control these muscles at will.  The reasoning is because of what happens when we over work our hamstrings, do too many leg raises, or spend too much time doing preacher curls.  We have all done it. Monday rolls around and its bench press day, and we are feeling pumped so we go at it hard, maybe a little too hard.  At the end of the work out and even the next day our chest and shoulders are super tight. When our pectorals fatigue, they tighten.  They become harder to move and our range of motion is reduced.  These muscles actually become over stimulated after so much stress the muscles fibers become contracted without us actively controlling it.

Phasic muscles include:




Upper Back



Phasic muscles can create a lot of power, but when they become fatigued they will become less responsive to stimulus.  They lose their ability to produce power and can actually start to lengthen.   Generally, most of our phasic skeletal muscles are extensors; muscles that create extension or open the angle of our joints.

Any of these muscles when over worked can experience Delayed Onset Muscle Soreness, but the difference here is the reaction of either becoming unable to relax or becoming too relaxed.

The biggest issue comes from the fact that our tonic muscles work opposite our phasic muscles. When our tonic muscles are fatigued as well as our phasic muscles it can lead to combined concerns.  One example is doing Deadlifts.  Over the course of a heavy deadlift workout, the abdominals keep tension on our trunk and allow us to maintain a neutral spine. As they fatigue, our hips have trouble maintaining good position and our erector spinae (low back extensors) take the load.  After the workout, the back is tight and sore and the abs are too tired to resist them.  If we never do anything to care for this,he will continuously over-tightened tonic muscles and weak phasic muscles that lead to chronic issues such as bad posture or over stressed joints.  The longer we take to realize this and take steps this will bother us.

Loading is not the concern. Over loading, Excessive volume, or unstable loading can all cause issues but we can train smart to battle this. The other things that we work against is the ravages of time. As we age, these same muscles will tighten or weaken.  On one end we can do too much and do ourselves wrong, but we can do just as much damage by doing nothing.

This isn’t always what we want to do because many of our tonic muscles are out “show” muscles. We want to do more biceps and pecs because they look good. On the other end, our phasic muscles are our “go” muscles, these are our muscles that we use to perform; to move fast and be powerful. These muscles aren’t the ones that get the girls, but are the ones that help your Fran time.  But the reward is the ability to move better even as we age.

So what do we do? The honestly the answer is simple. We have to stretch and work on the mobility of our tonic muscles and strengthen our phasic muscles. The answer is smart and balanced training. Stretch what gets tight, strengthen what gets weak. And don’t stop doing it!  75 year old You will thank me.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s