What are Tonic and Phasic Muscles?

What are Tonic and Phasic Muscles?

This blog will teach you the difference between Tonic and Phasic Muscles so you can increase your FitPro knowledge, pass your anatomy exam and train clients with confidence.

You’ll discover:

  • Why do you need to know about the Tonic and Phasic Muscles?
  • Muscles Work in Pairs
  • What are Tonic and Phasic Muscles?
  • Which muscles are tonic?
  • Which muscles are phasic?
  • How sitting might affect posture and tight muscles?
  • Fixing Imbalances

Why do you need to know about the Tonic and Phasic Muscles?

As a Personal Trainer, you need to know the difference between Tonic and Phasic Muscles for your Level 3 anatomy exam.

In your exam, you will be tested on your knowledge of 50 muscles, and that includes the relationship of those muscles to postural compensations,

which is basically talking about Tonic and Phasic Muscles.

You need to know the difference between Tonic and Phasic Muscles and which muscles are most likely to be tonic or phasic, but also HOW they affect our client’s posture.

This blog will tell you all you need to know about Tonic and Phasic Muscles, as a FitPro, both for your exam and for when you are training clients.

Let’s start with the foundations of understanding muscles…

Muscles Work in Pairs

They work together in a cycle of contracting and relaxing to bring about movement at a joint.

This partnership is ideally like a harmonious marriage, with both parties playing give-and-take. The problem is one muscle has the propensity to be shorter and tighter, with the other having a propensity to be longer and less activated.

This is like having one person in the relationship that loves to talk (all the time), and the other just has no chance to get a word in edge-ways.

Now replace the idea of talking, with the idea of contractions in a muscle – it is much the same.

They can’t talk and listen at the same time, so when the muscle is contracted the other Must listen, which really means it relaxes and lengthens.

The muscle that loves to talk, is definitely the dominant one, it finds it easy to contract and “fire” to the point it holds continual tension across a joint, causing a visible shift in the skeletal alignment.

In opposition, the antagonist muscle is spending most of the day relaxed, inactive and lengthened, based on the joint malalignment.

You could see this as a DOMINANT muscle and a RELAXED muscle in each muscle pair.

What are Tonic and Phasic Muscles?
what are tonic and phasic muscles

The Dominant muscle we spoke about a moment ago is what we will phrase as TONIC.

The antagonist to this is relaxed and we’ll call this PHASIC

Look at this diagram, can you see how the Tonic muscle is physically pulling the bone out of alignment?

This change in alignment is exactly why FITPROs needs to know about Tonic and Phasic Muscles.

Every muscle in our body has a propensity (tendency) to be either Short and Tightened or Long and relaxed.

This means that every muscle in the body has a propensity to be tonic or phasic.

So you can learn a list of tonic muscles and a list of phasic muscles in the body to predict which postural compensations are likely to occur naturally.

As a FitPro you can help prevent postural compensation and distortions with an effective exercise plan. Using strategic flexibility to relax the tonic muscles and strategic activation/ strength work to stimulate the phasic muscles is key.

Let’s look at which muscles fall into each category…

Which Muscles are Tonic and Phasic?

When asked which muscles are tonic and which are Phasic – research can vary considerably.

This table shows a summary of the main Tonic muscles in the body and the main Phasic muscles in the body.

The left-hand column lists the Muscles prone to be tight – these are TONIC muscles,

The right-hand column lists those prone to weakness – these are PHASIC muscles

what are tonic and phasic muscles table

These relationships are the key to understanding common patterns of postural compensations such as the upper-crossed (similar to kyphosis) and lower-crossed syndromes (similar to lordosis).  

What this really means is your understanding of tonic and phasic muscles can lead to a deeper understanding of strategic planning, which in turn can reduce joint and back pain associated with postural compensations.

You learn about how tonic and phasic muscles affect posture in your Level 3 Anatomy and Physiology exam.

Faulty Loading Patterns

Now, before we go on, let me clarify the term “faulty loading.” Faulty loading means any over-use, under-use, abuse (such as trauma) or disuse (such as not getting adequate exercise!).

Tonic muscles tend to shorten in response to over-use, under-use or trauma, whereas phasic muscles tend to lengthen and weaken in response to these types of stimuli.

These effects can lead to musculoskeletal imbalance and joint instability when postural and phasic muscles are located on opposing sides of the agonist-antagonist relationship.

You learn about how tonic and phasic muscles affect posture in your Level 3 Anatomy and Physiology exam.

To help you prepare for your level 3 Anatomy and Physiology exam and understand tight and weak muscles in the body, we have donated an entire video tutorial about posture within our Level 3 Anatomy and Physiology Revision Bootcamp.

Click the link below to find out more and learn everything you need for your exam:


How sitting might affect posture and tight muscles?

The problem is that the nature of our inactive clients’ environments is most likely to favour the recruiting of tonic muscles.

The body is a dynamic system (that just means it can change how it works), and it adapts to the things it is asked to do.

If you’re constantly asking it to sit at a desk all day and only squat when you need to go to the toilet, it’s going to begin to default towards using tonic muscles.

As a result the tight muscles get tighter and more active

and the weak muscles get weaker and more relaxed.

This is bad because when we start to exercise again, we won’t be able to use these muscles.

So as mentioned earlier joint stability will be poor, and furthermore, we’ll instead ask tonic muscles to do things that phasic muscles should do.

This is why people claim deadlifts and squats are bad for their backs. They can’t activate their glutes and hamstrings after years of sitting in a chair, so the low back muscles get OVER-Recruited to compensate for the low activation from the glutes and hamstrings.

Fixing Imbalances

So you and your client now understand what is happening. Now we have to address how we fix it.

As we mentioned, phasic and tonic muscles have some different properties.

Tonic muscles contract slowly and have great endurance. They’re also prone to overactivity and finally, tend to tighten when they’re overactive.

Phasic muscles contract quickly (yes this is due to fibre distribution) and have poor endurance. They’re prone to lengthening when they’re weak, and this coupled with the tightening of an overactive antagonistic tonic muscle about a joint can cause a misalignment in the joint.

Fixing this can have a massive impact on alleviating joint pain (especially in the back). Failing to fix it will result in you losing a client to injury.

So, next time you programme your client’s training session, think about Phasic muscles and which muscles need to be innovated in order to improve client technique.

In summary

  • You need to know about the Tonic and Phasic Muscles for your exam and also as a FitPro
  • Muscles Work in Pairs
  • Tonic muscles are those that have the propensity to be short and tight
  • Phasic muscles are those that have the propensity to belong and weak
  • Tonic and phasic muscles can create postural compensation
  • Imbalances between muscles can be caused by posture, sedentary or faulty loading
  • As FITPROs we can help fix imbalances
  • Posture is one of 8 modules inside your Level 3 anatomy exam

Test your knowledge with today’s mock questions:

[NOTE: The answers are below the 3rd question]

QUESTION 1: A Phasic Muscle is…

A. Lengthened
B. Shortened
C. Tightened
D. Overactive

QUESTION 2: Phasic and Tonic Muscles can contribute towards postural compensations in what part of the body?

A. Just in the spine
B. Just in the legs
C. Across the whole body
D. Just the front of the body

QUESTION 3: Which of the following is not a cause of postural compensations?

A. Faulty Loading Patterns
B. Injuries
C. A balanced exercise programme
D. No flexibility work

What’s the CORRECT answer?

Question 1= A, Question 2 = C, Question 3 = C

If you want more mock questions like this, then you can download more Free Mock Questions: DOWNLOAD NOW

Need More Help with your Level 3 Anatomy Revision?

or Trainee FITPROS Taking Their L3 Anatomy & Physiology Exam.

Learn, Revise & Pass Your Level 3 Anatomy & Physiology Exam In Under 10-hours

(Without Having To Spend Hours Revising Or Feeling Overwhelmed)

If you want to get your revision structured, learn everything you need to know and feel confident on exam day, then click the link below:

Dedicated to More

Hayley “what are tonic and phasic muscles” Bergman

Parallel Coaching

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3 thoughts on “What are Tonic and Phasic Muscles?”

  1. Eileen Newsome

    Thank you for a well thought out and descriptive review. As a massage therapist I always strive to help my clients understand their body mechanics.

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