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  • David Wadsworth

What Happens to the Back Muscles in Lower Back Pain?

There is absolutely no doubt that good muscle function is essential for a healthy pain-free spine. What happens to your back muscles once you hurt yourself? Do they magically heal and recover, or do you need to do some rehab to get things working properly again?


The back muscles are complex - which ones require rehab and which don't in your individual situation? image source: https://commons.wikimedia.org/wiki/File:1117_Muscles_of_the_Back.png

Whether rehab is required depends entirely on your own unique situation, and this can differ dramatically between people. Numerous scientific studies looking at low back pain suggest that there is progressive deterioration in the lower back muscles over time that is highly individual, which indicates a need for different rehab approaches for different people in order to return to full pain-free function.


Here is a summary of what happens to your back muscles once you have back pain:


In acute, first-time low back pain patients, the immediate change in the deep back muscles is inhibition, meaning that the nervous system is responsible for the impaired function. These changes tend to be more localised to the painful segment(s) of your spine.


Back muscle performance changes with pain, but this change is both highly individual and may be task specific (meaning whether the muscle over-activates or under-activates depends on exactly what task you are doing) (Hodges & Danneels 2019).


In sub-acute or episodic low back pain, the changes become more structural, and more widespread or generalised (Matheve et al 2023). Persistent inflammation appears to trigger fatty infiltration and fibrosis of the back muscles, which represents a structural change (muscle is being converted to fat and scar tissue). There also appears to be a conversion from type 1 “slow twitch” muscle fibre to the “fast twitch” type 2 fibres, which is associated with poor fatigue resistance.

These changes persist beyond the resolution of your pain.

This means that your muscles are weaker and fatigue faster, making it easier to “lock up” your back when moving since fatigued muscles have a slower reaction speed. This is an extremely common occurrence for people with recurrent low back pain – “I just bent down to…. and my back locked up again”.


MRI cross-sectional view of the low back: A) Normal back muscles (<10% fatty infiltration). B) Moderate change (11-50% fatty infiltration). C) Severe change (>50% fatty infiltration). Blue arrow: multifidus muscle. Green arrow: erectores spinae muscle group. Orange arrow: Psoas muscle. Pink arrow: Quadratus lumborum muscle. Source: https://creativecommons.org/licenses/by-nc/3.0/

As the pain becomes more chronic and long lasting, the structural changes become more widespread, involving both sides of the spine irrespective of which side the pain initially began. The changes remain highly individual, and more extensive muscle atrophy is often present. This implies a significantly longer and more challenging rehab pathway to achieve full normal function and resolution of pain. These changes are strongly correlated with low back pain (Kjaer et al 2007).


Rehab Implications

Due to the highly individual nature of muscle changes, and the progressive deterioration, early professional care and diagnosis is advisable. Starting rehab of muscle function early with the appropriate choice of exercise is likely to lead to a better result, meaning less pain and less recurrence.


In the acute phase, rehab is more specific, with localised, low load activation work targeting only those muscles that are inhibited. Often this may be all that is needed, and there is good evidence that this approach is effective. Not seeking care early and failing to do this is plausibly one of the reasons why recurrence and chronicity of pain develop. Thus, early professional care and rehab may have the major goal and benefit of preventing recurrence, which is the most common pattern for acute back pain (see my article here).


In subacute and chronic pain, the initial rehab may be similar to acute back pain consisting of low load exercises targeting specific muscles which are not functioning properly.

However, the low-load approach alone is unlikely to be adequate for reversing structural changes and atrophy.

A progressive resistance strengthening program is required. It is important to note that general physical conditioning (aerobic exercise performed over 6-18 weeks) can help promote resolution of muscle inflammation and conversion back to slow twitch type 1 fibres, but general conditioning cannot rectify specific weaknesses (Hodges & Danneels 2019). This is what the progressive resistance strength program is for.

Remember that the variation in muscle dysfunction between individuals, just like the variation in the causes for back pain, is highly variable so the strength program needs to be tailored to the individual.

For those that have been following my posts, you have undoubtedly heard me repeat this time and again – early intervention and following through with your rehab even after your pain settles are the keys to preventing recurrence, chronicity and minimising the need for a whole lot more treatment, making it a cost-effective strategy.


“Rehab right from the start”.

References:


Hodges P & Danneels L (2019): Changes in Structure and Function of the Back Muscles in Low Back Pain: Different Time Points, Observations, and Mechanisms. JOSPT 49:464.


Kjaer P et al (2007): Are MRI-defined fat infiltrations in the multifidus muscles associated with low back pain? BMC Medicine 5:2.


Matheve T et al (2023): The Role of Back Muscle Dysfunctions in Chronic Low Back Pain: State-of-the-Art and Clinical Implications. J Clin Med 12:5510.

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