There are a few muscles in the body that are often thought of as ‘magic’.
These ‘magic muscles’ seem to act differently from the other muscles in the body and we need to get them working at all cost by prodding and poking or performing bizarre exercises to ‘fire’ them up so that we can avoid ‘dysfunction’ and the inevitable contribution to pain from ‘sub optimal’ activation.
Examples of these muscles and their related ‘dysfunctions’ are the TvA and back pain, the rotator cuff and shoulder pain and the glutes and well every pain going from the knee to the lower back and shoulder.
Sometimes (this means often) we just blame the muscle automatically without even bothering to perform ANY kind of test.
Simply stating “Your back pain is because your TvA is not firing”
We have some wonderful theories about how and when ‘magic’ muscles should activate and what role they perform at a joint such as are they movers or stabilizers.
It has been suggested some deep muscles should activate first and independently of the movement being performed to provide stability rather than help move the body such as the TvA and rotator cuff whilst the glutes should activate first before other more superficial muscles of the hip to be the ‘prime mover’.
It has also been hypothesized that muscles may have specific jobs because of their fibre type. Muscles with a predominance of slow twitch fibres mean that they are better suited to a postural or ‘stabilizer’ role rather than movers witch have been proposed to contain a greater proportion of type 2 fibres. Sometimes we call them local and global or even fancier…..tonic and phasic.
This influences a whole bunch of what people do, both therapeutically and in gym settings. Do we have much evidence beyond the theory to support these common practices?
This gives us three questions to answer in this blog in relation to this line of theory about muscles and if it contrasts with what has ACTUALLY been studied?
• Do ‘magic’ muscles have specific firing patterns or onset times?
• Do ‘magic’ muscles act independently to carry out specific roles such as stabilization?
• Do ‘magic’ muscles display a distinct dominance of fibre type that mean they have specific roles such as moving or stabilizing?
Muscle ‘firing’
Glutes
How many times have you heard someone say “They told me my glutes weren’t firing”. The glutes not firing or ‘activating’ first have been blamed for lower back pain, SI pain, knee pain and my personal favourite, opposite shoulder pain (its true……myofascial slings baby) amongst other things.
There are a few purported reasons for this lack of 'firing'.
• ‘Inhibition’ of the muscle after injury.
• Excessive sitting causing inhibition
To determine if there should be a specific firing order it is important to look objectively at how healthy people activate their muscles. We can then see if there is a specific pattern or level of activity we should to try to aim for to be healthy and if there is inhibition OR the theory is incorrect.
The prone hip extension test has always been a favourite way of doing this. You know the one, get people to lie on their fronts and then the therapist or trainer pretends that their fingers are EMG machines and can tell which muscles activates first! This is known as the prone hip extension (PHT) test.
So, if pain was a consistent inhibitor of the glutes then we should see DECREASED EMG (with a proper machine) output from people who have had a previous injury during the PHT. This paper *HERE* actually found that the participants who had suffered a previous injury (in this case hamstring) had a GREATER EMG output. This was also the case here *HERE* where the chronic LBP group had a GREATER EMG signal in the PHT compared to those without. In fact any muscle ‘inhibition’ seems to resolve quite quickly *HERE* after injury.
As far back as 1990, so we really have ‘known’ this for a while, *HERE* they found that muscle firing was pretty variable in the PHT. Twenty healthy subjects each made 30 prone hip extensions with EMG used to measure the onset times of the right glute, biceps femoris, eractor spinae and left erector spinae. They found no significant difference in onset times between the muscles and variability in muscle activation within and between subjects. So the same person can perform the action differently and different people also perform it differently. This makes it really tough to have an objective firing order to base ‘dysfunction’ from.
This paper *HERE* DID find an objective order of muscle firing. They state very nicely their rationale for the need for the research.
“The development of low back pain is ascribed to changes of the muscle firing order in prone hip extension. There appeared to be no normative data on muscle firing order of the lumbar and hip musculature to provide a basis for recognizing variations”
They had healthy fifteen subjects and had a very similar testing procedure with a slightly different set of muscles around the hip but found the glute max actually activated LAST rather than first.
This was finding was supported *HERE* with a much greater sample size of fifty participants, 30 with back pain and 20 without. They looked at both onset time and amount of activation and found that glute activity was delayed significantly in both the symptomatic AND asymptomatic groups. Their conclusion being that the PHT was not able to discriminate between those with and without back pain.
They also made the great point that it is also tough to generalize what happens prone to standing or walking. This paper *HERE* found that changes in hip abduction altered the activation order and amount of activation (EMG amplitude) of the hip musculature. With 15 or 30 deg of hip abduction the glute max firing time was advanced with 0 deg delayed relative to the hamstring.
So slight alterations in hip position when prone changed firing order! What would happen with changes in body orientation (prone, supine, standing) or actually moving and at different speeds. Different terrains may influence hip width *HERE* and this would probably change firing order based on leg orientation.
In the Lieberman study looking at glutes during walking and running *HERE* we see alterations in timing and substantial activation changes between the two activities. This suggests that a single test is probably not transferable across multiple activities that may require different timings and activation levels.
As far as I am aware glute inhibition with prolonged sitting has not been studied. This is quite amazing considering how prevalent the theory is so it is tough to corroborate it. However WITHOUT evidence to support it and as we see delayed firing in those WITHOUT pain then we have to wait for more evidence before we can support this claim.
There is also the small point of can you really feel micro second differences in muscle activation with your fingers and if they are pretty minute do they matter anyway?
Although the 'glutes aren’t firing’ line sounds like an alluring and simple explanation for someone’s problems the actual SCIENCE really does not support such a simple answer.
TvA
The TvA is often a muscle that is blamed for not firing and inadequately stabilizing the spine leading to back pain. Although there certainly has been some early research showing delayed firing there is certainly not a well-defined causal link between delayed onset and lower back pain. Later work using both EMG and M mode ultrasound *HERE* have shown variable firing in healthy individuals and back pain sufferers with delayed onset not being a consistent finding for LBP patients.
Vasseljen et al state “Within and between subject variations need to be acknowledged in future studies” This paper found no association between changes in onset timing of the deep abdominal muscles and LBP.
This was finding was mirrored by Mannion et al *HERE*. They found no association between the ability to activate the TvA both before or after training and a good clinical outcome.
Much like the glutes, the onset of TvA muscular activity has been shown to be variable between tasks so generalizing what happens with a muscle in one research piece using a specific movement such as rapid arm movement cannot be used to create a generalized muscular model for human function.
Morris et al *HERE* found the role of the TvA to be more function specific with muscular activation to being dependent on direction and magnitude of an action being performed. It has previously been hypothesized that the TvA acts independently (not function dependent) like a corset to stabalize the spine and it does this bilaterally. The original research only looked at the contralateral (to the arm motion) TvA but extrapolated this to bilateral activity.
Morris et al reported that no one (in the admittedly small sample size of seven) used a bilateral activation strategy in response to unilateral arm movements, the TvA acted both reciprocally and asymmetrically and the authors suggested that training the TvA to act bilaterally may actually INHIBIT normal movement.
Again we see that it is just not that simple and giving a muscle a specific job with a timing strategy is a potentially problematic line of reasoning when we dig a bit deeper than the theory.
Rotator cuff
The rotator cuff (RC) has also be given the job of stabilizing the gleno-humeral joint with a hypothesized earlier onset time and co contraction to provide joint stability. This systematic review of the stabilizing characteristics of the rotator cuff *HERE* found only in 4 out of 10 of the tested movements that the RC onset time was earlier than more ‘global’ muscles around the shoulder joint. Co-activation was also not consistent across different movements.
The authors concluded here that there is no clear evidence for seeing the RC muscles as simply stabilizers but instead acting in a direction specific manner. This was supported by more co contraction between the supraspinatus and infraspinatus than between either muscle and the subscapularis. The stabilizing role of the RC maybe the limiting of translation within the joint in a direction specific manner.
This is supported by this paper *HERE* that found that the subscapularis being more active in shoulder extension and the supraspinatus and infraspinatus more active in shoulder flexion. Interestingly the level of external load did not alter muscular activation level! It was also supported *HERE* with different variations of shoulder exercises influencing the muscles role.
Boettcher et al *HERE* helpfully put their conclusion in the title of their paper “The role of shoulder muscles is task specific”. They wanted to see if the RC muscles displayed distinct rotator or stabilizer function and although the title somewhat suggested this was not the case this was confirmed in the paper with a task specific activation being apparent.
Muscle fibre type
The predominant fibre type of muscles has been cited as a reason behind why a muscle should be classified as a ‘stabilizer’ or a ‘prime mover’. A stabilizer would have a much greater amount of type 1 fibres whilst a prime mover a greater amount of type 2 and type 2A.
But do we see a predominance of fibre types within muscles?
Trunk muscles are often theoretically split into ‘inner’ and ‘outer’, the inner unit involving the deeper muscles that should have a much greater concentration of type 1 fibres. As we have looked at previously the inner unit muscles don’t appear to have a reflexive and independent activation or movement strategy. So what about their fibre type, is this more suited to stabilization?
This study *HERE* used bioposy to study the rectus abdominis, TvA and Obliques. They found large variation BETWEEN people but little difference between the individual muscles, in fact their words were “minor or non-existant”. They concluded that the muscles actually have a SIMILAR functional capacity.
This paper *HERE* looked at 36 muscles in a relatively young sample size. They found that “Most of the muscles studied were known to fulfil both tonic and phasic functions, however, and showed no striking preponderance of either fibre type.”
In an older population in the rotator cuff, this paper *HERE* also showed a mixed fibre with a maximum of a 60/40 split between type 1 and 2. This was also apparent in an older paper *HERE* looking at back muscles with around a similar split between type 1 and 2. Certainly not enough to warrant classifying muscles into specific roles.
A more recent systematic review paper *HERE* also suggests that there is little inter (between) or intra (within) differentiation of fibre types in either healthy people or LBP patients in the multifidus and erector spinae muscles.
Two muscles *HERE* that DO APPEAR to have a much greater predominance of type 1 fibres are the soleus and tibialis anterior that have around 80% of fibres being more ‘postural’ although we don’t seem to talk about these muscles in the same way but obviously they do a lot of endurance type work!
Conclusion
What does the research tell us?
• Muscle firing patterns do not appear to fit theories that are often discussed and that influence exercise prescription.
• Muscle firing, when looked at over repeat trials, seems to be variable within and between individuals and does not support a specific muscular strategy to base ‘dysfunction’ from.
• Muscles appear not to simply have ‘roles’ that are independent of the action being performed and can be consistently defined.
• Co contraction levels appear not to be consistent enough to support a singular role of stabalistation to a joint.
• Generalising activation patterns in one task to beyond that task appear unfounded.
• Fibre types although not a perfect 50/50 split do not really show a dominance of fibres that allow us to categorise muscles into postural or movers. Movement data also does not support this.
