First and foremost - HOW should you foam roll?
(A Technique suggested by the National Academy of Sports Medicine [NASM])
The idea is to position the body over a foam roller in such a way that specific sensitive areas can be targeted. The suggested areas to foam roll are the calves, the TFL (tensor fascia latae) including the IT band, inner thigh, piriformis (in the butt area), and the latissimus dorsi ("lats"). When in position you roll back and forth on the foam roller to identify an area, or areas, that are particularly tender. "It is crucial to note that when a person is using self-myofascial release [foam rolling] he or she must find a tender spot and sustain pressure on that sport for a minimum of 30 seconds." (page 177) This sustained pressure, a creation of pain, is very necessary to the beneficial effects of foam rolling as will be discussed more later.
Moving on - how does foam rolling WORK? (according to NASM)
The fourth edition of NASM's personal training certification textbook purports that foam rolling works through autogenic inhibition. By definition autogenic inhibition is "the process by which neural impulses that sense tension are greater than the impulses that cause muscles to contract , providing an inhibitory effect to the muscle spindles." (pg 647) Now, what does this mean?
To explain, we have two major sensory receptors in our skeletal muscles; the golgi tendon organs and the muscle spindles. Golgi tendon organs, most basically speaking, sense changes in pressure and tension, and when they are activated they cause the muscle to relax. Muscle spindles on the other hand sense changes in the length of muscle, and when they are activated they cause the muscle to contract. When we are stretching or foam rolling the neural impulses that sense tension (the golgi tendon organs) are greater than those that cause the muscle to contract (the muscle spindles). The result is that the activity, the contraction, cause by the muscle spindles is inhibited. This allows the muscle, ideally, to achieve an optimal length which can be beneficial during any stretching or training that may follow.
Following this reasoning of achieving optimal length in the muscle, NASM literature supports the idea that foam rolling helps to release knots and points tensions in the muscle. The idea, as explained by the textbook, is that through autogenic inhibition any knots, trigger points, or points of pain in the muscle are essentially broken up (imagine the knot being "untied," if you will) by the pressure applied via foam rolling. This break up, melting away, "untying", or release of painful knots occurs in our fascia; hence the name self-myofascial release for foam rolling.
However - why this logic isn't actually very logical…
I'm going to just quote this debunking directly from the article, because the author put it in words far better than I can: "Fascia is tough stuff. Sure it has some interesting adaptive properties, but at the end of the day its purpose is to form a solid structure for the body. Is it really plausible that we can significantly change our structure just by leaning on a foam roller a little bit? We must be made of stronger stuff than that. If fascia started to break down, or elongate, or “melt” every time it felt a little sustained pressure, we would be pretty fragile creatures. Every time we sat on a rock our posterior chain would lengthen. So for me the idea that foam rolling lengthens or melts some important structural stuff in our body does not pass the common sense test." In addition to this reasoning, Hargrove further points out that if the structure of our fascia is actually being altered by foam rolling then it doesn't really make sense that the effects are only temporary. Why is it that we feel the need to foam roll the same area again if the "knots" have been removed? Shouldn't the pain cease and stay that way? Do the knots in the fascia snap back in place over time? It's unknown.
Now, to be honest with you, I never even thought to question the rationale of "myofascial release" before reading the quoted article. However, after having read it I'm a bit embarrassed that it never occurred to me to be suspicious of the idea that something as feeble as a foam roller could make substantial changes to our fascia (the protective tissue surrounding our muscles), i.e. to our body's structure, and furthermore, to only do so temporarily. Surely, if this was possible then it would follow that humans be quite fragile in nature (which we aren't). So, I must concede to Hargrove's reasoning. But, just because one possible mechanism has been ruled out doesn't mean that foam rolling is not effective in other ways.
So - how might foam rolling work, then?
"Might" being the key word here to illustrate the reality that we don't actually KNOW. There are many theories and speculations about how foam rolling works and if it actually produces physical benefits, but there is not as of yet any universally researched and accepted understanding of these matters.
That being said, here is one of the (more reasonable, in my opinion) theories regarding how foam rolling actually works:
Diffuse Noxious Inhibitory Control (DNIC) is very likely the mechanism that allows those who foam roll to experience temporary benefit from their practice. In short, DNIC is the process through which the brain decides to ignore nociceptor activity. Nociceptors are the receptors in our bodies that cause us to feel what we know as pain. Therefore DNIC can be simplified as the process of the brain ignoring pain.
This phenomenon is triggered often by sustained nociceptive input, or sustained pain, which makes the necessity of sustaining pressure on a tender spot with the foam roller make a lot more sense. In addition to this, it is necessary to have an expectation that DNIC will work in order for DNIC to work. Essentially what this means is that in order to feel a decrease in pain we must believe that we are going to feel a decrease in pain. To put this in the context of the article, in order for DNIC to work when foam rolling we must believe that foam rolling will result in a reduction of pain in whatever area we're targeting. It can be likened to the placebo effect.
Well - are there any positive implications of DNIC and foam rolling?
Sure! This is kind of where the "if it works it works" school of thought comes into play. Whether it's through DNIC or through some kind of structural alteration, if a person experiences relief of pain from foam rolling then, in a way, it's worked. So assuming that foam rolling does provide a pain-reduction benefit we can conclude that training after foam rolling, during the time when that reduction of pain is experienced, can be quite effective. This is because, "[even] if you are feeling better only for an hour, this provides enough time to train movements that would not normally be accessible, learn new skills, develop new capacities, and reduce the perceived threat associated with certain movements." When applied to training, this effect of foam rolling could have longer lasting, maybe even permanent, benefits.
NASM Essentials of Personal Fitness Training. Michael A. Clark, Brian G. Sutton, Scott C. Lucett. 2014. Chapter 4, Exercise Metabolism and Bioenergetics. 79-81.
"How Does Foam Rolling Work?" Practical Science on Movement and Pain. Todd Hargrove. May 2013. LINK
"How Does Foam Rolling Work? And Why 'SMR' Should be Called 'SMT.'" Bret Contreras, CSCS, The Glute Guy. Bret Contreras. December 2013. LINK