Most people are amazed by the amount of sensation produced by pressure on their muscles. “I can feel that all the way down in my toes!” That’s a common sentiment expressed on a massage table — usually muffled by the face rest. How does that work? What’s going on?
Muscle is a sensory organ
Muscle tissue is about half of the average person by weight (most of us try not to think about the other half), and it is chock full of nerve endings. Also, muscle tissue is thoroughly mixed with connective tissues like tendons and ligaments and intricate wrappings called “fascia”1 — and connective tissue is also full of nerve endings.
All those nerve endings produce sensation — a lot of sensation. In fact, they produce more sensation than all the other senses combined.2
Muscle is a sensory organ.3 A big sensory organ, much more massive even than your skin. How your muscles feel is how your body feels! You are your muscles.
The sixth sense
Those nerve endings detect pressure, movement, stretch, velocity of muscle contraction, and much more. And some of it’s uncomfortable, too: like fatigue and pain during exercise, caused by nerves that pick up cocktails of metabolites.4 All together, about half of all sensory information sent to the spinal cord and brain comes from these nerves.
We couldn’t even stand up without this sensory information. Without it, you wouldn’t even be able to locate your body: you wouldn’t be able to feel it In fact, there is a rare neurological condition in which people lose their proprioception, and the consequence is a devastating and surreal disembodiment.5
Yet hardly anyone knows that it is a sense. Most people have never even heard the name for it.
Physiologists call it “proprioception.” Proprioception is the real “sixth sense” — not clairvoyance, but the very real and physical sense of effort, position and movement.
Strangely, though, most people aren’t aware of how their muscles feel until they start to hurt. If proprioception produces so much information, why aren’t we aware of it?
Raising sensory awareness
We aren’t normally aware of our proprioception because it’s always on, and there’s always a lot of it. It’s like trying to describe daylight in a world where the sun never sets or even goes behind a cloud. Proprioception is just there.
We can turn the other senses off, or easily turn the volume up or down. We can close our eyes and plug our noses. We can touch more or we can touch less. Taste doesn’t have much to tell us without food in our mouths. But proprioception is always on. No matter what position you are in, you are still in a position.
If we could switch proprioception off, it would suddenly be obvious what we were missing. But normal proprioception is a sense without a lot of contrasts. Until you get on a massage table.
Massage therapy produces a lot of unexpected and unusual proprioceptive data. You can’t turn your muscle sensations off … but you can give them extra stimuli, and new and interesting stimuli. And they love it!
Surprise! Novel sensory input
Your nervous system craves data. It’s like a drug.
Baby mammals, including us, literally die or grow up brain-damaged without lots of sensory stimulation.6 Their nervous systems need input in order to know how to grow. We don’t just learn about the world through sensation — we learn about our selves. The developing organism can’t define self unless it can feel other.
As adults, we no longer need that input to survive — but we still need it to thrive. We still want to entertain our senses. We like to touch, to be dazzled by sights and sounds, to stop and smell the roses and taste great food.
Touch is neglected in our culture, of course, and we tend to neglect proprioception completely. People engage in physical activity, of course, which is proprioceptively stimulating — anything from dancing to skiing — but they don’t do it for the proprioception, and the sensations are pretty much what you’re brain expected.
It’s friendly sensory surprises the nervous system really likes. You don’t know what a delicious meal is going to taste like until you put it in your mouth, but your body knows exactly what your next dance move is before you do it. The only way you can easily surprise your proprioception is with massage: with passive stimulation of your proprioceptive sense.
And we are surprised not just by the quality of sensations that massage produces, but by the quantity — as much sensation as all the other senses combined. What a rush!
And that’s why we tingle from head to toe when we’re getting massaged.
About Paul Ingraham
I am a science writer in Vancouver, Canada. I was a Registered Massage Therapist for a decade and the assistant editor of ScienceBasedMedicine.org for several years. I’ve had many injuries as a runner and ultimate player, and I’ve been a chronic pain patient myself since 2015. Full bio. See you on Facebook or Twitter.
- The clinical importance of fascia to therapy is faddishly exaggerated right now. See Does Fascia Matter? (Short answer: not that much, no.) But it certainly is richly innervated.
- This is a difficult statement to back up. I believe I read it somewhere once, in what was doubtless a credible source, or I wouldn’t have taken such care to remember it. Otherwise, it is based primarily on the observation that the proprioceptive nerve fibres bundled in the spinal column are rather substantive. However, visual, auditory, olfactory and taste sensations are not transmitted to the brain on nerve fibres in the spinal column at all, so an apples-to-apples comparison is not exactly straightforward. Suffice it to say that there is a lot of proprioceptive information, but I wouldn’t want to stake my reputation on the exact amount relative to the other senses.
- Graves JE, Pollock ML, Leggett SH, et al. Effect of reduced training frequency on muscular strength. Int J Sports Med. 1988 Oct;9(5):316–9. PubMed #3246465 ❐
Graves et al studied 50 men and women accustomed to strength training and tested them on 12 weeks of reduced training frequency, going from 2 or 3 days per week to 0, 1 or 2 days per week. Those reduced to zero lost strength as expected (about 70% over the 12 weeks), but for those who merely reduced their frequency? No loss at all: “Strength values for subjects who reduced training to 2 and 1 days/week were not significantly different … . These data suggest that muscular strength can be maintained for up to 12 weeks with reduced training frequency.”
- Pollak KA, Swenson JD, Vanhaitsma TA, et al. Exogenously applied muscle metabolites synergistically evoke sensations of muscle fatigue and pain in human subjects. Exp Physiol. 2014 Feb;99(2):368–80. PubMed #24142455 ❐ PainSci #53975 ❐
What specifically causes “the burn” in intense effort? Which molecules? Protons, lactate, and ATP — and only in concert. Individually, none of them really have that much effect. Surprisingly, this fairly unsurprising result is brand new information: “the first demonstration in humans that metabolites normally produced by exercise act in combination to activate sensory neurons that signal sensations of fatigue and muscle pain.”
- Oliver Sacks writes about this condition in chapter 3, “The Disembodied Lady,” of his classic book, The Man Who Mistook His Wife for a Hat and Other Clinical Tales. In this case, he describes the illness as “an acute polyneuritis, but a polyneuritis of a most exceptional type: not like Guillain-Barré syndrome, with its overwhelming motor involvement, but a purely (or almost purely) sensory neuritis, affecting the sensory roots of spinal and cranial nerves throughout the neuraxis.”
- NYTimes.com [Internet]. Goleman D. The Experience of Touch — Research Points to a Critical Role; 1988 Feb 2 [cited 15 May 16].
Touch is a means of communication so critical that its absence retards growth in infants, according to researchers who are for the first time determining the neurochemical effects of skin-to-skin contact.
The new work focuses on the importance of touch itself, not merely as part of, say, a parent's loving presence. The findings may help explain the long-noted syndrome in which infants deprived of direct human contact grow slowly and even die.
Psychological and physical stunting of infants deprived of physical contact, although otherwise fed and cared for, had been noted in the pioneering work of Harry Harlow, working with primates, and the psychoanalysts John Bowlby and Renee Spitz, who observed children orphaned in World War II.
The new research suggests that certain brain chemicals released by touch, or others released in its absence, may account for these infants' failure to thrive.