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There really is a sixth sense: it’s called proprioception. It is the sense of position and movement. It is produced by nerves in our connective tissues (ligaments, bone, fascia) and our 300-or-so muscles.
Without proprioception, you couldn’t stand up (standing up is actually shockingly complicated). You couldn’t so much as scratch your nose, because you wouldn’t be able to find it.
It’s like asking a fish what water feels like. We can’t know what proprioception feels like, because we cannot shut it off and it doesn’t change intensity the way sights or sounds do.
You can’t truly know what proprioception feels like until it’s gone.
Which only happens to a few people who get an extravagantly rare neurological condition that destroys proprioception (polyneuropathy of the neuraxis). Oliver Sacks describes a case of it in his book, The Man Who Mistook His Wife for a Hat:
She continues to feel the loss of proprioception, that her body is dead, not-real, not-hers … She can find no words for this state, and can only use analogies derived from other senses … .
This is an extremely high-quality slow-motion video of a cheetah running (well, five of them over three days actually). It is an amazing demonstration of proprioception at work. Note the lack of head movement — that is one exquisitely fine-tuned and effective motion control system! Cheetahs probably keep their gaze steady to help the hunt, so vision is a major part of the equation here: but it’s mostly proprioception that keeps track of where all the body parts are. Just fascinating. And beautiful. And cute too! All at once.
Cheetahs on the Edge — Director's Cut from Greg Wilson on Vimeo.
Proprioception is a large sense. It produces a tremendous amount of data, as much or more than all the other senses combined. So although it is a silent sense, it is a very important one. Just knowing that it exists is significant self-knowledge.
The nerves that generate proprioception are embedded in the tissues of our musculoskeletal system: in muscles, tendons, ligaments, joint capsules and cartilage. They send information to the brain about how much tension or pressure is being applied to them, and how quickly it’s changing. The brain uses this information to figure out:
You might expect the brain to be able to figure out the position of the eye based on what you are looking at. But that’s not how it works. You actually know the direction and focus of your gaze because you know the position of your eyeball, and the effort it took to change the shape of your lens. Without those nerves in the muscles of the eyeballs, we would be able to see, but we wouldn’t know where any of it was. Try to imagine that! You would, in effect, be virtually blind.
An understanding of proprioception is routinely applied in a variety of physical therapies. One familiar example is the relaxing effect of vibration. This works on a simple principle called “proprioceptive confusion.” If you move or shake the body at random, the brain gets a deluge of nonsensical proprioceptive data. The nervous system, overwhelmed by the random stimuli, effectively “gives up” and stops resisting the movement: providing you with deep, muscle loosening relaxation!
Similarly, proprioception is also responsible for most of the (delicious) feeling of sensory novelty that we experience when we are massaged. Although stroking sensations on the skin are certainly significant, most of us also crave the deeper sensations of our proprioceptive nerve endings being stimulated for us — the unfamiliar movements of joints, the pressures on our muscles in places we can’t reach ourselves, the incongruously effortless stretches of tendons and ligaments that are normally only stimulated by intense exercise.
Without proprioception, every massage would feel like just a skin massage: a poorer sensory experience.
I am a science writer, former massage therapist, and I was the assistant editor at ScienceBasedMedicine.org for several years. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook or Twitter.
This article is part of the Biological Literacy series — fun explorations of how the human body works, what I think of as “owner’s manual stuff.” Here are ten of the most popular articles on this theme: