Proprioception - Our Body Position Sense
In medical schools and textbooks, proprioception is defined as 'body position sense' - the ability to know where our body is at all times.
In other words, proprioception refers to our uncanny capacity to know almost exactly where our arm, leg, or finger is, without having to look at it. With proprioception, you can estimate the angle of your elbow, the position of your hand, and the spread of your fingers. If you had to touch your nose, you could do so, even with your eyes closed.
But, as impressive as this ability is, it is only part of the proprioception story. The ultimate purpose of proprioception is to control the way we move.
Proprioception gathers continuous input from the millions of sensors in our skin, muscles, joints, and ligaments... combines that with the input from our five main senses... then uses it all to control our balance, coordination, posture, and movement.
Whether you're picking up a glass, throwing a ball, watching television, or walking down a step, proprioception is constantly monitoring the input from the nerve sensors in your body, to make sure that the output to your muscles is perfect. This is a system so complex, diverse, and adaptable, that no amount of robotic or computing power can come close to duplicating the smooth and coordinated movements of the human body.
Your Brain and its Messages
Your brain is a processing centre. It doesn't actually generate anything; it just connects the mass of incoming signals in the right way. Our every thought and action starts life as an incoming nerve signal.
The barrage of incoming nerve signals is the raw material that creates all brain function and, therefore, all human function; most of those incoming nerve signals originate in the muscles themselves. Muscles, therefore are really the main power plant of the brain and central nervous system.
The brain's function is to turn those incoming nerve signals into action.
In human beings, action will mean making muscles work, and, to a lesser extent, controlling our glands and our digestion.
Thus, brain output is determined by its input-input that comes mainly from the nerve sensors and reflexes which control proprioception.
Nerve messages are classified as either incoming or outgoing. Incoming nerve messages are heading towards the spinal cord or brain and are known as sensory, or afferent. Outgoing nerve messages travel from the brain or spinal cord and are intended to make something happen; they are therefore known as motor, or efferent.
The ability for us to move at all - let alone perform somersaults, catch a ball, go to the toilet, or climb stairs - depends on the seamless integration of these two types of messages.
Our awareness of the outside world is maintained through the five senses of sight, smell, taste, hearing, and touch. Our awareness of our inside world is maintained through proprioception. It really is our 'sixth sense'.
Proprioceptors are a special group of sensors found throughout the body. They provide information on the movement, position, tension, and force in every area of the body.
Most proprioceptors, especially those in the skin, send messages only when they are stimulated, but a special class - known as muscle spindle cells - sends a constant stream of messages to the spinal cord and brain, even when the muscle is resting.
These spindle cells are smaller muscle fibres that sit alongside ordinary muscle fibres. Their job is to detect the slightest changes in the tension or length of the muscle.
Every muscle spindle cell constantly generates signals which travel from the muscle to the spinal cord. Like the rev counter of an engine, they are constantly active, responding to the activity of the muscle. Whether the muscle is fully stretched or semicontracted, the muscle spindles adjust their output to reflect the amount of tension in the muscle. This way, the muscle is ready to react at any time, no matter its position.
All proprioceptive information is sent to the spinal cord to perform two functions. First, it controls muscle tone, and, second, it supplies the brain and higher processing centres with vital feedback.
Although a great deal of sensory information reaches the brain, most of it is filtered before it gets to the consciousness. A majority of nerve messages never get as far as the brain; their reflex effects are managed within the spinal cord, totally independent of brain control.
Motor output simply refers to messages sent from the brain to the body. These messages are designed to produce an action.
Although most of our conscious movement occurs via muscles that are traditionally classified as voluntary, up to 90% of our 'voluntary' muscle activity is totally subconscious or involuntary, controlled by reflexes that originate in our proprioceptors. It is this involuntary reflex activity that stabilizes, holds, controls, and limits the movements of our bones and joints.
In other words, proprioception is our primary injury defence mechanism.
Most injuries actually occur with very little force or when doing normal, everyday activities. These injuries, especially the ones that become chronic, often have their origins in a failure of proprioceptive reflexes that control muscle tone. Usually, the muscles are inhibited and fail to act with enough speed or power.
All muscles are made of muscle fibres which contract when they receive an impulse from special nerve cells in the spinal cord called anterior motor neurons (which is a silly name because there are no posterior motor neurons).
Motor neurons are fascinating cells with only a single output. All they do is send messages - actually electrical impulses - to muscle fibres. A single motor neuron will supply one or more muscle fibres, so thousands of motor neurons have to act together to bring a muscle to full contraction. Every time they send a message, a muscle fibre, or fibres, will contract. The faster the messages are sent, the faster the fibre will contract. The more motor neurons that are sending messages, the stronger the contraction will be.
But what makes motor neurons really fascinating is not their output, but their input. Each motor neuron receives up to 10,000 different inputs - messages from other nerves - bringing information from other muscles, skin, tendons, ligaments, bones, and the brain. However, a motor neuron will not send a message to a muscle fibre until it has accumulated enough inputs. For your average motor neuron, this would be about 70mV (the work of nerves is measured in millivolts). Since each input supplies only one-half to one mV, it is impossible for any one input to cause an output.
Then, to make life even more complicated, not all inputs are positive (facilitatory). Some are negative (inhibitory).
So, each neuron waits until it receives enough facilitatory inputs to outweigh the inhibitory inputs. When the net total reaches about 70mV, the neuron fires, sending a signal to make a muscle fibre (or several fibres) contract. To put this in mind-boggling terms, we also need to remember that, even at rest, each motor neuron is firing fifty times per second. At maximum effort, each one is firing five hundred times per second.
Motor neurons (and therefore muscles) receive their instructions from three main sources:
- Their own sensors in the muscle (muscle spindles);
- The brain;
- Interneurons that receive inputs from proprioceptors in the skin, ligaments, tendons, and other muscles.
In other words, how and why we move comes down to proprioception, our most powerful injury defence mechanism.
Muscle tone gives us the ability to resist outside forces, forces that would otherwise cause injury. But inhibited muscles can't provide that resistance.
And inhibited muscles cannot be strengthened with exercise because the muscle tissue itself is already strong; it's the action of the muscle that is weak because the control mechanism has failed. Using an inhibited muscle is like trying to make a car go when the throttle cable is made of rubber. The car has potential power; we just can't use it.
When faced with any chronic joint or muscle pain, the questions should always be "Where is the weakness and why is it persisting?"
The answer is nearly always abnormal proprioception, i.e., problems with our bodies' internal sensor systems.
It does not matter how strong a muscle is; if the reflexes that control the muscle are compromised, the muscle will not function correctly.
For example, it doesn't matter how much you exercise, how toned your body is, or how strong you are - if someone sticks a knife in your back, you will move. Your back muscles will contract and your front muscles will be inhibited.
Exercising muscles inhibited by faulty proprioception only increases the damage to the joints. Indeed, exercising a muscle that has faulty proprioception cannot produce meaningful or lasting changes in strength, which is why so many injuries become chronic and so many athletes have their careers terminated through chronic or recurrent injury.
It is proprioception that generates muscle tone, and proprioception that governs robustness, not exercise. This is the folly of exercise regimes following injury. One study found that in most cases of knee injury, a degree of muscle inhibition remained in spite of extensive rehabilitation and exercise. In my opinion, the reason these researchers couldn't eliminate the inhibition they presumed was caused by the injury, is because the inhibition was there before the injury. In fact, the injury would never have occurred without the inhibition.
This inhibition came not from a lack of exercise, but from proprioceptive irritation. When the cause of the irritation is removed, the inhibition will vanish, whether or not the patient exercises.
Exercising after injury is like changing the oil every day when you have a flat tyre. You are doing something useful, but it's just not what the body really needs.
Correcting the Problem
The only way to correct muscle inhibition is to alter the proprioception that caused the weakness in the first place. Once muscle inhibition is corrected, joints are held in correct alignment and are able to resist whatever forces the body is subject to. Gradually, pains evaporate as ligaments heal. Even osteoarthritis pain diminishes as there is less strain on the joints.
With full muscle strength, people are able to work longer, play harder, run faster, jump higher, than they have been able to do for years.
Many people are told to rest or decrease their activities due to an injury. I hate to think how many millions of people have had their promising work or sports careers cut short because of abnormal proprioceptive input.
You cannot make your body work better if there is nothing wrong with your inhibition in the first place. Proprioception either works properly or it doesn't.
If you are one of the lucky ones whose proprioception works properly, then you probably wonder why other people get injured at all. You will have no understanding of the difficulties faced by those with persistent muscle inhibition because you have never felt what it is like to be weak.
Proprioception cannot really be trained, and it is difficult to treat with exercise, but spectacular results can be achieved by locating the source of the abnormal proprioceptive input and removing this permanently.
Belen Camacho said..
I work as a teacher with children with special needs and I have realized that many school problems have to do with the propioceptive sense. Unfortunately, they are misdiagnosed and this system is never addressed in therapy. What does this mean Proprioception cannot really be trained, and it is difficult to treat with exercise, but spectacular results can be achieved by locating the source of the abnormal proprioceptive input and removing this permanently??
Shawn Avery said..
I am a most difficult person when trying to understand some basic simple things. I will be soon graduating from occupational therapy assistance program. I am a much older student that has gone back to school, I'm 48 today. This is my problem Proprioception seems so complicated to me and I cannot get my head around it. I understand that it has to do with position in space. Position in space, I'm not sure if I quite understand that. My Occupational therapist had a patient sitting on the edge of the bed. She had her reach and grab a balloon, the patient was a little shaky. She then grabbed a bigger ball and had her grab it. This time she seemed not so shaky. She had asked me whatI thought she was looking at and I had told her core strengthening. She said actually proprioception. I didn't say anything, but I didn't get it. Can you please explain it to me where even I can understand the importance of proprioception and the significant of it when working with someone who perhaps has muscle weakness, or trunk weakness. Also what does this mean? Proprioception cannot really be trained, and it is difficult to treat with exercise, but spectacular results can be achieved by locating the source of the abnormal proprioceptive input and removing this permanently.