Just as we have mental maps of the world around us, we have internal maps of our bodies. We know what position our limbs are in even if we can’t see them, and they are not touching anything. This is Proprioception: one component of the sense of self.
Many of the proprioceptors are located in the muscles. Each muscle spindle collects information from 8 – 14 muscle fibers. It is specifically designed to give us constant information about changing muscle length and the speed of that change. They are alert and active whether the muscle is at rest or working. They constantly transmit information to the Central Nervous System.
In fact, running alongside the other fibers, they act in a contradictory manner, so that when they detect stretch, they stimulate contraction. This prevents overstretching and damage to the fibers and requires no conscious input. Thus, it is called the stretch reflex.
The muscle spindles are primary spatial monitors for us, aiding in accuracy of movement, in maintenance of posture, and in our sense of where our limbs are in space.
Another proprioceptor is the Golgi tendon organ. This receptor is located in the collagen where the muscle and tendon join. Where the muscle spindle concentrates on stretch, the Golgi Tendon organs concentrate on contraction, or tension. The Golgi Tendon organ is only active when the muscle contracts. Their goal is to protect joints from a too forceful muscle action. Their message to the central nervous system is “relax that muscle”.
These primary proprioceptors may be aided in sensing position by tactile receptors in the skin, notably the Pacinian Corpuscles. These are pressure sensors. When joints bend, they put pressure or release pressure on different points of skin.
All of these receptors send information to the central nervous system where it is combined with information from the vestibular system (balance sense) to create the internal picture of body position in relation to itself and more globally, in relationship to the earth.
How does this relate to ergonomics? Frequently, when there are repetitive motion injuries the sense of touch is affected. There is some evidence that the related pain may disorganize or create an inappropriate representation of proprioceptive stimuli.*
Disruption in Proprioception or diminished touch sensation (not uncommon) may contribute to difficulty in completing everyday tasks. In dressing, for example, there are many actions that are difficult to see. With vision blocked, we must rely on our other senses - so, buttoning or tying behind the back may be more difficult.
With impaired sensation even tasks where the eyes can be involved require more attention. If there is extensive impairment, then cooking may become more risky as the sense of hot/cold may be impacted.
When your therapy involves temperature modalities (hot / cold) you will need to pay more attention to other indicators (such as redness in the skin) to determine if the temperature you are using is correct. You can also use a non-affected body part to test temperature (such as an elbow, the opposite limb, etc).
At the same time, splint fit is a real concern. Any redness, shininess, or sensitivity should be given attention.
If you have an Occupational or Hand Therapist, he or she can quickly test your skin surface tactile sensation and determine if there may be an issue.
I’ve attached a link to my favorite book about Touch and the human senses. It is filled with interesting information and is a great read, even for the beginner. It helped draw me into the field of Occupational Therapy.
*Harris, A. Cortical origin of pathological pain: The Lancet, Volume 354, Issue 9188, Pages 1464-1466