Using effort (or exertion) to measure tension
If you've ever been sailing, and worked the sails, you've noticed how, when pulling on a sheet, how a strong wind working against the sails makes it harder to pull in, while working against a weaker wind is easier.
The actual act of reefing in a sail can tell you how strong a wind is just by how much effort you have to exert.
The same can be true tuning a guitar.
Turning a tuning peg while the string is slack, you can feel the peg turning with little resistance (the friction of the screw itself providing that resistance.) As the string gets tighter and string tension increases, the peg gets harder to turn. The increased effort required to tighten the string further tells you that the tension has increased.
To get further information, or confirmation of the state of the string, you could pluck it to hear how close it is to being tuned.
Work on truing a bicycle wheel and the same idea applies. This is assuming that it is the old type of bicycle wheel, the type with metal spokes, the greater the tension in the spoke, the harder it is to tighten the spoke further. Go around all the spokes and you can feel, just by trying to tighten or loosen each spoke, how much tension is in that spoke.
Measuring effort with a torque wrench
A torque wrench is a type of wrench with a gauge that measures how much torque is being exerted via the wrench. You pull on the wrench to tighten a bolt and as the bolt becomes tighter, the torque that the wrench measures becomes higher. But, it only measures torque when torque is applied.
Tension is Measured by Muscular Effort
In all the cases mentioned above, tension is measured by muscular effort.
By working to add tension or reduce tension on a sail sheet, guitar string or bicycle spoke you can feel how much tension there is. And changes in the amount of work effort tell you whether you are adding tension or reducing it.
You could continually measure the tension by continually exerting force, perhaps not quite enough to tighten or loosen a spoke, but enough that if you stop just short of actually tightening or loosening, that continual output could be used as a measurement.
Calibrating effort
Some people who use a torque wrench regularly as part of their job, particularly when tightening bolts to the same torque, can learn to recognize by feel when a bolt has been tightened to a particular torque. The sensations they can learn to recognize, and quantify, can include skin pressure, connective tissue tension and muscular effort.
A Comprehensive Proprioceptive Network
An important factor to take into consideration, and here the bicycle wheel is a particularly useful analogy, is that muscles don't act in isolation. They work against each other (on opposite sides of a joint) or with each other (along anatomy trains, or chains or meridians of fascia that contain muscles in pockets of connective tissue between joints.
Thinking just in terms of connective tissue (lets assume that the muscles fibers stay relaxed), moving a joint in one direction creates more space on one side of a joint and reduces it on the other. This lengthens or stretches connective tissue on one side and shortens the space across which connective tissue spans on the other side.
You could think of this in terms of increasing or decreasing stretch. You could also think of this in terms of increasing or decreasing connective tissue tension (which isn't the same as muscle activation.)
Passive Proprioception via Stretching
With a bicycle wheel, add equal tension to all spokes and the hub will be equidistant from the rim at all points, no matter how the tension that is added to each spoke. However, vary tension in the spokes so that some have more tension and others less, and you could displace the hub relative to the rim (or vice versa.)
As an example, tighten the spokes on the right side of the wheel and loosen them on the left and you'll slide the hub to the left relative to the rim.
Now lets say the change was driven by an outside force. Push the hub to the left while keeping the rim stable and you might just dispace the hub relative to the rim. How do you then figure out that displacement via the spokes? By the amount of tension in the spokes on the right side compared to the tension in the spokes on the left.
Using Stretching to Proprioceive
Lying on your back with your eyes closed, and imagining that all you can feel is changes in tension in your connective tissue (your muscles stay relaxed), if someone picks up your right leg and moves it towards your chest while keeping your knee straight, at some point you'll feel an increase in hamstring tension that tells you that your leg is being flexed at the hip.
Lying on your belly and having someone bend your leg at the knee so that your heel moves towards your butt, you may feel at some point your quadriceps and hip flexors being stretched, and again that can tell you what is happening to your body.
The only problem with these two scenarios is that they require your tissues to be stretched to a particular degree before you can get any sort of sense of what is happening.
And so for someone who is "tight" they'll get sensory data a lot sooner than someone who is loose.
In either case, there has to be a certain level of stretch before proprioception occurs.
Why We Need Muscular Effort for Proprioception
Imagine driving at night on a road that has it's lanes marked by cat's eyes. Imagine that the only way you could sense where you were with respect to the lane is to actually drive over the cat's eyes. To know where you where with respect to the lane you'd have to continue to drive with one set of wheels going over the cat's eyes.
This would be the equivalent of trying to operate the body while only relying on connective tissue stretch for proprioception. We'd only be able to feel parts of our body when connective tissue is being stretched. And that tends to happen only when a joint is near it's end range of motion.
How then does our brain sense the disposition of our body when we are doing the equivalent of trying to drive in the center of our lane, i.e. without running over the cat's eyes?
I'll suggest that muscle output, or "muscle activation sensation", provides the necessary information so that our brain can potentially sense the disposition of our body through all ranges of movement, not just the end ranges.
It's like playing with your iphone. It only works if it has power. You can touch your iphone all you want but if it isn't turned on it isn't going to sense your loving touch.
Likewise with our body. While there is some passive sensitivity, if we want to fully activate our powers of proprioception, we need to activate and use our muscles.
In terms of feeling what is going on inside our body, proprioception has two components. One is connective tissue tension. The other is muscle output.
Published: 2020 08 17
Updated: 2020 11 04
