If you've ever tightened a bolt or a screw, you've probably noticed that the tighter a bolt gets the harder it is to tighten it further.
You may feel extra pressure where your skin is pressed between bone and the metal of the wrench, but in addition your muscles have to work harder.
The same can be observed tightening or loosening the strings of a guitar or violin or adjusting the spokes of a bicycle wheel. The tighter the strings, the harder you have to work to tighten them further.
Pulling on the sheets of a sail boat, the stronger the wind, the harder you have to work to pull the sheet in. (Sheet is the nautical term for the ropes used to adjust sail tension).
In all of these examples, it's variations in muscle output that help you to notice how tight a bolt or string or sail sheet is.
There's a device called a torque wrench. If you've never used one, it's a wrench with a dial on it. It actually uses your muscle power as a means of measuring torque output. As the bolt you are tightening gets tighter, the dial shows an increase in torque. And so via this device you can tighten the head bolts of car engine the specified amount.
In order to measure how tight a bolt is you have to actually use the wrench. You have to apply muscle power to get a measurement.
Note that torque is a measure of force at a perpendicular distance from a center point. It's force that is applied in such a way as to cause a rotation (or resist it).
You could just skip using the dial and use your muscles to measure the torque instead. It's just that the wrench has a dial that is calibrated.
With enough experience, say on an assembly line or in a repair shop, some mechanics know when the torque is right by the feel. Repeatedly doing the same actions, they've effectively calibrated themselves for measuring particular levels of torque.
Measuring torque using our own bodies, it's more than just muscle activation sensation that helps us to measure how much force or torque we are generating. There's connective tissue tension also.
If you've ever tried "truing" a bicycle wheel, after riding over a bumpy road or perhaps after getting stuck in a sewer grating you've more than likely noticed that variations in spoke tension change the shape of the rim. Tighten a spoke with enough tension and you can use that spoke to pull its section of rim towards itself. This can be used to distort a rim or, if it was already distorted by, say, a clumsy rider, then it can be used to un-distort it.
One way to tell how tight a spoke is (or for that matter the string of a guitar or violin) is to pluck it. Another way to tell how much tension is present is to notice the distortion produced as a result of that tension.
Overly tight instrument strings bend the instrument. Overly tight spokes can bend a wheel (or as mentioned, be used to help straighten it).
Over and above a particular tension level distortion can give you a clue as to how much tension is present. But from another point of view, the amount of tension can give you a clue about distortion. Noting the tension in a guitar string, an overly tight string will give you a hint that perhaps the guitar has started to bend.
Within our bodies spoke tension would be the equivalent of connective tissue tension.
Connective tissue is the stuff that makes up our ligaments, tendons, joint capsules. It can also be found within the belly of each muscle.
When a muscle is lengthened, stretch can be felt at the ligaments and tendons and along the connective tissue within the belly of the muscle itself.
An easy way to tune into this type of sensation is to practice relaxed stretching.
In relaxed stretching, the idea is to keep the muscle being stretched relaxed.
An important point is to anchor one end of the muscle being stretched. It's like telling the brain that you want to move in a specific direction. Anchoring one end of the muscle by stabilizing it helps to give your brain the necessary information. So for example, to stretch thigh muscles you could stiffen the knee to anchor muscles at the knee or you could stabilize the hip joint to anchor muscles at the hip.
Depending on the body part being stretched, stiffening the foot and ankle is another way to anchor leg muscles.
Lengthen a muscle sufficiently, while at the same time anchoring one end, and you'll create a stretch in the ligaments and tendons as well as the connective tissue within the muscle itself. Thus you get a feeling of "stretching" or lengthening.
Because the muscle being stretched is relaxed, muscle fiber doesn't inhibit stretching of the connective tissue within the belly of the muscle.
If the muscle being stretched isn't relaxed, then it's muscle fibers may resist stretching of the connective tissue within the muscle. However, it may still be felt as a stretch. However you may also get muscle activation sensations as well. It depends on the amount of stretch and the density of the muscle fibers in question. What you may find is that past a certain point the sensation of muscle activation is so weak as to be relatively unnoticeable compared to the sensation of stretch.
One way to do relaxed stretching is to use body weight to help drive the stretch. In a standing forward bend you could use the arms to support the weight of the upper body initially, then gradually reduce the support of the arms so that you can keep the hamstrings relaxed as you work at stretching them.
Using arms to support the weight of your body initially, muscular tension in your arms can help inform you that your arms are activate and are active in working against the weight of your body. Another strong hint that this is the case is a feeling of your hands pressing down into the floor (or if you are less flexible, whatever support you are using for your hands.)
As mentioned, muscular activation is an active way of feeling your body. However, in order to sustain activation, a muscle or set of muscles needs an opposing force to work against.
Sticking with the standing forward bend, you could try to relax your arms but instead of trying to keep your hamstrings and glutes relaxed, you could actually activate them. Working to prevent your upper body from moving as you relax your arms, your hamstrings and glutes can activate and work against the weight of your upper body. In addition your spinal erectors and other back muscles will also activate against body weight to prevent your upper body from sagging.
If you do a forward bend with one leg in front of the other in pyramid pose, with both legs straight and the arms reaching back, you may find that the glutes and hamstrings of your front leg not only give you a stretching sensation since they are both lengthened you also get a muscle activation sensation from both muscles. (You may get muscle activation sensation in the back leg too.) And if you work at keeping your chest open you'll also get spinal erector activation sensation. Reach your arms forwards and you increase the moment arm that your muscles are working against. And you'll feel this as increased effort in your legs and spine. Thus your muscles are helping you to measure an increase in moment arm or torque.
In the absence of external forces, muscles can work against each other to mutually sustain muscle activation. There's a game where you lean your hands against a partners hands and step your feet back from each other. Both of you have to keep your hands and bodies active to keep you both upright. With muscles, in the absence of any external force to work against, they can work against opposing muscles which has the effect of not only creating sensation but of stabilizing the joint they are working across.
Flex your biceps without allowing any elbow movement and your triceps will also activate. Activate your quadriceps without allowing knee movement and either your gastrocnemius or you hamstrings will activate to oppose the activation at your knee. In either case you get sensation that can inform you of where your legs are in space.
With smaller muscles muscle activation sensation is less obvious. With the hips for example, you don't need a lot of muscle power to stand on one leg. Standing with legs slightly bent and shifting to one foot, you may notice, if you pay attention not only an increase in muscle activation in the thigh of the supporting leg, but you may also connective tissue tension in and around the hip joint.
In this case smaller hip muscles may be activating to help support the weight of your body, but because these muscles are smaller, muscle activation sensation is not as obvious and so instead what you can learn to identify is connective tissue tension.
Most breathing exercises rely on the sensations created by muscle activation and connective tissue stretch. Bar exercises where the focus is on the actual orifices that you breath through you are generally focused on the sensations generated by the muscles that you use to breathe with.
One of the ideas then of muscle control is that it isn't just about "controlling muscles" its also about focusing on and improving proprioception by focusing on the things that generate sensation in the first place, your muscles.