Sensory awareness or proprioception
Before I go on, I should say right from the get go that for me proprioception ties in directly to the muscles. Our muscles allow us to proprioceive and they allow us to move.
Without muscles we wouldn't be able to proprioceive. But we also wouldn't be able to move either. And so nature has equipped us so that the same things that cause us to move also allow us to feel movement.
These may or many not be the qualities that Dave is pointing at when he talks of sensory awareness.
As for the stretching aspect, Dave apprenticed with Kit Laughlin, who teaches stretch therapy. And Dave's developed his own brand of stretching. If you follow his facebook page, you'll often see pictures of people being stretched by one or two or even three other people all at once.
My own exposure to being stretched by other people is fairly limited. I've done some partner stretching. With a partner who knows what they are doing it can feel really nice. When you've got a partner who is, say, less than delicate in the application of their strength, (a brute in other words) then the experience is less than fun and I dare say less than beneficial.
So how might this type of stretching be similiar to tai ji?
When you are being stretched by someone else, you have the opportunity to relax the muscles being stretched. When you are being stretched by a few people who know what they are doing, then you are being supported while you stretch. And that makes it easier to relax and as a result you get a relaxed stretch.
Note that part of the set up may be making sure that whatever muscle or muscles is being stretched has a fixed end point, so if your leg is being stretched, then the torso, and in particular the hip, is stabilized.
The main point is that you can relax and be stretched. The stretchers do the work. Their muscles do the work, guided by their experience, understanding, and guided by the fact that they are tuned in to the body being stretched.
Assuming you are the body being stretched, what you can then experience is connective tissue being lengthened while the muscle it is a part of is relaxed.
Dynaments and a partial model of a proprioceptive sensor
My own model of a muscle (how I understand it) is based in part on Jaap van der Wal's idea of a muscle . A basic muscle includes the belly of the muscle, plus both tendons and ligaments at either end.
In general, tendons attach to bone while ligaments attach to joint capsule. When a muscle is activated, ligaments add tension to joint capsules while tendons move or stabilize bones.
In some cases, tendons also affect joint capsules. And in some cases ligaments affect the relationship between bones.
Basically, the difference between ligaments and tendons isn't always clear cut.
Van der waals calls this basic building block a muscle dynament, and he talks, in his paper, about how ligaments are active whenever muscles are active. And he talks thus how sensing happens not just at extremes of movement, but throughout them.
and targeted stretching of connective tissue within the belly of a muscle
At one stage of my yoga teaching career I taught meridian stretching. Here, the idea for stretching the meridians, was to relax the muscle being stretched so that you could stretch the connective tissue within the muscle.
Lung meridian, right, large intestine meridian, left.
Since meridians lie or are contained within connective tissue (at least that was the theory) you could best stretch the portion within the belly of a muscle by relaxing that muscle. In the process you then stretched it along with the associated ligaments and tendons.
Three stretch sensors in series
(connected end to end!)
Based on that understanding, I formulated a slightly modified model of the muscle. In it, the ligaments and tendons at one end could be represented by one stretch sensor, at the other end by another. And finally, the connective tissue within the belly of a muscle would be represented by another.
So, each dynament could contain three stretch sensors connected in series (end to end). There could be variations of this just as there can be variations of the basic dynament that van der waals proposed, but that was the basic model.
Relaxed stretching and the sensation of stretch
When a muscle is relaxed, but anchored at one end, you can stretch all three segments of connective tissue. Thus you get a stretch sensation. And in my limited experience, this is what you can get when someone else is stretching you and you are relaxed (but with a stable foundation for the muscle being stretched).
While I've not done that much partner stretching, I have done a lot of relaxed stretching, both within the meridian yoga context and outside of it. My approach to relaxed stretching generally involves using gravity to provide the driving force of the stretch. As such the muscle being stretched can be relaxed (with one end anchored). Thus it was easier to feel the stretch without worrying about muscle sensation from larger muscles getting in the way.
While I started with relaxed stretching, now, my tendency, due to various pains and problems I've experienced both in my body and while trying to teach others, is a mix of active and relaxed stretching.
Before going on, I should say that with relaxed stretching (whether the driving force is provided by a partner or by gravity) that stretching sensation is proprioceptive feedback that connective tissue provides! However, it is only half the proprioceptive information that your brain needs in order to figure out how the parts of your body relate.
What is active stretching?
Active stretching is a type of stretch where either the muscle being stretched is active, or muscles that oppose the muscle being stretched are active.
Another type of active stretching involves repeatedly resisting the stretch, and then relaxing. You can then go deeper into the stretching position during the relaxation phase.
Now before going on, what provides the other half of the propioceptive information that your brain needs in order to figure out how the parts of your body relate?
Muscle activation sensation, it's proportional
Muscle activation sensation is the sensation generated in the belly of a muscle when it is active. It is the feeling in your biceps when you flex them. It's the feeling in your pecs during the active phases of the pec pop of love. It's the burn in your quads (and/or hams and/or glutes) when you are holding a squat with thighs level.
In the same way that connective tissue stretch sensation is proportional to stretch, muscle activation sensation is proportional to activation. The more a muscle is activated the stronger it feels. (But at the same time, it's inversely proportional to the amount of stretch. So, the greater the stretch a muscle is under, the less sensation it puts out if it is active.)
You won't get a stretch unless you stretch>
Connective tissue only gives out a stretching signal if it is being stretched
Connective tissue needs be stretched to some degree (which requires an anchored end point) to register stress and deformation. When there isn't stretch, the brain can't read information about the relationship in question.
To accurately read how the parts relate, the brain also needs muscle output information, how much work each muscle is doing. That in combination with stretch helps the brain figure out the position of the body.
Note that muscles won't output information unless they are turned on. This is like an engine. The RPM gauge doesn't register RPMs unless the engine is on. And since RPMs are only useful when the engine is on, it kind of makes sense.
The difference between sensing larger muscles and smaller ones
When larger muscles activate, like the quads for example, sensations tend to be dominated by sensations generated by the belly of the muscle. With smaller, thinner muscles, like serratus anterior, sensations tend to be connective tissue sensation dominant.
What's the difference?
With larger muscles it's generally easy to feel them activate via their muscle activation sensation. That's how we learn to flex our biceps, and after doing bench presses a few times it's also how we learn the pec pop of love.
With smaller thinner muscles, muscle activation sensation is harder to feel.
How you feel when these smaller muscles are activated is via the connective tissue stretch generated in opposing muscles.
For example, spreading the shoulder blades can be driven by the serratus anterior, but you feel it not in the serratus anterior but in the rhomboids, which are being stretched (while at the same time they may be active, resisting the stretch and also providing a force for the serratus anterior to activate against.)
Generally, it's easier to feel muscle activation sensation than it is to feel connective tissue stretching. However, if you've done tai ji, and you've had a good teacher, or are smart enough to have figured it out by yourself, or you've learned to stretch while being aware, then you've possibly learned to feel and differentiate sensations that result from connective tissue tension.
Note, based on conversations with one of my teachers, connective tissue tension is one type of qi. It's a subtle sensation and when you can feel it, it generally means that your larger muscles are relaxed.
So how is proprioceptive sensation generated while doing Tai Ji?
In tai ji, the focus is on relaxing muscles as much as possible. Obviously some are active to keep you upright and moving, but the focus tends to be on smaller muscles that have less of a muscle activation signal. Thus it's easier with these muscles to feel connective tissue tension. That is generated by the weight of bones hanging and being suspended from other bones.
Their weight stretches the connective tissue within the muscle that supports them. These muscles are active, their bellies are stretched, but because these are smaller or thinner muscles, their lack of bulk makes it easier to feel the stretch because it isn't dampened out by the sensations generated by active muscles.
Note that in tai ji, connective tissue sensation isn't as loud as it is while being stretched because the stretch isn't so great. I'd go on to state here that the last thing you want while doing tai ji is a stretch. Or at least not a maximal stretch.
The benefits of moving slowly and smoothly
Another key quality of tai ji that lends itself to improve proprioception is it's slow, and smooth, movement. And I daresay in any decent stretch the last thing you do is sudden or abrupt movement.
Slow and smooth, and even slight movements, make it easier to focus on feeling your body and I'd say that they actually force you to focus on what you are doing. Most of us haven't turned slow and smooth movement into a habit and so we have to focus to do it. In the process we can become more aware of our body.
And so with "being stretched" assuming it is mostly passive stretching (either using gravity to drive the stretch or the muscle power of somebody or somebodies external to yourself) then what you can learn to tune into is connective tissue tension.
Can active stretching also improve proprioception?
Now what if the type of stretching being done isn't passive, but active? Can it still improve proprioception?
With active stretching, one possible mode is to activate the muscle being stretched, and allow it to gradually lengthen while it is active. Here, the belly of the muscle is lengthened, thus the sensation it creates, is less strong. and so you get a sensation of being stretched but also of muscle activation at the same time. An alternative is to activate muscles that oppose the muscles being stretched.
In either case you are activating muscles, and since muscles activation and connective tissue stretching are both a part of the mechanism of proprioception, yes, active stretching can help improve proprioception.
The approach I take is to teach proproception and muscle control first. (You can't have one without the other.) And I generally teach it in non-stretching actions or positions so that then it can be gradually applied within the context of a stretch.
What are some tips for improving proprioception?
Teaching muscle control and proprioception, I generally teach students to turn muscles on and off. It's akin to turning a light switch on and off, repeatedly. You thus see which light the light switch works.
Doing this with muscles gives you a repeated change in sensation that you can focus your awareness on. Additionally it potentially gives you the benefits of both relaxed and active stretching and addtionally you can use it even when you aren't stretching.
In all cases, whether stretching or not stretching I'd suggest once you've learned to turn a muscle on and off, focus on doing the activation and the relaxation slowly and smoothly. If the activation creates a movement, then do the movement slowly and smoothly. This makes it easier to notice your body. It also makes it easier to keep it safe.
The nice thing about this method is that it, along with other body awareness basics like weight shifting and touch sensitivity, gives you proprioception and muscle control that you can apply to anything that you do.