The muscle spindle is a type of muscle fiber embedded in connective tissue that is designed to sense tension and changes in tension.
Portions can sense rapid changes in tension.
Portions can sense sustained tension.
Connective tissue doesn't have the elasticity of, say an elastic band. When a muscle contracts, the muscle spindle has to actively shorten with it so that it can sense when the muscle is subsequently lengthened.
If it didn't, it would go slack.
So the muscles fibers within the muscle spindle aren't for strength, but rather to keep the muscle spindle at the same length as the muscle it is embedded in.
One important idea is that when one muscle shortens, the muscle (or muscles) that oppose it lengthen.
Beyond a certain point it may not be feasible to contract a muscle spindle. But, the opposing muscle will be lengthened as well as the muscle spindle within it.
The type of sensation that a muscle spindle generates could be described as a stretching sensation.
This sensation is easy to notice during a relaxed stretch with at least one end of the target muscle anchored.
During active stretching, the targeted muscle can be active and resisting the stretch. In this case, muscle activation sensation is attenuated because of the stretch and it can still be relatively easy to feel sensations generated by the muscle spindles within the muscle being stretched.
In some muscles, muscle activation sensation is not very loud, even when the muscle is contracted. An example of this is the serratus anterior muscle. And so to sense when this muscle is active, the stretch sensation generated in muscle spindles of opposing muscles can give a sense of the serratus anterior activating.
In practices like tai ji where the focus is on "relaxation", smaller muscles tend to be used to keep the body upright. The weight of, say the arms, hanging from the upper body can create tension in the muscles spindles of smaller muscles. These muscles may still be active, but because they are smaller and/or their muscle signals aren't that loud, it can be easier to feel the sensations generated by muscles spindles being stretched.
Another way to generate muscle spindle sensation is to focus on making a body element feel long. For example, making the spine feel long can cause spinal muscles to activate in such a way that the spinal elements are stacked. Muscle tension is used to make the spine feel long. But at the same time, the lengthening action of stacking the vertebrae also stretches muscle spindles and this helps to register as the spine feeling long.
A similiar sensation can be generated by making the fingers feel long.
Muscle spindle stretching could be likened to stretching the investing fascia of the muscle being stretched. Investing fascia is the fascia within the belly or working part of a muscle. It's the fascia that contains muscle spindles.
The meridians in Chinese traditional medicine may in part (or wholly) relate to the muscle spindles.
Because the muscle spindles are embedded in fascia and fascia can be continuous from joint to joint and even across opposite sides of the joint, muscle spindles may part of a network that enables proprioceptive information to be promulgated locally, i.e. from muscle to muscle, and joint to joint, without having to wait for commands from the brain and Central Nervous System.
In plain English, if a muscle spindle is tensioned, it can transmit that tension to other muscle spindles directly. If two muscle spindles are in series, where the end of one connects across the joint to the end of another, tension in one muscle spindle can add tension to the connected muscle spindle.
Note some specialized terminology related to the muscle spindle.
It is associated with a gamma neuron meaning that it sends information to the brain. This is as opposed to alpha neurons which transmit information from the brain.
And so, gammas are muscle outputs and brain inputs. You could think of gammas as travelling upwards (assuming an upright body). Meanwhile, alphas are muscle inputs and brain outputs. You could think of alphas ad travelling downwards.
Note that if the brain is relied on to synchronize the muscle spindle with the muscle, movement can tend to becomes less smooth, less well coordinated. It's like movement is being micromanaged.
Muscles that are single joint and/or closer to the bone or joint may tend to have more muscle spindles. These are the muscles that are targeted in tai ji or chi gong or any other movement practice where body awareness is the goal or a necessary part of attaining the goal.
Larger more powerful muscles may have a lesser quantity of muscle spindles. However, these muscles are easier to feel and control for someone less well trained.
That doesn't mean that these muscles aren't important. However, if these muscles are relied on then the movements will be brutish in nature.
One suggestion for training the smaller muscles is to start with, where possible, unweighted movements with a focus on feeling and controlling the smaller or muscle spindle dense muscles. This can be done with particular muscles isolated and then trained in different poses actions so that control of these muscles and abiilty to sense them can be done without having to think about how to do it. The idea then is to practice in a whole body context, adding body weight, and if desired, further testing with weight or with accelerated movements or both.
What you then end up working towards is using both the muscle spindle dense muscles as well as the larger force generating muscles.
Note that the larger the forces being dealt with, the more the body has to work together as a coordinated and integrated whole both to protect muscles (by distributing forces among them) and also to protect joints (by tensioning joint capsules sufficiently to maintain lubrication and "friction free" operation.
The friction free operation is then what allows muscles to share and distribute tension.
To get to this state, a simple idea is to lengthen and/or create space in order to add tension to the muscle spindles. Note that with the idea of lengthening or creating space the tendency will be for muscles on opposing sides of a joint to activate against each other. The amount of activation can be adjusted by feel. Adding more muscle tension (by making joints or sets of joints "feel longer" or "more spacious") will tend to create a stronger activation but at the same time may also create a stronger stretching sensation in the muscle spindles because they are being pulled on with greater force.