Anatomy trains (or myofascial meridians) are lines of connective tissue which in part resemble the pathways of the meridians of Traditional Chinese Medicine (TCM). The proponent and discoverer of these anatomy trains is Tom Meyers who first theorized them based on his own therapeutic experience. He then commissioned specifically designed cadaver dissections to see if these structures actually existed.
His team found proof of these myofascial meridians. They are continuous sheets of fascial/connective tissue containing discrete pockets of muscle. While there are many ways of dividing the connective tissue network into potential meridians, Meyers picked his based on clinical applicability and usefullness.
Whether you learn the myofascial meridians or not one very important concept to understand with respect to connective tissue and muscles in general is that muscles are laid on top of and within sheets of connective tissue.
(Tom Meyers describes muscles as being located within pockets with this web.)
Because of the way it is nestled within this network, when muscle tissue contracts or relaxes, it creates or releases tension in the sheets of connective tissue to which it is attached. Thus muscle tissue can shape or adjust the connective tissue web potentially affecting the body as a whole.
This same connective tissue web doesn't just form tendons, it also forms the ligaments that hold joints together. This means that muscle tension doesn't just affect the relationship between the bones it acts on, it also directly affects the joint capsule.
(Another possible interpretation is that the state of the joint affects ligaments and via those ligaments the muscle itself (since it is directly situated on top of the connective tissue that connects to those ligaments.)
Because muscle cells are located with connective tissue sheaths, lines of muscle can form a train (an anatomy train!) with muscle tension from one muscle potentially affecting other muscles in the same train.
And like in a mechanical train, you can liken the connective tissue to the undercarriage of a train carriage and connection between each pair of carriages. Meanwhile the muscles would be like the carriages themselves.
A better model would have the carriages able to contract or relax, bringing sets of wheels on the same carriage closer together or further apart, thus affecting the length of the train as a whole.
If you are having trouble contracting a muscle or getting it to relax (so that you can stretch it) the solution may lay in contracting another muscle in the same anatomy train.
An anatomy train is literally a train of muscles connected via the same "sheet" of connective tissue.
One example (which corresponds in part to the inner leg meridians of TCM, most specifically the kidneys) is the "deep front line."
A portion of the "Deep Front Line" anatomy train creates a connection between the diaphragm and the psoas among other muscles.
Knowing this, one way that you can help to stretch or lengthen your psoas is to pull upwards on the back of the diaphragm.
Because the diaphragm is anchored by the lower edge of the ribcage as well as the lumbar spine, and because these attachments are below the dome of the diaphragm, you can contract the center of the dome of the diaphragm downwards (while inhaling). This action creates an upwards pull on the ribs and lumbar spine. (Think of holding on to a kite string while the kite is flying. You pull down or back on the string to pull the kite towards you. However, the wind resists and the feeling you get is of the string being pulled away from you.)
If you focus specifically on the part of the diaphragm that attaches to the lower back ribs (ribs 11 and 12) you can use the diaphragm to lift these ribs. This same part of the diaphragm can at the same time be used to pull the upper fibers of the psoas upwards.
I was extremely surprised when I first felt this first hand in myself.
In most anatomy books the furthest upwards extent of the psoas is T12 (the lowermost thoracic vertebrae) and so just looking at muscle attachments with no consideration of connective tissue connections you'd get no idea that you could use your diaphragm to stretch your psoas.
Because I could feel the connection I assumed I'd missed this connection in my anatomy books. But then I found that none of them mentioned a connection of the psoas to the lower ribs or diaphragm.
The only connection that I could find documented (and there I had to infer it) was that the psoas connected to the diaphragm via the renal fascia (the connective tissue covering the kidneys.
And becuase I experienced this lengthening under the guidance of Richard Freeman, I knew that there must be a connection. The only one I could find, and even that was guesswork, was that the diaphragm had a connective tissue connection to the psoas (and quadratus lumborum) via the renal fascia.
(Note that "spreading the backs of the kidneys" or "lifting the backs of the kidneys" is another good cue for learning to lengthen the upper fibers of the psoas upwards.)
The deep front anatomy train showed that there was a connection between the psoas major and the diaphragm and it was a very direct connection.
Another important connection made apparent by the deep front line anatomy train is between the psoas and the upper adductors (brevis, longus, and the pectineus).
Doing a bench press I was trying to stabilize my upper body in part by expanding the backs of my kidneys. (This is the same action I mentioned above. I've found it helpful when trying to lift up into a handstand with straight legs and so I thought it might also be helpful in bench pressing.)
It didn't seem to help my lifting technique. But then I thought of trying to engage my adductors while reaching my back ribs away from my pelvis. In particular I found that squeezing the upper adductors (the contraction feels like it is in the upper half of my inner thigh compartment) helped to solidify my upper body making bench pressing that much easier. I felt stronger. Also sharp shoulder pain which had been present before was absent.
(Note that even though I was using my adductors, I wasn't focused on squeezing my legs in. My thighs where parallel with each other with shins vertical and feet more or less parallel to each other. When I activated my adductors I could feel them contracting but I didn't feel any inward pulling on my knees. It felt like I was using them to add tension to the deep line connective tissue train as opposed to using them to pull my thighs inwards.)
I suspect that in this case adding tension to my adductors (without pulling my legs in) helped to stabilize my psoas and the rest of the deep line making my ribcage and upper body stable enough to make bench pressing relatively easy.
So one benefit of understanding anatomy trains, is that you can use muscles in one part of the train to help stabilize other muscles (and give them a firm foundation) in the same train.
And if one thing doesn't work, you can experiment and try something else, using your understanding of the anatomy trains to guide your experimentation.
Prior to learning about the extent of the connective tissue web, and also prior to learning about anatomy trains, one general principle that I believed in was that of the need for creating a firm foundation. My notion was that a firm foundation allows "targeted" muscles to either contract or relax as required.
Why does muscle require a foundation to relax or contract?
One analogy is standing on the side of a slippery mountain with a big drop below. The normal tendency is to tense up unless we have good awareness and control in which case we might be able make ourself relax.
For the person who hasn't got that control, it isn't until they reach solid ground that they find it easier to relax and even then it may take some time.
On solide ground we can relax or tighten up at will. When the ground is less stable we might not have much of a choice, we tighten up to create stability within ourselves.
How did this apply to controlling muscles. I figured that if a specific muscle hasn't got a firm stable base then it tenses up to create that stability. At the time I thought that by stabilizing one set of bones to which a muscle is attached, I was giving that muscle a stable foundation so that it could either relax or contract as required.
As an example, if moving the leg relative to the torso, I would suggest stabilizing the pelvis and lumbar spine. Moving the torso relative to the legs, stabilize the foot, ankle, shin and bottom half of the knee.
Understanding even a small portion of the connective tissue network (and the anatomy trains offer one way of understanding it) makes creating a foundation easier.
Now to create a stable foundation from which muscle control is easier, tighten a portion of the anatomy train to which that muscle is a part of.
If parts of a muscles anatomy train is slack then that muscle may tighten up automatically making stretching difficult. It might also make it difficult to tighen muscle, (say while laying down on a bench while doing a bench press.)
And so the answer in either case is to add tension to some part of a muscles anatomy train.
One way of visualizing this is via a rather special tug-o-war team.
Imagine a two part tug of war team. One team holds the rope but stands on a carpet that can slide relative to the ground. This carpet is attached to a rope which the second part of the tug-o-war team holds on to (the "carpet pullers"). If the carpet pullers don't pull on their rope and the carpet, the first part of the tug of war team can't pull effectively on their rope because the carpet gets pulled with them. However if the carpet pullers act to pull on their rope (and the carpet) the first half of the tug-o-war team now have a firm foundation so that they can pull effectively on their own rope (and the two together act to win.)
If you learn the anatomy trains, you can figure out which part of your body acts as the carpet pullers so that another part of your body has a firm foundation. So, to gain better control over your muscles, whether stretching, strengthening (or stabilizing) be aware of your body, feel it, notice it, and tighten up any slack spots.
If you understand the anatomy trains then you can use that knowledge to help you figure out where to take up the slack.
Or you can just experiment with adding tension until you find the application of tension that works.