That being said, let's talk about your adductors. The adductors play a crucial role in the squat as taught in Starting Strength. There are other versions of the squat that don't work the adductors, but this version provides knee stability... and works the adductors, which you should do.
The adductors are commonly known as the groin muscles. They run along your inner thighs, connecting your femurs to your lower pelvis. The adductors attach to the pelvis along either side of the pubic bone and to the adjacent sit bones (or "butt blades," in medical terminology). These work to bring your thighs together, and this process is called adduction. Abduction, when not referring to kidnapping, in this context refers to the opening of the thighs, usually driven by your glutes. More on those later probably.
The adductors and the hamstrings are vital for knee stability and safety during the squat. A squat that doesn't at least engage the hamstrings is dangerous for the knees. I recommend you read Rippetoe's chapter on the squat for more information on this, because he explains it better than I can. However, the gist is this: when you squat, your quads (tops of your thighs) pull upward on your knees, while the hamstrings and adductors are supposed to pull in the opposite direction, thus providing stability. Failure to engage the hamstrings and adductors can lead to knee injury. I've noticed that if I don't squat correctly, I hear my knee pop.
|Technically this is considered a sumo squat - wider stance, bar apparently not over mid-foot|
The squat works as a stretch for the adductors (as it does for the hamstrings), and the tension provided by the hamstrings and adductors at the bottom of a squat creates a "bounce" that helps you drive up and out of it. However, this bounce is only felt when one's back is in extension and the knees are pointed outwards, aligned with the feet. If the knees are too close together, the adductors are never engaged. A common problem for beginning lifters is actually that the adductors are too tight, making it difficult to keep one's knees out at the bottom of the squat. If one's knees are too close together in the squat, it makes it difficult to reach proper depth, protect the knees from strain, and stop one's back from entering flexion ("butt wink," as mentioned in part 2).
A handy way of thinking about stretching your adductors is thinking of it as pulling your pubic bone away from your knee(s). I know this may sound counter-intuitive. However, the more distance between your pubic bone and your knees, the more your adductors are stretched directly. This is part of the reason why it's important to keep one's back in extension while squatting: in lumbar extension, the pelvis is locked in place, and as one descends in the squat, the pubic bone is naturally drawn away from the knees (unless your hamstrings are too tight or your back is too weak!).
The best way to stretch the adductors for the squat is also the best way to start practicing for the squat; you squat. More specifically, you do a squat stretch. In yoga, this is called malasana. This is simply the position you'll end up in at the bottom of the squat. From standing position, make sure your heels are about shoulder width apart (or wider depending on flexibility needs), and your toes are pointed out at a thirty degree angle. Then squat, lowering your butt as far as you can. In no part of this should your heels leave the floor. When you're at the bottom, put the palms of your hands together and force your knees apart with your elbows. This will stretch your adductors and give you a nice preview of the bottom of your squat. You get extra points if your feet don't change position on the way down, if your thighs are aligned with your feet, and if your back is in extension, because you'll be doing all that in the actual squat.
Frog Pose is another helpful stretch for learning the squat, and it's a bit easier than the actual squat stretch. Here you're stretching the same muscles and engaging the same hip flexors, only without the pressure of gravity. Start on your hands and knees then spread your knees apart. Engage your lumbar and thoracic spine, putting your back in extension. Then slowly push back, feeling and pushing past the tightness of your hips. Do this slowly so you can get a feel of how flexible your hips are. This is a great pose for gaining some kinesthetic sense for what kind of pressures you should feel in your hips at the bottom of your squat, and for discovering your current range of flexibility. I recommend trying this stretch with and without engaging your psoas muscles. To do it without the psoas, relax them (if you can) and put all of your weight on your arms. Keeping your back in extension, you then push yourself backwards into the hip stretch with your arms.
The other adductor-stretching pose that anybody can do is butterfly pose. Sit on the ground, touch the soles of your feet together, and pull them towards you. For a deeper stretch you can try to fold your torso over your legs.
As with any of these muscle groups, there are a ton of yoga poses out there that stretch the adductors. Simply look up "yoga adductor stretch." Though at this point in this series, you can probably just start looking up "yoga hip openers" and you'll find poses that engage any of the muscle groups I've covered so far. Please be mindful to take safety precautions such as warming up and doing less intense stretches before the more intense ones.
Next in this series: I think there will probably be two more posts. The next one will probably be about calisthenics and yoga sequencing, taking all of these yoga asanas and putting them into a solid routine one can practice for squat preparation. The final post will put all of this information together and walk you through the squat itself. I'll probably go over some more hip openers in the former and Starting Strength in general in the latter.