I remember when I first started doing Ashtanga Yoga, and for a good while afterwards, I always used to have trouble touching my hands over my head in poses like Virabhadrasana (Warrior 1) and Utkatasana (Chair pose or Awkward Pose). Even worse was holding my hands together. Some teachers were mean and didn't allow me to interlock my thumbs.
(And that's one of the joys of being a yoga teacher, making students suffer because you know it will be good for them in the long term.)
Are there easier ways to learn to touch the hands together? Hopefully this article can help. Of course it is based on a little bit of anatomy plus doing things in slightly different ways.
A very simple way to get the required flexiblity to touch the hands together over the head in Virabhadrasana is to stretch the shoulders. Use one arm to stretch the other using this shoulder stretch. If you can't straighten one arm then try it with both elbows bent to begin with.
Another approach (you can use both) starts with learning to feel and control your ribcage, neck and your shoulder blades.
Basically to lift the arms overhead your shoulder blade has to be positioned in such a way that the arm bones can clear the accomion process. That's the piece of bone that crowns the shoulder. You can feet it at the top of your shoulder muscle (deltoid). It's a part of the shoulder blade that covers the shoulder joint. It's also the point to which the middle fibers of the trapezius attach as well as the fibers of the middle head of the deltoid.
(The upper fibers of the trapezius and the front head of the deltoid both attach to the outer end of the clavicle.)
Three basic ways for the upper arm to clear the accromion process in the arms overhead position is for the upper part of the shoulder blade to move inwards while the lower part moves outwards. Because of the connection of the shoulder blade to the sternum via the collar bone this could happen in a few ways. One is for the bottom part of the shoulder blade to move outwards more than the upper part as the shoulder blade moves outwards. Another way is for the outer edge of the shoulder blade to move upwards more than the inner part as the shoulder blade moves upwards.
With arms down and then up, you could practice pulling upwards and inwards on the accromion process. With the arms up (but hands initially separated) you may find that just pulling upwards and inwards on the top of the shoulder blade helps to bring the hands closer together and that's an action you can use in both warrior 1 and chair pose.
Note that the muscle that does this action is the middle fibers of the trapezius which attaches to the neck. So that this muscle can work effecitvely you may find it helps to lengthen the back of the neck by pulling the head back and up and by lifting your chest.
With the accromion process pulled up and inwards you can then work on using your anterior deltoid (the head that attaches to the clavicle) to pull the upper arms inwards.
When moving the shoulder blades forwards, the collar bones will prevent the top of the shoulder blades from moving too far forwards. As a result the bottom edge can move outwards more than the upper edge. This can also result in the accromion process moving inwards giving clearence for the arm bones. And so another method involves first practicing spreading the shoulder blades. Start with the arms down, then after spreading the shoulder blades reach the arms forwards while keeping the feeling of separation between the shoulder blades. Then reach the arms upwards keeping the spread feeling. To help the accomion process move inwards, and the bottom tips of the shoulder blades to move outwards, you can recruit the bottom fibers of the trapezius muscle by pulling downwards on the inner edges of the shoulder blades.
So if coming up to warrior 1 from downward dog as in sun salutation b, first spread your shoulders then reach your arms forwards and up. If sinking down into chair pose from standing you can first bend the knees, then reach shoulders forwards, then arms forwards and up.
If using the first method (using the traps), start by moving top of the the shoulders up and inwards. Thne lift the arms and touch the hands overhead.
Either action can also be used in the first movement of sun salutation a, lifting the arms up over the head. In either case lead the action by initiating with the shoulder blades. Or better yet, initiate with ribcage and neck followed by shoulder blades followed by arms.
With practice you may find that lifting the arms and touching them overhead becomes easier. You don't have to focus on first moving the shoulder blades, and then the arms, it happens automatically.