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 flexibility 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) for touching your hands together in Warrior 1 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 (the humerus) can clear the accromion process. The accromion process is the piece of bone that crowns the shoulder. You can feet it at the top of your shoulder. It's where the collar bone attaches to your shoulder blades. It covers the shoulder joint helping to protect it. It also acts as an attachment point for muscles such as 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 or collar bone.)
In order for the upper arm to clear the accromion process in the arms overhead position (and thus avoid shoulder impingement), the shoulder blade has to tilt in such a way that the socket of the shoulder looks up. Another way to think of this is that the accromion process moves inwards while the bottom tip of the shoulder blade moves outwards.
Apart from avoiding shoulder impingement, this action is important because it gives the muscles that work on your arms from the shoulder blades room to pull your hands together when you arms are up over your head.
One way to achieve this upwards tilt of the shoulder socket is to spread the shoulder blades. Past a certain point the bottom point of the shoulder blade will move outwards more than the accromion process.
Another action that in isolation can help to move the accromion process inwards is to lift the shoulder blades. Again because of the collar bone attachment, the higher you lift your shoulders, the more the accromion process moves inwards. This is in part also because of the upper and middle trapezius muscles which create an inwards pull on the accromion process and collar bones as they pull pull the shoulders upwards.
If you like lifting your arms forwards and then up, you could lead both actions with your shoulder blades. Spread your shoulder blades as you reach your arms forwards. Keep them spread and lift your shoulders as you reach your arms up. You can lower your arms by reversing these steps.
If you're arms are already up, whether in virabhadrasana/warrior 1 or chair pose, you can start with the arms relaxed. Focus on lifting your shoulders. Better yet, focus on pulling your accromion process inwards as you lift your shoulders. You may find that it is then easier to move your hands towards each other, perhaps even getting them to touch.
Note that the muscle that pulls upwards and inwards on the accromion process, the middle fibers of the trapezius, 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. Try doing this prior to lifting your shoulders.
With the accromion process pulled up and inwards, the muscle that works on pulling your upper arm inwards is possibly some combination of the anterior deltoid and the upper fibers of the pectoralis major.
The important point is that in order for these muscles to pull your hands together they need a stable foundation from which to work. The pectoralis major attaches to the ribcage so you probably don't have to worry too much about it. But for anterior deltoid to function effectively, the clavicle and shoulder blade need to be stable. If you are pulling inwards and upwards on the top of the shoulder blade (accromion process) and the outer ends of the collar bones, that could be enough to give the anterior deltoid a stable foundation from which to work.
If you still have trouble bringing your hands together even though your shoulders are lifted, then it may help to accentuate protraction of the shoulder blades. That means moving them apart while your shoulder are lifted.
The basic instruction is to lift your shoulders, then move them forwards while keeping them lifted. Then keeping your shoulders forwards and lifted then working at moving your palms towards each other. Why might these shoulder actions be helpful?
As your shoulder blades protract the shoulder joints are moved forwards relative to the ribcage. That may actually cause the external rotation of the upper arms to increase. The fronts of the upper arms (the biceps side) will move away from each other. That in turn increases the distance that the fibers of the anterior deltoid work across giving the muscle more room to contract.
Note that one additional tip for avoiding shoulder impingement in virabhadrasana and any other "arms over the head" yoga pose is to pull downwards on the inner edge of the shoulder blades. This is in addition to spreading the shoulder blades. You still lift the shoulders, in particular the outer portion, which is where the accromion process attaches, but at the same time you create a downwards pull on the inner edge.
The net effect is to rotate the shoulder blades in such a way that the top of the shoulder blades move inwards and the bottom moves outwards. This creates space for the upper arms to move inwards while lifted withoug creating shoulder impingement.
Doing this, you recruit the bottom fibers of the trapezius muscle which attaches to scapular spine, close to the inner edge of the shoulder blades.
It's a tricky activation but it may happen automatically if you focus on spreading your shoulder blades to maximum as you lift them.
When getting ready to lift your arms in virabhadrasana, first spread your shoulder blades, as you reach your arms forwards. Keep your shoulder blades spread as much as possible as you lift your arms. As you move your palms together, then may find it helpful to create a downwards pull on the inner edges of your shoulder blades. But keep your shoulder blades spread as you do so.
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.