Most people probably think of standing forward bends as hamstring stretching poses. Of course you know better! If you do them properly, forward bends don't just stretch the hamstrings they strengthen them and the glutes. And if you do them even more properly, you can use them to strengthen your arms and even your torso.
Standing forward bend yoga poses tend to be easier with feet hip width apart and even easier with feet wider. In contrast they can be more difficult (or more uncomfortable) with feet together.
So as you get more practice with forward bends you can work at doing them with feet together. Using them for arm strengthening, I'd suggest feet parallel and hip or shoulder width apart.
Assuming that you can touch your hands to the floor with knees straight, one way to strengthen your arms (and legs) while bending forwards is to grab the big toes.
The general grip is with the first two fingers and thumbs, with palms facing inwards, towards each other. Pressing down with the big toes to provide some resistance, you can then pull up with your hands to strengthen your arms in a pulling action.
For a different action, try pulling forwards on your toes (as if trying to pull then out of your feet). This action can then help to strengthen your shoulders.
Note, there is no need to be a brute when doing this. Don't just yank on your toes. Instead, do these actions slowly and smoothly and sequentially, so that you can learn to feel and control parts of your body at the same time. For instance:
Next try it while grabbing your big toes.
Your lats extend all the way to your pelvis, with some strands attaching to the opposing glute so what you may find is that as you activate your arms and toes against each other your leg muscles also activate. You may find that your glutes activate to provide a foundation for the pulling action of your lats and your hamstrings may activate also.
Now what do you do if you can't reach your big toes? Get longer arms! Or, extend your reach with a belt or towel or even creative use of your yoga mat.
Stand sideways on your mat if you need to and grab a handfull of yoga mat on each side of your feet. Now instead of pressing down with your toes, resist with your forefeet.
Note that because of your lack of flexibility your torso will be closer to horizontal. That means your glutes will have to work harder and this is going to put more stress on your back. So be careful. Gradually increase the tension and stop if you experience pain. If you add tension (and then reduce it) slowly and smoothly you may find that you automatically make posture adjustments so that your body feels stronger.
If that doesn't happen then you need more focused training to help develop that ability to automatically adjust.
Now, rather that holding the action (no matter what your strength and/or flexibility level) try to gradually activate and then relax, and smoothly repeat.
Note that even if you are flexible, try the mat or belt option anyway. It gives you some more options for playing with. And thats one way of looking at flexibility. The option to do more things. For example:
Wow, you really like making this difficult don't you. Not only can't you reach your big toes, you don't have a mat either! Well gosh darn is, grab your ankles! Hook your hands behind your ankles or calfs, or wherever you can reach.
If I haven't already pointed this out, one benefit of using a mat in this way is that it helps you to improve grip strength, something we don't practice much in yoga class unless you are doing aerial yoga. Of course, now that I think about it, binding yoga poses are another way to practice improving grip strength, but only if you actually pull while you are gripping.
Find a position and grip that allows you to use the strength of your arms easily.
What direction to you pull in? For this exercise, pull forwards.
But even as you pull with your arms, resist the pull so that you strengthen your glutes and hamstrings as well as your arms.
I don't show it in the picture above, but you can also try gripping (and then pulling) on the inside of the ankle or shin.
And that goes for using a mat as well. Try a variety of different grip positions (palms in, as shown, palms back, palms facing outwards etc) to strengthen your arms in a variety of positions and to get used to using your grip in a variety of different positions.
The above arm strengthening standing forward bend variations can be used to strengthen the arms in pulling actions. But what about pushing actions?
Here again if you can't reach the floor you could just place your hands against the fronts of your shins. Push against your shins but resist your body moving backwards. You may find that this is actually a good way to train your abs and hip flexors.
If you can't touch the floor then you could also use a chair or yoga blocks.
With your hands on a chair, shift your weight forwards so that your hands automatically press down. Then, actively push your hands into the floor without allowing your torso to move upwards or rearwards. You could do this with the elbows straight (torso higher) or with elbows bent.
Practice both options.
If you are flexible enough to get your hands to the floor you can try the same exercise with your hands forwards. Shift your weight forwards then press your hands down.
Here again you can do this arm strengthening standing forward bend with elbows bent or with elbows straight.
You could also try it with your hands beside your feet. Here you don't have to shift forwards. Just press your hands down.
If using a chair you can simulate this position by positioning your hands closer to your thighs on the chair.
For more all-round arm and shoulder development you might choose to practice with fingers pointing forwards, inwards and even out to the sides. You could also practice with fingers pointing rearwards.
There is definitely nothing wrong with wanting to use yoga to get stronger or more flexible. But so that it is still yoga, or at the very least, so that it is still mindful, pay attention while doing these or any other exercises.
To the changes in sensation that occur when you shift your body and/or engage muscles, and to the sensations that occur when you relax.
It's a little like the fourth or fifth time you had sex. When you've had enough experience to actually notice what is going on. You where probably focused on the parts of your body that where touching someone else's body.
Paying attention in yoga is more or less the same thing except that you are paying attention to the sensations that are occurring inside your body. Where can you find sensation? Look at your muscles activating and relaxing. Look at changes in tension both within the muscles and across your joints. And look for changes in pressure where you contact yourself or things outside of yourself. And look for changes in touch, particularly since you are hooking your toes, your shins, your mat, or pressing yourself, a chair or the floor.
As a yoga teacher, I'm constantly exploring new exercises, new ways of doing yoga poses.
There is no single "right way" of doing a yoga pose. Instead, there are options. And the better you are at "feeling" your body, the better you can get at choosing the right option for your body as it is now.
For any technique, the point of practice is to learn feel it and to control it, so that it can be used without thinking about how to use it.
And that is more or less the approach taken in all of my ebooks and videos. They help you to feel your body and control it so that you can work towards using it effectively in anything that you do.