The way I see it, the pose as you drew it works, better understandable as a complete animation, because this is an "all-in" power punch. This means the power comes from the core of the body, rather than just extending the arm. So the force of the fist comes mainly from rotating the body and falling forward. The other arm backwards is not meant to be a punch as such, but flinging the other arm backwards helps rotate the core body / shoulder faster, which flings the forward arm harder into the front. The whole body is shortly tensed for control and conducting the core power into the tip of fist. Something like that I guess.
Something to improve in the drawing of it could be making the rotational aspect of the body more obvious, so that the body is not facing front in the end but sideways, left shoulder back, right shoulder forward. For actual animation: the correct power punch is an incredibly complex movement. the main force comes from coordination skill more than total muscle, in that the physics of momentum on whole body mass are fully utilized and make the actual power leading into the fist. As far as raw strength goes, the most important force is originated by the hips actually, on the lower body, initiating the rotational momentum of body, which travels slightly delayed into a rotation of upper body, where it is further exploded, and has a loose and leading shoulder sling shot the arm forward, almost like a trebuchet. It's very snappy, and the right timing of being loose and tensing up for a moment is key. But I'm not sure how much of this finesse makes for a good pa frame animation.
The body mass momentum / rotational / hips aspect is always most important, but sometimes more or less pronounced, depending of how much you want to expose yourself in the end as price for committing. So sometimes you barely even see it, but it always makes the brunt of force of any given punch from a skilled fighter.
