The left leg, the extended one during the kick, could do with some exaggerated curvature to the line.
This isn’t realistic, but it is effective in conveying dynamic kinetic motion.
Look at any Spiderman comic where he’s swinging from building to building. Even when his legs are just “dangling there” they’re curved in an unrealistic fashion, especially in the calf/shin. This conveys an expression of motion to the line. There is an appearance of slight curvature to the leg from the knee down to the ankle and curve is typically over-emphasized in dynamic posture via enhancing the curve or exaggerating it.
In plain english, take a look at that ref image you posted. Notice how from her feet to her knee there is a hell of an arc? Even if you measured from the tip of the toe to her knee there is a straight line, describing that line is the arc of shin…which anatomically speaking would indeed be a straight line, in terms of motion expression will be better rendered by beefing up that arc.
Haha, I said it would be “plain English” there in the last part, that was a lie. Anyways I see what you’re going for in the straight leg, the “snap” of the kick, and yes it may be more true to real life, but for the arts we want dynamism because we want to break through from reality into the realm of emotion and to do that you need to harness the power of line, form, energy, motion and convey it even if it means breaking the rules.
Anyone can take a photo of what is real, a child can do it without even knowing more than how to press a button. To make something that is static and lend it an impression of …dynamism…that goes beyond the norm, you can look at distorting reality for a bit, abstracting it from line to curve is a tried and true method used to great effect. Start there. Look at the lines, the main shapes, break them up into angles relative to one another, look at the negative space in between those angles. Imagine them moving, interacting with each other, playing against and off one another to form an interplay of motion and excitement. Get the people that you are intending this to be shown excited. Perspective, foreshortening and exaggeration to convey motion are all ways to get this to happen for you. A simple curve of her calf/shin here would do wonders. Then her arms, look at them, look at your own arm extended, then hyper extend it, then render that.