its clear to me that you may understand a bit about various muscle groups - but you've failed to study how they flow together. here is an image I whipped up to help demonstrate what would take much too long with words...
Anatomy:
I wont say much here other than study my edit, but more importantly - study from photo-reference. I don't claim to be perfect at anatomy, but you cannot go wrong with photos.
Palette:
firstly, I adjusted your palette to be a bit more even across the ramp. Your last shade made no sense in how it jumped to a ridiculously high saturation. some colors were too close together, and others were too far apart
Composition:
You need to grasp a sense of depth and forms. utilize your palette to show these things. Currently you're barely making use of it at all - barely even touching your darks only for the outlining of things. Try not to think of lines, but rather shapes.... and how the light would react to these shapes. Also, try flipping your canvas horizontally occasionally while you work. This will help resolve a lot of symmetry issues you're currently having.
-Dan
