I've seen worse.
The best way to get a handle on perspective and foreshortened limbs is from a book I have, called 'Dynamic Figure Drawing', by Burne Hogarth.. (I'm reminded I need to go dig that book up now... If I even still have it.) but in thinking of the joint, one must consider which 'face' of the protruberance to highlight (patella or elbow) with a curved 'accent' line. Think about which part of the limb is dominant, to the front, the one that overlaps the other the most. (I'm talking the relation between the upper and lower limb) Right now I can't tell of the left arm is going forward or backward, because their's no overlapping muscle masses to cuew my eye. If the triceps were over the deltoid a little, then it would be coming toward the viewr. It'd be the opposite for having the arm away from the viewer. Then look at the elbow. A 'U' shape to define the knob at the back, that would show the arm as being upper dominant, the lower arm would be more behind it. an 'n' shape there would have the forearm more or less continueing along the line of the arm, as opposed to being bent. In your case, you have a very sketchy 'U', Ned to accent it a little more. Make the joint 'pop' out of the paper.