My advice is to practice gesture drawing first. The idea of gesture drawing is to give you a feel for the motion in the image and get you comfortable with recognizing the proper proportions necessary to make drawings that look right.
In case you haven't seen them yet, I recommend giving these articles a look-over:
You have to keep in mind that for many many artists, gesture drawings (including people who draw cartoons) are the core foundation of what makes a realistic image. Stiff images, even when anatomically correct, are not too appealing in most circumstances.
You will also find out that once you have captured the motion of the image, it's MUCH easier to build on that gesture sketch and create a detailed work of art. Almost every artist I know of starts with the basic 'circle for head, shape for torso, shape for hipbones, attach joints and limbs in the form of small circles and stick lines', and if you get this base wrong, you're much more likely to have a deformed or disproportionate end result.
Knowing anatomy by heart takes time and practice, especially if you want to get more detailed. You already cover about 50% of anatomy with gesture drawings though!
I hope this was helpful to you. :) Let us know if you have more questions!