Capturing human anatomy is a lifelong pursuit. A great first step in that journey, as well as a fun daily warm-up, is gesture sketching. There are many different approaches to quick-pose sketching, and this video explains a 'constructive anatomy' technique. Based largely on the methods of Andrew Loomis, this technique combines observational sketching with memorized anatomy to capture poses in less than one minute.
To give this exercise a try, grab your sketchbook and open up www.posemaniacs.com. Though the default time interval is 30 seconds, I'd recommend selecting 60 seconds. Remember: the goal is learning, not creating beautiful drawings. Good luck!