I agree with pretty much everything that has already been said. I advise for you to study more on anatomy, including both the muscular and skeletal structure. Your character's backs are simple one stroke, along with many other parts of the body coming off as so, which is why they come off as looking so stiff. Look at your hand alone and notice all the curves in them, don't rush your work out for the sake of completing it. Quality over quantity.
"You are not special. You are not a beautiful or unique snowflake. You are the same decaying organic matter as everything else." - Tyler Durden