If you want to bring up a lagging body part or muscle group, the trick is not to jump into over-training it. You actually want to UNDER train it, train it consistently, and add the intensity, and then volume of intensity. The surrounding anatomy is generally proportionately lagging which can result in immediate injury, or more likely a strain or sprain. And of course that immediately sets you back again.
Likewise, if you believe your entire body is under-trained or out of shape, in the new year do not jump right into training full tilt. Not only is it rarely sustainable, it’s a straight shot from weak, to weak AND injured.

css.php