MUSCLE IMBALANCE can mean more than looking like a crab. It can mean a disfunction between an agonist and its antagonist muscle, resulting in pain and/or mobility issues. For instance, a muscle imbalance could cause knee, shoulder, or low back pain. To avoid this, each time you train, try training your weak side first and matching--not exceeding--the reps, weight, and sets on your stronger side. I have done this since I got my first, concrete-filled plastic, weight set as a 14-year-old, and it has served me well. Also, work the muscles you cannot see (e.g., postural muscles) as much as you work the ones you can (those mirror muscles). Additionally, remember that the core is more than the muscles that comprise that illusive washboard look. Just as a lifting belt wraps all the way around your waist for maximal support, so do your core muscles. Lastly, if you already have pain, get evaluated by a medical professional. If you don't know one, let me give you a referral to one in my network.