Most gym-goers learn about the body in constituent parts (i.e., the foot independent from the thigh independent from the core) and, thus, train it that way. We have "arms days," "legs days," core routines, etc. But there is a more efficient and effective way to train, be it from the metabolic perspective (burning calories during the session and post-workout), stability/strength perspective (building full-body foundational strength/support) or even from the cognitive performance perspective (boosting the power of brain performance). This mode of training is full-body, integrated training.
What I mean by full-body training is doing exercises that require the whole body to contribute to the drill. Doing a lunge or squat is not enough. If you are doing a lunge or squat, consider adding a task such as shoulder-to-overhead press with the lunge/squat, for example.
Incorporating full body exercises into your routine will strengthen not only the muscles, but the connections between them as well, making the body strong and stable for activities and life. Incorporating full-body integrated exercise into your routine is easy. No matter what your goal and/or exercise selection, simply add 5-10 minutes of full-body exercises to your workouts. It could be a forward step with a one arm cable press, a bend-over row to a bicep curl, a transverse step with your tricep pushdown or essentially any movement with resistance, which requires a full-body contribution to get the exercise accomplished.
I recommend that you keep your exercises simple to begin with (i.e., forward movements before lateral movements before movements requiring a turn), and incorporate 5-10 minutes of these movements in each of your workouts, varying the exercises from session to session.
You'll find that when you move the whole body, you start to feel and move better, and in much less time than you'd normally spend on each of your "ab days," "arm days" or "legs days" because you'll be training consistently with the body's inherent design.