Working your abdominals doesn’t have to be sweating in the gym doing crunches and leg lifts. Sometimes doing ab exercises can hurt your lower back I focus on the athlete and the non athlete. If you want to tone your abs there are several ways most everyone knows them. But if I am not in the gym and sometimes I’m not in the gym, I will do crunches on my bed because the floor hurts my back, are they effective yes, if you only wanting to tone and tighten your core. A yoga ball is great a great workout for your core also. You can also do twist for your oblique muscles. For lower back you can lay on your stomach half of the bed and do lifts.
If you are in the gym you usually have a choice of machines,plank exercises and chairs. You may have a back lift machine. I have worked my obliques with a cable crossover I just grabbed both sides and twisted. Abdominals can be worked out everyday if you want, weight to me isn’t that big of a factor reps are the more the better. Sometimes I like to invent my own exercises just to try something different.