I'm a 50 y.o. female using dumbells (usually 8 or 10 pounds) and exercising to tone my muscles. I do 3 sets of 12 on most of my weight exercises. Sometimes, I don't feel the fatigue at the end of my sets. I found that doing 2 sets of 18 (still 36 reps) does fatique the muscle. Is this an acceptable option? Or should I be increasing the weights to 10 or 12 pounds?