Do you go by the feel of an exercise or do you go for the pump or a certain weight for reps or what?

I have trouble isolating certain muscle groups like lats, rear delts, upper pecs, lower triceps, outside of the calf and if I use heavy weights I just don’t feel like I’m working these areas. How do you use heavy weights and still isolate muscle groups?

Would it be better to use less weight and feel it in the muscle more or is the weight more important?

You cannot always get the exact feel you want. Don’t just work for heavy weights. Try moderate weights for a while using perfect form.

For the lats do a variety of pull downs, concentrating on the stretch.

Rear delts: give the incline bench lateral raise (body facing bench) a try.

Upper pec are worked well with dumbbell incline bench set at a shallow 30° angle.

Single arm triceps extensions work the lower triceps and the outside calf is worked best with standing calf raise putting stress over the big toe.