About 10 years back, when I went to my first gym, the trainers told me to do 3 or 4 sets with about 8 to 15 repetitions on 3 different machines for each group of muscles, depending on the goals I want to achieve.
Now I wonder if this rule is true for every muscle group, because the muscles are so different. The small biceps gets exhausted pretty quickly while I can do a lot more repetitions when I train my stomach muscles. It feels like different muscles need a different amount or type of training. Is this correct and are there some guides I can use for orientation? Or am I over thinking this and my standard approach of 3 sets with 12 repetitions on 3 machines works well for everything? I am most concerned about stomach training and training my leg muscles.
Edit: Adding more information as requested. I am currently trying to do the first exercise without a machine (barbells for example) and the other two with a machine. I am currently targeting my upper body (breast, shoulders, upper and lower back, stomach) to gain some muscle and stability and also train my legs to gain more elasticity for sprinting, because I play flag football and need to be fast.
My usual workout consists of 3 groups of muscles. Yesterday I started with shoulders (3 exercises, 3 sets, 12 reps), proceeded to upper back (3 exercises, 3 sets, 12 reps) and finally stomach (3 exercises, 3 sets, 15 reps). Warm-up consists of about 10 to 15 minutes on the cardio bike and cool-down is the same, with additional stretching.