Good amount of sets and repetitions for different muscle groups

Question

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.

Answer 1

Your goals determine the number of sets. (Your goals also may require you to change the exercises you do.) It's possible that the goals of the trainers at your gym are not the same as your goals.

Possible Goals

Training for strength, power, endurance and hypertrophy all require a different number of sets and reps:

• Strength (how much your muscle can move) is best developed with 5 or less reps. The weight must be heavy enough to make more than 5 sets very difficult. 3x5 or 5x5 are the two most common set/rep schemes.
• Power (how much your muscle can move quickly) is roughly similar to strength. You'll use slightly less weight so that you can move it faster.
• Endurance (how long your muscle can keep doing its job) requires many, many more reps: at least 15. You'll have to use a lot less weight.
• Hypertrophy (how big your muscles are) is best developed with a moderate amount of weight, done for 6-12 reps. You should be able to get 5 or so sets in.

This is well explained by a chart in the article I linked to:

The next crucial step (which you didn't ask about) is making sure that the exercises you do are in line with your goals.

• For strength, you'll want to do compound exercises across entire ranges of motion. So: squats, deadlifts, overhead presses.
• For power, you'll want to get a baseline level of strength, then work the Olympic lifts: clean, snatch, jerk.
• Endurance is activity-specific, so you should do exercises that mimic whatever you want to do.
• For hypertrophy, you'll want to get a baseline level of mass by doing strength training (as above), then work specific muscle groups using both machines and free weights. Using machines prematurely will make it harder to gain substantial muscle.