Hello. Exercise should be "composed" based on personal experience, level of physical fitness and the body's ability to recover. Take a beginner's workout as an example. So, his training week will consist of two workouts. Usually they are divided into the first, which includes exercises of all muscles of the upper body, and the second includes the muscles of the legs.
Sometimes deltoids, as well as biceps and triceps can be added to the second training day. For those who have just come to the gym, workouts are not divided at all and they perform exercises for all muscle groups at each workout.
Naturally, this is done with minimal weight and minimal number of approaches. But, and for those who have already reached the next level and his training consists of three to four workouts per week, in each of which six to eight exercises, muscle groups should be divided in more detail. For example, the first workout is aimed at working out the pectoral muscles and triceps. The second is for the muscles of the back and biceps, the third is for the deltoids, and the fourth is for the muscles of the legs.
As a rule, this is the use of synergistic muscles in one workout. Exercises are basic, or multi-joint (several muscles of the joints are included) and isolating (one joint is involved in the exercise). If you train three times a week, it will be correct to mix the basic exercise and the isolation exercise in each workout. For example, on the day of your chest workout, you do a barbell (or dumbbell) bench press. In this exercise, the triceps acts as a synergist, so it is advisable to pay attention to it in this workout. Biceps is actively involved in back exercises, so after, for example, various traction, it makes sense to pump it (one or two exercises maximum, biceps is a small muscle, overwork it - it will not grow). On the third day, when you train your legs (and this is half of our body, you need to devote a lot of time to them), add, for example, training for deltas (because in the case of a high-quality leg training, you have little strength left for anything else). p>
But here I will note that everything written above is one of the classic training options, and is not an axiom.