You can gain weight in this way, but the effect won't be too obvious, and you have to figure out whether you simply gain weight or gain muscle. The parts you practice like this are too single. The body is a balanced body. If you practice the strength of one part strongly and ignore the practice of other parts, then the other parts will become weak links. For example, the body is like a wooden bucket filled with water, but if a wooden board in this wooden bucket is short, it will also be unable to hold water. If you only practice the abdomen and arms singly, these parts will become thicker, but you neglect to practice its antagonistic muscles.

And you should pay attention to whether your training method is correct or not. A simple movement can't be completed by one muscle, while a complex sports movement, under the coordination of several muscles or muscles, can make links produce various movements or make people maintain a certain posture. According to the function of muscle in exercise, it can be divided into prime Mover, active muscle, secondary Mover, antagonistic muscle, fixed muscle and neutral muscle. So I suggest you develop in a balanced way.

You should control the number of times in each group. The principle of weight gain is to give enough stimulation to the muscles, break the muscle fibers, and then absorb nutrients to thicken the muscle fibers, so as to achieve the purpose of weight gain you want. To sum up, you should reduce the training times of abdominal muscles and increase the intensity of training. For example, when doing sit-ups, put fast barbells or other heavy objects on your abdomen to increase the stimulation. See table for details:

load strength reference table

in most cases, 12 times is appropriate. Moreover, each group had better be trained in a pyramid way, which can be continuous weight increase or continuous weight reduction, and do four to five groups.

,,,,, Anything you don't understand, you can continue to ask me.