Hello, when doing sit-ups, doing it a few times more can enhance muscle endurance and reduce fat, but it will not stimulate muscle fibers.

If you want to make your muscles bigger and thicker, you need to stimulate them with heavy objects. For example, pushing the chest, without heavy stimulation, strength and circumference will not increase significantly.

When doing sit-ups, put your hands on your chest and hold something 10KG. Do it again. Slowly increase the weight.