The muscles that are built up during push-ups are the muscles in the core, chest, shoulders and back. The muscles of the arms also benefit from this exercise, which requires no equipment whatsoever.
The muscles in the core are those that support the stomach area and the lower back. These include the transversus abdominis, the diaphragm, the longissimus thoracis, the muscles in the pelvic floor and the internal and external oblique muscles.
Shoulder muscles include the muscles of the rotator cuff, the deltoid muscle, the latissimus dorsi and the teres major.
Muscles in the arm include the biceps brachii, the triceps brachii and the brachialis.