The pushup is an excellent movement for several muscles at the front of the upper body. These muscles include the pectoralis major sternal and clavicular heads, anterior deltoid, triceps brachii and anconeus. When you do the pushup using a wide grip, which is wider than shoulder-width apart, you work the same muscles as if you used a shoulder-width apart grip. However, if you have long arms, it is recommended you use a wide grip, so you can perform a better full range of motion.
The pectoralis major of the chest, more specifically the sternal head, is the prime mover of the pushup. This muscle originates at the sternum and inserts at the humerus, or arm bone. The other muscle of the pectoralis major, which is the clavicular head, is involved, although to a lesser degree, during the movement. This muscle originates at the clavicle, or collarbone, and it also inserts at the humerus. Both heads of the pectoralis major function to adduct and flex the shoulder in the transverse, or horizontal, direction, the latter of which is the main movement of the pushup. However, the angle of the pushup places most of the emphasis on the pectoralis major sternal head.
The anterior deltoid of the shoulder also flexes the shoulder in the transverse direction, among its many movements. This muscle, at the front of the shoulder, originates at the clavicle and inserts at the outer part of the humerus. The anterior deltoid acts as a secondary mover during the pushup, similar to the pectoralis major clavicular head.
Unlike the previous two muscles, the triceps brachii acts on the elbow. This is a three-headed muscle, comprising a long head, lateral head and medial head. The former originates at the scapula, or shoulder blade. The latter two muscles originate at the back of the humerus. All three heads of the triceps brachii insert at the ulna, or outer forearm bone. The muscle group as a whole functions to extend the elbow, which is another movement in the pushup. So, the triceps brachii also acts as a secondary mover.
The anconeus, similar to the triceps brachii, works to extend the elbow. This muscle originates at the back of the humerus and inserts at the olecranon of the elbow and the ulna. Similar to the triceps brachii, the anconeus is a secondary mover in the pushup.