How muscles work

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Abdominals
The rectus abdominis and transversus abdominis contract continually while performing push-ups to hold the body off the floor and keep the legs and torso aligned. The rectus abdominis spans the front of the abdomen and is the most prominent of the abdominal muscles. The transversus abdominis lies deep within the abdomen, wrapping around the entire abdominal area. Both muscles compress the abdomen, and the rectus abdominis also flexes the spine forward, although it does not execute this function when performing push-ups.

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The anterior portion of the deltoid muscle is one of the major shoulder-joint horizontal adductors, moving the upper arms toward the chest during the upward phase of a push-up. It also helps control the speed of movement during the downward phase. The deltoid attaches to parts of the clavicle and scapula, just above the shoulder joint on one end, and to the outside of the humerus bone on the other. Along with horizontal adduction, the anterior deltoid assists with flexion and internal rotation of the humerus within the shoulder socket.

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The push up requires the work of many muscle groups, with one of the primary muscle groups being the chest muscles, the pectoralis major and the minor.[6] These are the two large chest muscles and the main pushing muscle group of the upper body. When pushing and lowering the body during a push-up, the pectoralis major is doing most of the work. As a result, these muscles become very strong and can become defined as lean muscle after doing push-ups regularly.

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The push-up depends on stabiliser muscles as the body is pushed and lowered. The erector spinae is the main stabiliser muscle in the back. Made up of three muscles including the spinal, longissimus, and iliocostalis. The spinal runs adjacent to the spine, the longissimus runs adjacent to the spinal and the iliocostalis runs adjacent to the longissimus and over the ribs. Two muscles called the gluteus medius and gluteus minimus stabilise the upper leg. The medius and minimus sit under the largest butt muscle, the gluteus maximus.

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While the anterior deltoids and pectoralis major muscles work to horizontally adduct the upper arms during the upward phase of a push-up, the triceps brachii muscles, or triceps for short, are also hard at work extending the elbow joints so the arms can be fully extended. The triceps also control the speed of elbow-joint flexion during the downward phase of the exercise. The closer together the hands are placed during a push-up, the harder the triceps work. The muscle is divided into three heads — the lateral head, long head, and medial head. The lateral and medial heads attach to the back of the humerus bone, and the long head attaches just behind the shoulder socket on one end; all three heads combine and attach to the back of the elbow on the other.

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Stabilizers include wrist and forearm muscles, the knee extensors, and the hip/spine flexors, which all work isometrically to maintain a proper plank position in the standard prone push-up.