A push-up is a basic calisthenics exercise that works the pectoral muscles, triceps, and anterior deltoids. While primarily targeting the chest, arm, and shoulder muscles, push-ups also involve the abdominal muscles to keep the body aligned as well as muscles in the back, glutes, biceps and forearms that act as stabilizers. Military recruits often perform push-ups as part of their physical training to build upper body strength. Correct form engages the chest, shoulder, arm, core and back muscles through flexing and extending the elbows to lower and raise the body in a straight line.

1. push-up
A push-up is a common calisthenics exercise beginning from the prone
position. By raising and lowering the body using the arms, push-ups exercise
the pectoral muscles, triceps, and anterior deltoids, with ancillary benefits to
the rest of the deltoids, serratus anterior, coracobrachialis and the midsection
as a whole. Push-ups are a basic exercise used in civilian athletic training
or physical education and commonly in military physical training. They are
also a common form of punishment used in the military, school sport, and
some martial arts disciplines.
While the push-up primarily targets the muscles of the chest, arms, and
shoulders, support required from other muscles results in a wider range of
muscles integrated into the exercise.
Military recruits will often perform push-ups as part of their physical training.
Here, U.S. Marine recruits at Marine Corps Recruit Depot San Diego perform
push-ups in May 2005, as a part of their basic recruit training.
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.
Deltoid
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

2. 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.
Chest muscles
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. 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.
Stabilisers: back body
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 stabilises the upper
leg. The medius and minimus sit under the largest butt muscle, the gluteus
maximus.
Triceps brachii
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.
Forearms
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.
Biceps
During the push-up exercise, the short head of the biceps brachii muscle acts
as a dynamic stabilizer. This means the muscle activates at both ends—the
elbow and the shoulder—to help stabilize the joints.
Joints and tendons

3. Inner muscles that support the operation of the fingers, wrists, forearms, and
elbows are also worked isometrically. Some push-up modifications that
require to have the arms at different heights effectively engage the rotator
cuff.
video tutorial:
https://uii.io/pushup1
https://uii.io/pushup2
https://uii.io/pushup3
https://uii.io/pushup4