The shoulder is one of the most complex joints in the body. The shoulder joint is formed where the humerus (upper arm bone) fits into the scapula (shoulder blade), like a ball and socket. This allows a wide range of motion at the shoulder joint. The shoulders comprise of 2 muscle groups – the superficial deltoid group and the deeper rotator cuff group. The shoulder is involved in every upper body movement that involves the arm. All pushing or pulling movements that we perform in day to day life involve the shoulders. They are used for sports such as tennis or badminton, where a fast change of direction is required. But, if we look at the structure of the shoulder joint, it is highly unstable and also happens to be overused. Hence, it is more prone to injuries. Balanced and proper training for equal development of all the heads of deltoid and the rotator cuff is crucial to shoulder health.
The largest of the shoulder muscles is the deltoid. Deltoid muscles form a triangular shape at the shoulder which explains their name. It was previously called the deltoideus and the name is still used by some anatomists. Deltoid is also further shortened in slang as "delt". It gives the shoulder its rounded-off shape. The deltoid group adds enormously to the aesthetic appeal of the human body. Well rounded well developed deltoids make a body look leaner as it gives the illusion of a smaller waist. The deltoid muscle is constructed with three main sets of fibers: anterior, middle, and posterior. These fibers are connected by a very thick tendon and are anchored into a V-shaped channel that is attached in the shaft of the humerus bone in the arm.
ANTERIOR DELTOID – These fibers originate on the clavicle and insert in the middle of the lateral aspect of the humerus shaft on a bump known as the deltoid tuberosity. The anterior deltoid is commonly called the front delt. The function of the humerus bone is to flex the elbows at the shoulder joint as we can see in front dumbbell raise and abduction of humerus when arm is externally rotated. Anterior deltoid also assists your chest muscles while doing a bench press.
MIDDLE DELTOID - Also known as the lateral head of the deltoid, these fibers originate from the lateral upper surface of the acromion and inserts into the deltoid tuberosity. The function of this head is the abduction of humerus at the shoulder joint as we can see in lateral raises. It does not come into play in any of the torso movements, either pulling or pushing; neither do we train this muscle in day-to-day life. The “wide shoulders” look can only be achieved with the full development of the middle deltoid.
POSTERIOR DELTOID – Known as the rear head. These fibers originate from the spine of scapulae and inserts in the deltoid tuberosity. Often neglected and overlooked, development of this head absolutely crucial to shoulder joint health. A lop sided development in favour of the frontal deltoid will cause instability in the joint. The function of this head is shoulder hyperextension as in when the elbow is pulled behind the body and abduction of the humerus in the horizontal plane as in the case of high rows or what are popularly known as Face Pulls.
As the name suggests, these muscle together hold onto the ball of the humerus and keep it firmly within the socket like a cuff. Stability of the shoulder, to a large extent, is maintained by a healthy rotator cuff.
Rotator cuff is the most injury prone area of the shoulder complex. Hence, strengthening the rotator cuff is quintessential to maintain shoulder health. By nature, they are more like tendons than muscles and thus are largely avascular.
Most of the injuries in the shoulder joint are rotator cuff based and, due to their avascular nature, the prognosis for these injuries is not good. Hence, rotator cuff injuries have to be avoided at all costs. Strengthening the often neglected rotator cuff muscles will, to a large extent, prevent injuries and improve one’s functionality. Rotator cuff is made up of four heads.
1. SUPRASPINATUS - This muscle originates from the posterior face of the scapulae and inserts at the head of the humerus. The function of supraspinatus muscle is to initially help in the abduction of the humerus at the shoulder joint. Like in case of the cable lateral raise, the work of lifting the cables sideways to almost 30 degrees is done by the supraspinatus.
2. INFRASPINATUS - This muscle originates from the posterior face of the scapula and inserts on the lateral head of the humerus head. It causes external rotation of the humerus at the shoulder joint. This muscle gets trained in dumbbell or cable external rotation.
3. TERES MINOR - This muscle originates from the posterior face of the scapula and inserts on the head of the humerus but inferior to the inferior of the infraspinatus (should this be interior?). The teres minor also causes the external rotation of the humerus at the shoulder joint. Teres muscle could be trained on dumbbell or cables.
4. SUBSCAPULARIS - This muscle originates from the anterior surface of the scapula. These muscles cause the internal rotation at the shoulder joint. Generally, there is no need for strengthening internal rotation as it is achieved by the big muscles of chest and back assisting the subscapularis.