A working knowledge of the anatomy of pitching and throwing is important for every player and coach to have.  The more informed you are about your body and how it works, the more you can control your training and adjust it to get maximum results.   Once a player realizes where he needs to be strongest to throw harder and decrease injury risk, it is often easier to motivate them to do the additional work.

So, today we’re going to discuss the main upper torso and shoulder muscles that accelerate and decelerate the baseball…

Anatomical Terminology

Anterior: in or toward the front of the body (when divided by the coronal plane)

Posterior: in or toward the rear of the body (when divided by the coronal plane)

Internal Rotation: to bring toward the body’s midline by  joint rotation. (When the arm is abducted, this brings the hand forward)

External Rotation: to carry away from the body’s midline by joint rotation (When the arm is abducted, this brings the hand backward)

Adduction: to bring toward the body’s midline on the coronal plane (ex: bring hands to your sides

Abduction: to move away from the body’s midline on the coronal plane (ex: raise arms to the side)

Medial: (describing the position of a bodypart) toward the midline of the body

Lateral: (describing the position of a bodypart) away from the midline of the body

Showing the 3 planes of movement

The Accelerators

Most of the upper body and shoulder muscles that accelerate the arm are internal rotators. They take the arm from…

1. Subscapularis

Subscapularis (Anterior View)

The most powerful muscle performing a given movement is called the prime mover, or agonist. The prime mover of internal rotation is the subscapularis, which is a member of the “rotator cuff” group.  This muscle covers the anterior portion of the scapula (shoulder blade) and inserts on the lesser tubercle of the humerus (upper arm).  The subscapularis, along with all of the rotator cuff, contributes in stabilizing the shoulder joint which helps prevent injury.

2. Teres Major

The Teres Major is another internal rotator, which originates on the posterior aspect of the scapula and inserts on the medial lip of the intertubercular groove of the humerus.

Muscle action can be figured out by knowing the origin and insertion, as they act just like hydraulics – lengthening and shortening to create movement.  Since the teres major inserts on the front of the humerus, when it contracts the arm will rotate internally.

3. Anterior Deltoid

Anterior Deltoid and Serratus Anterior

The anterior portion of the deltoid (anterior and middle deltoid shown) originates at the clavicle and inserts on the deltoid tuberosity of the humerus. Because the insertion point is on the more lateral aspect of the humerus, the anterior deltoid is a relatively weak internal rotator, but does assist nonetheless.

4. Latissimus Dorsi

The Mighty Latissimus Dorsi

The “Lats” originate from a large fascial attachment extending from the posterior illiac crest, to a few ribs and up to the thoracic vertebrae.  The lats are huge, powerful mucles that connect the hips to the core to the arm, and snake around the teres major to insert on the medial aspect of the humerus.

Most people are unaware of the lats’ role as an internal rotator, but as an extremely influential muscle, its incredibly important to have strong lats to aid in the acceleration of the throwing arm.

When you perform a chin-up you will feel your arms’ natural inclination to pronate inward as the lats attempt to internally rotate the humerus as it brings the body upward.

5. Pectoralis Major

Stronger Pecs… Stronger Fastball

The “Pecs” originate from the sternum and clavicle to insert on the lateral aspect of the humerus.

When the arm is abducted, as in the cocked throwing position, the lateral insertion point on the humerus allows the lower fibers of the pectoralis to aid in accelerating the arm forward.

Exercises that strengthen and develop the chest are often thought of as unnecessary vanity exercises, but this is far from the truth.  The pitcher who wishes to reach his velocity potential should be as strong and flexible as possible in all of the muscles that assist in internal rotation, which is a major factor in pitch speed as well as injury prevention.

6. Serratus Anterior

The serratus muscles are so named because of their sawtoothed appearance, and act to pull the scapula toward the anterior part of the body.  The serratus anterior originates on the upper ribs and inserts on the anterior portion of the scapula’s medial border.  In throwing, the feeling of fully extending and “really reaching out toward the target” is the serratus anterior pulling the scapula forward.

The Decelerators

These muscles are responsible for cocking the arm back and slowing it down after the ball is released.  Make note that there are more muscles performing internal rotation than external rotation, which makes strengthening the external rotators crucial to preventing injury.

1. Infraspinatus

The Infraspinatus originates on the medial aspect of the scapula and inserts on the posterior of the humerus.  It is the prime mover of external rotation. It is a member of the rotator cuff group along with the Supraspinatus, Subscapularis and Teres Minor.

2. Teres Minor

The Teres Minor originates on the lateral aspect of the scapula and inserts on the posterior aspect of the humerus.  It sits right next to, and strongly assists, the infraspinatus in performing external rotation.  Both the teres minor and infraspinatus also help in maintaining posterior shoulder stability.

3. Posterior Deltoid

The posterior deltoid originates on the spine of the scapula and inserts on the lateral aspect of the humerus.  The deltoid is subdivided anatomically into three parts: the anterior, middle, and posterior.  This is because all three divisions perform different actions depending on the position of the humerus and the plane in which they are active.

Supporting Roles in Deceleration

The trapezius, rhomboids, biceps, levator scapulae all attach to the scapula and help stabilize and slow it during follow-through.  While the infraspinatus, teres minor and posterior deltoid are the main muscles slowing the arm down, throwing is a whole body motion and recruits a very large amount of muscles in varying degrees.

Part II of this anatomy series will discuss the lower arm muscles, including the hand, forearm, triceps and all the interactions between them.

But in the meantime, start strengthening these muscles! Learn how in my articles on the LYTP Shoulder Circuit and The Pitcher’s Complete Shoulder & Rotator Cuff Prehab Circuit.