The Biomechanics of Baseball Pitching — Part II

This work is part of my dissertation submitted at the University of Aberdeen in partial fulfilment of the BSc in Physics in May 2021.

During this research I was supported by physicists and experts to whom I am grateful. In particular I would like to thank Prof. Jan Skakle, Alessandro Rosa Colombo, Gary McCoy, Brent Strom and Ron Wolforth.
I want this work to be publicly available to athletes, coaches and researches as a source of information to support athletes development.

Chapter 2 — Basic Anatomy of the Upper Body Limbs

When analysing the biomechanics of baseball pitching and the forces acting on the shoulder and elbow joints, it is fundamental to refer to the anatomy of the upper body limbs. It will be shown in later chapters that the pitching motion requires voluntary contractions of the musculature as well as compensatory forces that prevent the bones in the joints from detaching from one another. In order to discuss those biomechanics aspects, a detailed presentation of the anatomical structure is required.
The upper body limbs are very complex to describe, therefore, emphasis will be placed mainly on the structure of bones and ligaments, and muscles and tendons that are essential for the good functioning of the shoulder and elbow joints, as these are also strongly involved in the baseball pitching motion.
The whole anatomical presentation will come together in the last section of the chapter, as the natural range of motion of the upper limb during every-day activities is described and a brief introduction to compression forces will set the ground for the upcoming chapter on muscle recruitment during the throwing motion.

2.1 Structure of the Upper Body Limbs

Upper body limbs are an essential component of the human body, as they allow us to grasp and manipulate objects.
The upper body limbs are divided into five regions: shoulder, axilla, upper-arm, forearm and hand. For the purposes of this dissertation, all the constituents of shoulder, upper-arm and forearm will be presented in detail.
The skeleton of the upper limbs is designed to provide two features: great range of motion, and stability and strength when moving objects.

2.1.1 General Bone Structure

Human bones are divided into five categories based on their shape: long, short, flat, irregular and sesamoid bones. The arm is mainly composed of long bones to provide great mobility. There are over 30 bones in the upper limb, with the main five being the clavicle and scapula (in the shoulder region); humerus (in the upper-arm); and radius and ulna (in the forearm), (Fig.1). Beside the scapula (which is a flat bone), all these bones have a longitudinal shape, (Jacob, 2007; McCausland et al., 2018).

The clavicle connects the upper limb to the trunk, creating a “bridge” from the sternum, on the front, to the scapula, on the back of the thoracic cage. The ensemble of these two bones (clavicle and scapula) forms the “shoulder girdle” where the arm is anchored, (Fig.1).
In the shoulder girdle, a ball-in-socket joint is formed between the glenoid cavity (cup) and the head of the humerus (ball): the glenohumeral joint. This joint is far more shallow than other ball-in-socket joints (such as the hip), and allows for a greater range of motion of the arm. The shoulder girdle is coated with a synovial membrane to minimise friction when with the humerus head, (Fig.1), (Jacob, 2007).

The humerus is the longest bone in the upper limb and the main component of the upper-arm. As in many other long bones, the diaphysis or shaft of the bone, is made primarily of cortical bone, a rigid and dense material that avoids bending and breaking, and where the yellow marrow is contained; the epiphysis or heads of the humerus, are made of cancellous bone, a more flexible material with an open honeycomb structure that contains red marrow, (Fig.2), (Villa & Mahieu, 1991; Gerber & Nyffeler, 2002; DoITPoMS, 2011). It is also referred to as trabecular or spongy bone.
At the extremities, the bone widens to better connect to other bones, and the cancellous material around the heads is covered by an articular cartilage that provides shock absorption and minimises friction, (Fig.2), (DoITPoMS, 2011).
The good functioning of the humerus is essential for the health of the upper limb as it is the only bone that appears in the structure of both the shoulder and the elbow, (Jacob, 2007; Innerbody Research, 2017).

The elbow is the area where the upper-arm (humerus) and the forearm (radius and ulna) come together. These three bones connect to each other in three joints, that together, form the complex of the elbow joint, (Fig.1A). As described later (§2.3.1), each one of these joints is essential for the stability and mobility of the elbow, (Jacob, 2007).

Similarly the wrist joint is the point of connection between the forearm and the carpal proximal bones of the hand, (Fig.1A). In particular, the scaphoid bone and the lunate bone are placed at the border of the radius and ulna respectively. The hand is the final region of the upper limbs and provides prehensile capability with its multi-fingered structure. The hand bones are divided into carpal, metacarpal and phalanges bones, (Fig.1B).

2.1.2 General Muscle Structure

The upper body limbs contain two types of muscle: sheet-shaped muscles, guaranteeing support and solid attachment of the arm to the trunk; and fusiform muscles, coordinating the movements of the limb, (Jacob, 2007).

The bone structure of the upper limbs works in strong synergy with the sheet shaped muscles around the shoulder, such as back and chest muscles. In particular, the trapezius, running from the base of the skull to the thoracic vertebrae in the spine, supports the weight of the entire arm (Fig.3A), and the latissimus dorsi (largest muscle in the upper body) supports the shoulder during extension and adduction movements, (Fig.3B). Anteriorly, the arm is supported by the pectoralis major, that extends from the sternum, to the clavicle to the costal cartilages and to the lateral lip of the humerus in the shoulder girdle (Fig.1, 3C), (Jacob, 2007; Lee et al., 2016).

The shoulder presents three main muscles: the rotator cuff, the deltoid and the teres major, (Fig.4). These three muscles are the main source of stability between the humerus and the shoulder bones, as well as the source of most mechanic processes, (§2.2.2), (Jacob, 2007; Innerbody Research, 2021).

Along the humerus, three fusiform muscles (the tricep brachii, the bicep brachii and the brachialis) can be found, allowing flexion and extension of the arm through a connection to the shoulder region, ulna and radius, (Fig.4), (Ruland et al., 2005).
In the forearm, it is possible to find a group of small flexors and extensors that extend from the base of the humerus to the wrist. Among them, worthy of note are the brachioradialis, that supports the elbow flexing motion and the palmaris longus, that squeezes the palm of the hand, (Jacob, 2007).

2.2 Shoulder Joint

The shoulder is an elegant piece of machinery and has the greatest range of motion of any joint in the body, which unfortunately, can lead to shoulder joint injuries. The structures of the shoulder joint relevant to this study can be divided into two categories: bones & ligaments, and muscles & tendons.

2.2.1 Bones and Ligaments in the Shoulder

The shoulder is composed of four joints, (Kent, 1971; Culham & Peat, 1993; Jacob, 2007):

1. Glenohumeral joint: main shoulder joint with a ball-and-socket joint structure.Formed where the head of the humerus fits into the shallow cavity of the scapula (glenoid); this shallow socket is called the glenoid cavity, (Fig.5; 6.1).
2. Acromioclavicular joint: gliding joint formed where the clavicle meets the acromion. The acromion is a protuberance of the scapula that extends over the shoulder and meets the clavicle on the anterior side of shoulder (Fig.5).
3. Sternoclavicular joint: saddle joint attaching the clavicle to the sternum. It creates a small protuberance at the base of the neck and supports the connection of the shoulders to the main skeleton on the front of the chest, (Fig.6.3).
4. Scapulothoracic joint: false joint formed where the shoulder blade glides against back of the rib cage.

The shoulder joint is covered by a joint capsule, that protects and stabilises the joint and is made of two components: the fibrous outer layer and the inner synovial layer. The fibrous layer of the shoulder joint is made of ligaments. Ligaments are soft tissue structures that connect bones to bones. In the shoulder, ligaments are the main source of stability and keep the shoulder from dislocating. The main ligaments attach the humerus to the glenoid in the glenohumeral joint, and the clavicle to the acromion in the acromioclavicular joint. The synovial inner layer is made of synovial membrane: a soft tissue with a lubricant function. Synovial membrane can be found in the glenohumeral joint, where the humerus connects to the scapula; here the synovial membrane coats the extremities of the two bones, (O’Connell et al., 1990; Burkart & Debski, 2002).

The glenohumeral joint is the most important part of the shoulder and is known for its particular “almost flat” shape, due to the disproportion in size between the head of humerus and the shallow glenoid cavity. This is the joint that allows the majority of movements of the shoulder, (Gerber & Nyffeler, 2002).

The glenohumeral joint presents a few unique characteristics. In this area of the shoulder, the humerus and scapula are covered by a thin layers of articular cartilage of about 0.25 cm thick in their contact areas, (Fig.5). This is the layer on top of which the synovial membrane is attached and contributes to the bones sliding with less friction than bone on bone thanks to its rubbery, smooth consistency, (Jacob, 2007).

Around the glenoid, a special type of fibrocartilageneous ligament creates a deeper hollow for the humerus: the labrum. Because the head of humerus does not fit tightly in the glenoid, the labrum is essential to prevent shoulder dislocation, (Rodosky et al., 1994).

2.2.2 Muscles and Tendons in the Shoulder

On top of the bony structure of the shoulder joint is attached the muscular structure. The muscular structure of the shoulder is complex as it is made up of about 20 different muscle fasciae connected on different levels. As mentioned earlier, (§2.1.2), the three main muscles groups are: the rotator cuff, the deltoid and teres major, (Jacob, 2007).

The rotator cuff is an ensemble of four separate muscles: supraspinatus, infraspinatus, teres minor and subscapularis, (Fig.4). These extend just above the glenoid labrum and attach on the head of the humerus and the joint capsule. The rotator cuff is responsible for raising the arm from the side and turning the shoulder in many directions. Moreover, it is the muscle that contributes the most to the stability of the shoulder joint once other muscles groups, such as the deltoid, are contracting, (Dugas et al., 2002).

The deltoid is the largest and most powerful muscle in the shoulder and can move the arm past head height. It is divided into three muscles: the anterior, intermediate and posterior deltoid, based on its attachments. All of these three muscles attach to the humerus on one side, but on the other, they are linked to the clavicle, the acromion and the scapula spine, respectively.
The teres major is a small, but important muscle attached to humerus and scapula. It is often confused with the muscles of the rotator cuff because of its position, but differently from them, the teres major is not attached to the joint capsule. The teres major helps the latissimus dorsi in adduction movements, (Jacob, 2007; Donohue et al., 2017).

The function of each muscle depends on the bones it is attached to. Tendons resemble ligaments, except that tendons attach muscles to bones. Muscles move the bones by pulling on the tendons creating strong tension forces. The most important tendons in the shoulder joint are the rotator cuff tendons, attached deeply to the bone structure, and the bicep tendon, (Dugas et al., 2002).
The rotator cuff muscles are connected, on one side, to the joint capsule through four distinct tendons and, on the other side, they come together into a unique tendon attached to the head of the humerus.
The biceps tendon runs from the biceps muscle across the front of the shoulder to the glenoid. At the very top of the glenoid, the bicep tendon attaches to the bone, becoming part of the labrum, (Dugas et al., 2002; Jacob, 2007).

2.3 Elbow Joint

The elbow is an ensemble of joints in the arm, which connects the ulna and radius bones to the humerus, (Fig.1). The elbow allows to bend and extend the arm as well as pronating and supinating the hand. The next section describes the structure of bones and ligaments, and muscles and tendons of the elbow joint.

2.3.1 Bones and Ligaments in the Elbow

The elbow is made of three different joints, (Fig.7), (Jacob, 2007; De Haan et al., 2011):

1. Humeroulnar joint: main component of the elbow, its hinge-joint structure allows elbow extension and flexion. In this joint, the trochlear notch of the ulna is locked in the humeral trochlear; this space is also known as the olecranon fossa.
2. Humeroradial joint: ball-and-socket joint formed by the head of the radius and the capitulum of the humerus. Despite its structure, it does not contribute to movements directly but rather prevents any separation between humerus and radius during the motion.
3. Radioulnar joint: pivot joint formed where the head of radius and the ulnar notch meet. This joint is recruited when pronating and supinating the hand.

Similarly to the shoulder joint, the elbow joint is covered by a joint capsule. The fibrous outer layer is made of ligaments and the inner synovial layer is made of the synovial membrane.
The ligaments of the elbow have a slightly tighter structure than in the shoulder, this is to impose strict limitations on abduction, adduction, and axial rotation of the elbow. The ulnar collateral ligament and the radial collateral ligament, respectively part of the humeroulnar and humeroradial joints, are the largest ligaments in the joint and they experience the greatest tension during the motion of the joint, (Cohen & Bruno, 2001).

In the inner layer of the joint capsules, the synovial membrane of the elbow is extensive: it covers the head of the humerus and it is prolonged up to the neck of the radius and radioulnar joint. This is essential to provide freedom of movement and stability for all movements of the elbow, (O’Connell et al., 1990; Burkart & Debski, 2002; Ruland et al., 2005).

Contrary to common belief, the radius and ulna do not lie on the humerus longitudinal axis. The elbow joint, in fact, creates a small angle in the direction of the thumb when fully extended. It is possible to witness this deviation by standing up, placing the arms along side the trunk and supinating the palm of the hands (point outward with the thumbs). This is the carrying angle and it measures between 5° and 15° to avoid the hand hitting the hips while walking, (Fig.8), (Van Roy et al., 2005).

2.3.2 Muscles and Tendons in the Elbow

The elbow joint can perform different functions based on activated muscles. The four main muscles that contribute to moving the elbow joint are: brachialis, brachioradialis, bicep brachii and tricep brachii.
The brachialis is just underneath the bicep and attaches the halfway point of the humerus to the ulna (instead of the radius where the bicep is attached) and it contributes exclusively to elbow flexion.
The brachioradialis is connected in the humerus next to the brachialis, but terminates in the radius, and contributes to both flexion of the elbow and supination/pronation of the forearm, (Dugas et al., 2002; Jacob, 2007).
The bicep brachii is a two-headed muscle. The short head connects to the scapula, while the long head starts in the glenoid, and they come together in a tendon attached to the radial tuberosity. The bicep is the strongest muscles in the elbow joint, andrepresents the only connection between elbow and shoulder, contributing to elbow movements as much as shoulder stability, (Ruland et al., 2005).
The tricep brachii is the three-headed muscle that performs the extension movement of the arm. The long head attaches to the scapula, lateral and short heads are attached to the humerus radial sulcus (about midway in the humerus); they all connect in the olecranon (ulna bone). Given the different directions in which the three heads develop, the forces exerted by the tricep can vary based on the angle of the shoulder, (Dugas et al., 2002).

The important tendons of the elbow are the biceps tendon and the triceps tendon, as these are linked to the largest muscles in the elbow joint, (Jacob, 2007).

2.4 Natural Range of Motion of the Joints

Despite some similarities between lower and upper body limbs, such as their limb girdles and joint capsules, the structure of the arm is far more versatile because of its wide range of motion, (Liewluck & Milone, 2018).

2.4.1 The Motion of the Shoulder and Elbow Joints

The shoulder can perform three main movements: arm abduction and adduction, arm flexion and extension, and shoulder internal rotation, (Murray & Johnson, 2004); and is sufficiently strong to perform all of them with an external resistance force on the glenohumeral joint that can reach up to 164% the bodyweight of the individual, (Klemt et al., 2018), (§2.4.1).
Arm abduction and adduction move the arm laterally away from and towards the trunk respectively, on the frontal plane and requires the activation of the rotator cuff and the deltoid. This is the only movements that the shoulder joint can execute in full autonomy, (Jacob, 2007).

In both arm flexion and extension and shoulder rotation, the support of the pectoralis major and the latissimus dorsi (as well as other surrounding muscles) is required.
With the combination of these three movements, the shoulder is also able to perform arm circumduction, (Jacob, 2007).
According to Murray & Johnson (2004), daily tasks usually require the shoulder to have a certain range of motion. Some examples of movements that place the shoulder in maximal range positions are presented, (Table.1; Appendix.A).

Notice that a combination of the three movements (flexion, abduction and rotation), puts the shoulder in a weaker position, as the shoulder joint can exert less force when approaching maximal ranges of motion. It is close to these maximal ranges of motions that the shoulder can get injured, (Murray & Johnson 2004).

The elbow can perform only two movements: flexion and extension, and pronation and supination. The range of motion of the elbow is larger than in the shoulder when comparing single dimensions, but is overall smaller, as the elbow joint cannot bend the forearm in the frontal and transverse plane without the interaction of the shoulder joint, and therefore, has only two planes of motion (Murray & Johnson 2004; Jacob, 2007).
Elbow flexion is performed by the brachialis, brachioradialis and bicep brachii, while extension is performed by the tricep brachii. The pronation and supination movements is performed by the supinator and pronator muscles in the forearm. The elbow flexion ranges from 15° to 165° and pronation from -54° to 66°, (Table.1), (Murray & Johnson 2004).

The main tasks of the elbow is to lengthen and shorten the upper limb to bring the hand close to objects. For this reason, the elbow joint is exposed to smaller external forces than the shoulder joint, which rather focuses on carrying most of the weight on the arm, (Jacob, 2007).

2.4.2 The Glenohumeral Compression Force (GHCF)

A recent study by Klemt et al. (2018) has shown that the shoulder joint is able to sustain significantly larger weights than previously estimated. In particular, the glenohumeral joint is responsible for balancing the majority of forces acting on the upper body limb.
There are three forces acting on the glenohumeral joint: the glenohumeral compressive force (GHCF), the anterior-posterior shear force and the superior- inferior shear force, (Fig.9).

The GHCF pushes the humerus head towards the centre of the glenoid socket and opposes the two glenohumeral shear forces that are warping the glenoid labrum. If the forces are not balanced, the head of the humerus will translate out of the socket causing a dislocated shoulder.
Klemt et al. (2018) studied the ratio between each individual shear force and the GHCF, concluding that during regular day life, human bodies undergo several “extreme movements” (i.e. movements in which either the anterior-posterior or superior-inferior shear forces exceed the 0.50 ratio with the compressive force). In particular, it was identified that the most dangerous movements include raising objects with fully extended arms towards position of maximal range, such as raising objects above head height; steering a driving wheel at high speed; and supporting the body with the arms on a desk or chair while trying to sit down gently.