At the Fascia Institute and Treatment Center, we specialize in fascia care, but are also experts in biomechanics. Because of the Fascia Institute’s association with Tulane Orthopedics we have developed a collaborated biomechanical analysis system spearheaded by Buddy Savoie and Andre Labbe. Using the Noraxon system, overhead throwing athletes of all levels are seeking evaluation at the Fascia Institute. As experts in the field, the Fascia Institute have authored a detailed review of elbow and throwing biomechanics.
Jacques Courseault, MD, Andre Labbe MPT, Felix Savoie, MD
Bony Anatomy
The humerus is the proximal bone of the elbow joint. The distal humerus bears two condyles, medial and lateral, which articulate with the bones of the forearm (Morrey, 2000). The epicondylar areas of these two bones, the radius, and ulna, serve as the attachment sites for the ligamentous and dynamic support (Fornalski, 2003). The condyles’ articulations are humeral structures known as the trochlea medial and capitellum lateral (Karbacj, 2017; Fornalski, 2003); the shapes and orientation of these articulations are important to the configuration and structural integrity of the elbow complex.[CAM1].
The ulnohumeral joint, identified as the articulation of the spool-shaped trochlea of the humerus with the ulna’s sigmoid notch, provides massive stability in full extension The sigmoid notch is comprised of the coronoid process anteriorly and the olecranon posteriorly (Fornalski, 2003). These structures play a large role in maintaining the static stability of the elbow as they receive the majority of the elbow’s compressive forces (Wilps, 2020). In extension, the ulnar olecranon is prevented from excessive posterior motion by the olecranon fossa of the humerus; in flexion, the ulnar coronoid fits elegantly into the coronoid fossa of the humerus (Wilps, 2020). This anatomy, in combination with the compressive forces of the muscles, provides an osseous blockade to posterior subluxation even when other stabilizers are damaged (Armstrong, 2015; Wilps, 2020). In elbow dislocation injury, the elbow may remain reduced because of the presence of the coronoid process blocking the elbow from posterior subluxation and the “compressive forces generated by the muscles that cross the elbow” that maintain that anatomy (Armstrong, 2015l Wilps, 2020). The strength of this osseous interaction is crucial to the elbow joint’s static stability.
The radial head has three articulations: the radial notch of the ulna proximally, the ulnar head distally, and the sphere-shaped capitellum of the humerus (Fornalski, 2003). Biomechanically, the radial head itself transmits approximately 60% of comprehensive forces across the radiocapitellar joint and creates stress to the lateral ulnar collateral ligament (Wilps, 2020). Additionally, the radial head provides osseous stability and contributes to the varus-valgus stability and kinematics of the elbow (Beingessner, 2004). The movements afforded by the radius and ulna are crucial to the normal biomechanics of the elbow.
Ligaments/Capsule
The elbow’s soft tissue components include the anterior and posterior joint capsule as well as the medial and lateral collateral ligament complexes. These tissues are crucial to static stabilization, particularly as the elbow moves away from the endpoints of flexion and extension (Safran, 2005; Wilps, 2020). Additional soft tissue static stabilizers include the triangular fibrocartilage complex (TFCC) and the interosseous membrane (IOM).
The medial ulnar collateral ligament complex (UCL) originates anteromedially from the medial epicondyle and inserts at the humerus. The UCL particularly contributes to valgus stability in extension ( approximately 30% of total stability) and flexion (approximately 50% of total stability) (Karbach, 2017). Support and tension from the UCL increase as the elbow flexes (Safran, 2005). The UCL is composed of an anterior bundle, posterior bundle, and transverse segment (also known as the Cooper ligament). Due to the UCL not originating from the elbow’s axis of rotation, the tension in the ligament does not remain constant throughout the flexion arc of motion (Fornalski, 2003). As a result, the anterior band becomes taut in extension while the posterior band is held taut in flexion (Fornalski, 2003; Karbach, 2017).
The UCL’s anterior bundle contributes the most to the ligament’s overall valgus stability and is sometimes referred to as the medial ulnar collateral ligament (MUCL) (Fornalski, 2003). This bundle can be further subdivided into the anterior and posterior bands. It is the anterior band that serves as the “primary constraint to valgus and internal rotatory forces” throughout the flexion movement while the posterior band and posterior bundle contribute only minimally (Floris, 1998). The posterior bundle is “best defined at 90 degrees flexion” (Fornalski, 2003). Reconstructive studies indicate that the presence of the posterior bundle lessens posteromedial rotational instability in the context of coronoid fractures to and prevent elbow dislocation in a “terrible triad injury” (Gluck, 2018; Shukla, 2016). The transverse segment of the UCL often has its origin and insertion on the ulna and as such provides minimal support to the elbow complex (Fornalski, 2003; Morrey, An, 1985).
The lateral collateral ligament complex (LCL) contains the radial collateral ligament (RCL), annular ligament, lateral UCL, and accessory RCL. The RCL originates from the lateral epicondyle and inserts in the annular ligament. This complex appears to work uniformly to stabilize the bony articulations against posterolateral instability (Karbach, 2017). Unlike the UCL, this ligament complex is near the elbow’s axis of rotation, resulting in a nearly isometric ligament throughout the motion arc (B&K, Ch25; Fornalski, 2003). Without this complex, the elbow is unstable in supination (Karbach, 2017).
The other components of the RCL complex contribute to variable importance to the elbow’s stability. The annular ligament originates and inserts anteriorly and posteriorly within the lesser sigmoid notch and maintains articulation between the radial head and the ulna during forearm pronation and supination rotation (Wilps, 2020; Fornalski, 2003; Morrey, 2000). During supination, the anterior portion becomes taut, while in pronation, the posterior origin is taut (Morrey, 2000). The accessory LCL blends into and stabilizes the annular ligament, particularly during varus stress (Fornalski, 2003). The interosseous membrane (IOM) which spans across the radius and ulna stabilizes the distal radioulnar joint throughout forearm rotation (Adams, 2017). The IOM appears to remain isometric throughout the range of motion and provides stability to the structural integrity of the forearm (Adams, 2017). Although its functions are dynamic, the anconeus is mentioned in this section due to its’ considerable stabilizing characteristics alongside the LCL complex against varus stress and posterolateral rotatory instability (Bryce, 2008).
The remaining component of the RCL, the lateral UCL (LUCL), originates at the lateral epicondyle and inserts along the ulna to serve as the primary lateral stabilizer of the ulnohumeral joint (Fornalski, 2003). Like the RCL, because of its origination point, the LUCL provides consistent ligamentous tension throughout elbow flexion and extension (Karbach, 2017). The LUCL is responsible for preventing varus and posterolateral rotatory instability (Karbach, 2017; Werner, 1993).
Finally, the anterior capsule has the most tension in extension (Fornalski, 2003). The anterior capsule appears to provide resistance to distraction as well as hyperextension and valgus stress (Karbach, 2017). Some studies show that the capsule is responsible for 85% of resistance to distraction when the elbow is in extension (Morrey, 1985).
Muscles
The muscles of the elbow joint are responsible for the motions of the forearm. Supination and pronation are the forearm’s primary motion, followed by flexion and extension (Fornalski, 2003). Muscles crossing the elbow also stabilize the joint by offsetting forces exerted on the ligamentous and osseous structures. Of these muscles, the brachialis and triceps brachii have the largest work capacity (Ann, Hui, 1981). The dynamic stabilization of arm and forearm muscles ensures that, regardless of the forces at the hand, distraction loads are not placed on the elbow while in motion (Wilps, 2020).
The radial nerve and posterior interosseous nerve innervate muscles that cross the elbow joint and allows for elbow and wrist extension. Posterior muscles include the triceps brachii, anconeus, extensor carpi ulnaris (ECU), and extensor digitorum communis (EDC) (Fornalski, 2003). Muscles of the lateral joint are also innervated by the radial nerve and include additional extensors (extensor carpi radialis brevis and longus), a flexor (brachioradialis), and the supinator. Medial muscles are innervated by the median nerve and are responsible for wrist flexion: flexor digitorum superficialis (FDS), and median innervated flexor digitorum profundus (FDP). The anterior interosseous nerve innervates the flexor pollicis longus and flexor digitorum profundus to the index finger. Anterior muscles flex the wrist and forearm and pronate the forearm: biceps brachii, pronator teres, flexor carpi radialis (FCR), palmaris longus, flexor carpi ulnaris (FCU) (Fornalski, 2003).
The muscles which cross the elbow joint and originate from the forearm assist with resisting valgus forces (Wilps, 2020). The flexor pronator mass refers to the medial muscles including the flexor carpi ulnaris (FCU), flexor carpi radialis (FCR), flexor digitorum superficialis (FDS), and pronator teres. Because of the proximity of the FCU and FDS to the UCL, this flexor pronator mass can assist the MCL in resisting valgus forces medially in normal movement and in overhead throwing motions (Labbott, 2018; Wilps, 2020; B&K, Ch25). In fact, studies are beginning to show that loading of these muscles, the pronator teres, in particular, may resist valgus stress at the elbow (Wilps, 2020). The FCU in particular has been described as the primary stabilizing structure in an MCL-deficient elbow (Wilps, 2020).
Muscles originating from the humeral shaft maintain elbow stability during flexion and extension (Funk, 1987). The extensor muscles are located laterally to the flexor pronator mass. The extensors include the brachioradialis, extensor carpi ulnaris, extensor digitorum communis, extensor carpi radialis brevis and longus, and the anconeus. The extensors originate at the lateral epicondyle, allowing them to resist varus forces (Karbach, 2017). The triceps brachii has three heads: the long head originates from the infraglenoid tubercle of the scapula, the lateral and medial heads originate from the humerus. Together, they insert into the olecranon. The anconeus is also located in the posterior group and is considered a compressive stabilizing force to the bony structures (Fornalski, 2003). The biceps brachii has long and short heads that originate at the scapula at the supraglenoid tubercle and coracoid process respectively. These heads then form the distal biceps tendon which inserts at the radial tuberosity. The biceps works with the brachioradialis to flex the elbow; the brachioradialis flexes the elbow primarily when the forearm is in a neutral position (Fornalski, 2003). The biceps also supinates the forearm when the elbow is flexed.
Kinetics & kinematics
Physiologically, the elbow rests in a slightly valgus position, referred to as the carrying angle. When the elbow is fully extended and supinated, the forearm should hold a carrying angle of between 9-14 degrees in men or 12-17 in women (B&K, Ch25). As the elbow flexes, the carrying angle changes from valgus to varus (Morrey, 2000). Changes in this angle alter kinematics and put the elbow at risk for injury.
Contributions to Movement at the Elbow
The elbow moves in the planes of both flexion-extension and supination-pronation around the humeroulnar, humeroradial, and proximal radioulnar joints. The normal range of motion for flexion-extension is about 0 degrees to 150 degrees, and the normal range of motion for rotation is about 75 degrees of pronation to 85 degrees of supination. With this wide breadth of possible movement along 2 different axes, the muscles of the elbow complex are vital not only for movement along the joint, but also for dynamic stabilization.
The elbow complex contains three separate bony articulations which work in tandem to create the motions of the forearm: ulnohumeral joint, radiohumeral joint, and radioulnar joints. Furthermore, as a gingliotrochlear joint, the elbow operates within two movement axes, rotating, and hinging (Fornalski, 2003). The ulnohumeral joint is the primary stabilizing articulation, facilitating flexion, extension, and hinging motions of the forearm (Fornalski, 2003; Zimmerman, 2002). The radial articulations allow for forearm rotation and pivoting about the ulna (Fornalski, 2003). These structures are the most important static stabilizers in full flexion and full extension (Wilps, 2020). The radiocapitellar articulation allows for the flexion and extension of the forearm as well as 180 degrees of pronation and supination (Karbach, 2017); the radioulnar joints help “guide” the rotational movement (Wilps, 2020).
Transverse Plane Motion
The primary motion of the elbow is supination and pronation which is afforded by the rotational capabilities of the radius and by a shifting axis of rotation (Fornalski, 2003). The normal range of motion for this rotation is between 70-75 degrees pronation and 75-85 degrees supination with most activities of daily living accomplished with 50 degrees pronation and 50 degrees supination (Morrey, 1981; Fornalski 2003). This movement is associated with the “axio-rotational force transmission through the elbow joint” (Hwag, 2018). The centerline of this rotation is established by the articulation between the trochlea and the trochlear notch and is located along the medial UCL anteriorly (Wilps, 2020; Graham, 2019). As the forearm pronates from a supinated position, the radial head “translates anteriorly, proximally, and laterally” to the humerus, while the trochlear notch “translates medially” (Omori, 2016). Additionally, the force transmitted between the radius and humerus at the radiocapitellar joint remains constant, but the area contact between the bones decreases as the forearm pronates, suggesting that pronation plays an important role in maintaining the congruence of this articulation (Hwang, 2018).
Pronation and Supination
There are two major pronators of the elbow: the pronator teres and pronator quadratus. Each muscle has two heads. The pronator teres consists of the humeral head and the ulnar head. The humeral head is the larger of the two, and it arises near the common flexor tendon on the medial epicondyle of the humerus, while the smaller ulnar head arises from the medial coronoid process. Both heads form a common tendon that attaches to the lateral side of the convexity of the radius (pg. 292 JS&F).3 The pronator teres also has a unique function in translational stabilization of the radial head against the capitellum (JS&F pg. 293)3 in addition to having a small contribution to the force of flexion at the elbow. The pronator quadratus consists of a superficial head and a deep head. It is a quadrangular shaped muscle located on the distal end of the forearm that not only assists the pronator teres in pronation but also acts to maintain compression of the distal radioulnar joint to stabilize the radius and the ulna relative to each other.
The two major muscles of supination are the biceps brachii and the supinator. The supinator has a broad origin over the lateral epicondyle of the humerus, the radial collateral ligament, the annular ligament, and the lateral ulna. It crosses the posterior interosseous membrane and inserts on the radius medially and laterally to the radial tuberosity. The biceps brachii is able to assist in supination via its attachment to the radial tuberosity (JS&F pg. 313).3 The biceps brachii and supinator always act together in elbow flexion. In elbow extension, only the supinator contracts, the biceps brachii does not.
The average maximum isometric torque of supination is about 90 kg-cm in men and 55 kg-cm in women, while the average maximum isometric torque of pronation is about 80 kg-cm in men and 35 kg-cm in women. Pronation and supination are both maximal when the elbow is flexed to 90 degrees. The maximum torque of supination is achieved at roughly 20 degrees of pronation. Without resistance, the supinator may act autonomously to supinate the elbow, especially when the elbow is extended. However, the biceps will be recruited as resistance increases, and/or the elbow approaches 90 degrees of flexion (JS&F pg. 287).3 Conflicting studies have shown that the maximum torque of pronation is achieved either in the neutral or supinated position. Furthermore, it is important to note that the average maximum isometric torque for each flexion, extension, pronation, and supination has been found to be roughly 5% – 10% greater in the dominant arm than in the non-dominant arm.
Sagittal plane Motion
The elbow supports flexion and extension of the forearm within a normal range of motion of 0 degrees to 140 degrees facilitated by the ulnohumeral joint (Morrey, 1981; Fornalski, 2003). This motion occurs along a “flexion axis” found “between the centers of curvature of the arc of the capitellum and the greater sigmoid notch” (Zimmerman, 2002). The axis is oriented slightly valgus to the humerus and is internally rotated (Zimmerman, 2002). However, this centerline changes as the joint moves through its arcs of motion due to the “obliquity of the trochlear groove of the distal humerus”; the magnitude of this axial rotation change is less than 4mm (Zimmerman, 2002). This results in variable tension throughout the isometric collateral ligaments (Wilps, 2020).
Flexion
The brachialis, biceps brachii, and brachioradialis are the active flexors of the elbow. The largest contributor to these is the brachialis, which inserts on the ulna. It has consistently proven to provide the highest force of flexion regardless of the angle of elbow supination, pronation, flexion, or extension, although its highest moment arm is at a little more than 100 degrees of flexion. Unlike the biceps, the brachialis is a single-joint muscle.
The biceps brachii and brachioradialis are considered secondary flexors of the elbow. The biceps is more active in the supinated position, has its highest moment arm between 80 degrees and 100 degrees of flexion, and, due to its distal attachment, when the elbow is extended beyond 100 degrees it demonstrates more of a distracting force than a flexing force (pg. 286 JS&F).3 Additionally, since the biceps crosses both the shoulder and the elbow, the action of the biceps is directly related to shoulder position. Mainly, the force of flexion by the biceps is diminished when the shoulder is also in a flexed position (pg. 286 JS&F).3 The brachioradialis is more active in the pronated position, and its moment arm is highest between 100 degrees and 120 degrees of flexion. Due to its attachment near the styloid process of the radius, it plays a key role in the compression and stability of the elbow joint throughout flexion (pg. 287 JS&F).3
Additionally, unlike the brachialis, each the biceps and the brachioradialis has contributions to other types of movement. The biceps brachii contributes to supination of the forearm, as will be discussed in the ‘Pronation and Supination’ section, and flexes the shoulder, due to its attachment at the glenohumeral joint. The brachioradialis contributes to both supination and pronation, as will also be discussed in the ‘Pronation and Supination’ section. Furthermore, hyper-flexion beyond 150 degrees at the elbow is prevented largely by the counteractive stretch-reflex of the triceps and the anconeus, but also by the muscle bulk of the anterior compartment flexors themselves (pg. 43, Morrey).4
Extension
The triceps and anconeous are responsible for triceps extension. The triceps has three heads, the long head, the medial head, and the lateral head. All three heads combine to form a common tendon that inserts on the olecranon process. Combined, the triceps has the largest physiological cross-sectional area of any of the muscles of the elbow and is thus the strongest elbow muscle.5 However, because of the differences in origins of the three heads, they do not necessarily function in unison. The medial head of the triceps is active in all forms of extension, while the long and lateral heads only become active in extension against resistance.6 In addition to extension at the elbow, the triceps also serves, to a lesser extent, as an extensor of the shoulder joint, in conjunction with the latissimus dorsi, deltoid, teres major, and teres minor.7 The anconeus is a triangle-shaped muscle that originates on the lateral epicondyle of the humerus and inserts on the lateral aspect of the olecranon. It is a secondary extensor of the elbow but also assists in the dynamic stabilization of the elbow complex throughout extension, pronation, and supination.3 The attachments of the anterior flexors of the elbow can contribute to resistance to hyperextension of the elbow, but the end-range of extension is typically limited by bony contact of the olecranon process in the olecranon fossa.3
According to Morrey et al, the average maximum isometric torque of flexion at the elbow is about 7 kg-m for men and 3.5 kg-m for women. This torque is typically reached between 90 degrees and 110 degrees of flexion, which correlates with the fact that the moment arm for each the brachialis, biceps, and brachioradialis lies between 80 degrees and 120 degrees of flexion. Additionally, maximum torque strengths are generated when the elbow is supinated,12,13 which correlates with the fact that the biceps muscle is most active when the elbow is supinated. However, according to Kasprisin et al, with increasing loads, and thus an increase in the required torque of flexion, the biceps can be recruited to become active at all positions of pronation and supination.
The average maximum isometric torque of extension at the elbow is about 4 kg-m for men and 2 kg-m for women. The greatest isometric extension force is achieved around the 90-degree position, however, the total amount of quantitative torque at this 90-degree position is dependent upon the positioning of the shoulder. With the shoulder in hyperextension, the ability of the long head of the triceps to produce torque tends to be diminished. However, in spite of differences in activation between the different heads of the triceps, just like with the flexors, when load (and thus required torque) increases, all three heads can be recruited to activate synchronously regardless of position.
Dynamic Stability
While much of elbow stabilization is due to the inherent anatomy of the joints and ligaments, the muscles of the elbow play an important role, especially in stabilization against extrinsic forces and loads. The triceps have been proven to have multiple different possible functions in the dynamic stabilization of the elbow. One instance is in the lowering of one’s body against gravity, as in the first part of a push-up. This forces the triceps into an eccentric, rather than concentric, contraction, and helps control the rate and direction of elbow flexion in spite of little activation from the elbow flexors themselves (JS&F pg. 309).3 Additionally, the triceps has also been implicated as a synergistic antagonist of the biceps when the action of supination is desired without elbow flexion (JS&F pg. 309).3 The anconeus has also proven to support dynamic stabilization throughout extension, pronation, and supination. However, dynamic stabilization of the elbow complex is largely due to contributions from the muscles of the forearm.
Lin et al examined how loads to various muscles of the forearm relieved strain on the medial ulnar collateral ligament during valgus stress with the elbow flexed at 45 degrees and 90 degrees. They found that the flexor carpi ulnaris is the primary dynamic stabilizer of the medial ulnar collateral ligament against valgus force, with further contributions from the flexor digitorum superficialis and the flexor carpi radialis.20 Similarly, An et al showed that the extensor digitorum communis, extensor carpi radialis brevis, extensor carpi radialis longus, extensor carpi ulnaris, and anconeus all contribute to the dynamic stabilization of the lateral collateral ligament against varus strain. However, the majority of resistance to varus stress comes from osseous and ligamentous structures.21,22 The extensor carpi ulnaris and extensor carpi radialis brevis also each have further contributions to elbow stability during pronation-supination movements. Throughout supination and pronation, the extensor carpi ulnaris exerts a depressive force on the dorsal ulnar head to help maintain proper positioning of the ulnar head, while the extensor carpi radialis brevis helps to stabilize grip strength during pronating torques and helps to extend the wrist during supinating torques (pg 315 JS&F).23
Biomechanical Analysis of Sport
Overhead Motion at the Elbow
Overhead motion, particularly the act of throwing, requires complex coactivation of the muscles of the elbow as part of a larger, full-body kinetic chain, and results in a number of different forces applied to the elbow complex. Due to a high incidence of injury, one of the most studied overhead motions has been that of the baseball pitch. In particular, a study by Loftice et al broke down the act of pitching into distinct phases and examined each phase’s effects on the elbow.24
The two initial phases, the ‘windup’ and the ‘stride’ have minimal elbow involvement and serve largely to set one’s body up to properly execute the kinetic chain events of throwing. By the beginning of the third phase, called the ‘arm cocking’ phase, the pitcher’s lead foot has contacted the ground, their throwing shoulder is externally rotating, and their throwing elbow is flexed between 80 degrees and 100 degrees. At this point, the kinetic chain of throwing begins, and the initial force of this chain is actually generated from the legs, pelvis, and trunk. Zattara et al demonstrated that force in rapid overhead throwing begins in the gastrocnemius and soleus and then proceeds into the trunk.29 Hirashima et al analyzed the sequential activation of the core and arm muscles during rapid overhead throwing and showed that force is carried and further generated through the contralateral internal oblique, external oblique, and rectus abdominus, followed by the ipsilateral stabilizers of the scapula, pectoralis major, deltoid, and rotator cuff. It is the sequential co-activations of these muscles that result in the pitcher’s pelvis and trunk having rotated to face the batter, with their elbow flexed and their shoulder externally rotated, in the ‘arm cocking’ phase. (Loftice).24 Thus, it is during this phase that the elbow begins to experience moderate flexion and valgus torques. In order to resist valgus torques, which have potentially injurious consequences, the arm creates a counter-acting varus torque of 64 +/- 12 N-m. Much of this varus torque is provided by an innate resistance to valgus stress from the UCL. However, muscle activity, especially at the flexor-pronator mass, bone-bone articulation, and concomitant compressive force at the lateral elbow also contribute to counteracting varus torque.25,32
Naturally, the ‘arm cocking’ phase is followed by the ‘arm acceleration’ phase. This phase consists of rapid elbow extension in the context of continued upper torso rotation and lasts from maximum shoulder external rotation until ball release. It is the shortest of the phases of throwing but generates intense forces at the elbow and the shoulder. During this phase of the overhead throw, the elbow is capable of reaching an angular velocity of up to 2700 degrees per second.25 Since the muscles of the elbow are unable to achieve this high velocity alone, most of the velocity achieved during rapid overhead throwing is a result of the torque generated from the aforementioned kinetic chain that begins in the legs, trunk, and shoulder. This concept has been corroborated both by the aforementioned study by Hirashima et al, which showed that contractions of the triceps and forearm muscles did not occur in sequence with contractions in the rest of the kinetic chain during throwing,30 and a study by Roberts et al, which showed that 80% of throw speed could still be attained after induced paralysis of the triceps.34 Each of these studies implicates the triceps and forearm muscles of the elbow complex are dynamic stabilizers that act to fine-tune smaller movements necessary for throw accuracy, in conjunction with muscles of the hand and fingers. These fine-tuning actions are collectively called “voluntary focal movements”.35 Additionally, due to the high angular velocity produced during acceleration in rapid overhead throwing, the elbow can experience forces of up to 1000 N at the moment of ball release. It is estimated that the UCL itself is exposed to force of up to 290 N (Kibler)36 which predisposes an individual to “valgus extension overload.” To resist this high stress, the flexors of the elbow are activated in this phase to help stabilize the elbow complex in addition to the aforementioned varus resistance provided by the UCL, flexor-pronator mass, bony articulations, and compressive lateral elbow force.24
From the point of ball release until the shoulder reaches maximal internal is considered the ‘arm deceleration’ phase. The activation of flexors which began in the acceleration phase becomes most important here, as an elbow flexion torque reaching up to 35 N-m is generated throughout deceleration to dissipate the energy of the throw.31,25 Ultimately, the elbow extension is terminated approximately 20 degrees before full extension.25,27 Once the shoulder reaches maximal internal rotation, the throwing motion concludes with the ‘follow-through’ phase. This phase finishes once the pitcher has established himself in a balanced position, and is characterized by complete forward trunk rotation, flexion of the elbow into a comfortable position, and dissipation of energy throughout the rest of the kinetic chain.
While the motion of baseball pitching is arguably the most studied overhead motion at the elbow, research has been conducted on other aspects of overhead motion at the elbow. One of these is the tennis serve. Similar to the baseball pitch, the tennis serve depends largely on kinetic chain contributions for the generation of force. A study by Kibler et al showed that between 63% and 74% of the kinetic energy delivered to the hand originated in the hip, trunk, and shoulder segments.38 The tennis serve also generates large amounts of angular velocity of extension and forearm pronation39,which, similar to the baseball pitch, produces valgus torque and requires kinetic and kinematic counteractive varus torque.* Another similar motion is that of the football pass. However, studies by Fleisig et al and Shapiro et al have shown that there are fairly significant differences between the kinetics of these two motions.40,41 In the ‘arm cocking’ phase of the football throw, elbow flexion tends to range between 100 degrees and 120 degrees, as opposed to 80 degrees to 100 degrees in baseball. Maximum varus torque in acceleration, maximum extension velocity in acceleration, maximum proximal force in deceleration, and maximum flexion torque in deceleration were all found to be lower in football passing than in baseball pitching. The lower levels of force on the elbow in a football pass allow for the passer to have an abbreviated follow through as compared to a baseball pitcher, which serves more practical in a game-action situation. Additionally, the lower levels of force on the elbow may be the reason for a decreased incidence of elbow injury in football quarterbacks compared to baseball pitchers. However, Loftice et al posit that, due to the kinetic and kinematic differences between the two motions, performing each in the same season may lead to faulty mechanics in one or the other and may ultimately lead to a higher chance of injury.
Pathomechanics
The role of abnormal force relationships in creating diseased states is crucial to rehabilitation professionals for the purposes of injury prevention and earlier return to sport. The study of pathomechanics, or the process in which abnormal kinetics leads to disease, is of great value to the rehabilitation process (Hertel, 2002). In the elbow joint, overhead kinetics in throwing athletes is of particular importance in the study of pathomechanics of the elbow. Overhead throwers generate tremendous amounts of valgus stress that must be counteracted by rapid elbow extension. This process generates significant stress in the compartments of the elbow. Inability to generate sufficient elbow varus torque to overcome the tremendous valgus stress generated in throwers may result in medial tension, lateral compression, or posteromedial impingement injury (Fleisig et al., 1995). Repetitive throwing exposes the elbow joint to repeated near-failure valgus stress, shear stress, and impingement of the posterior compartment. In regard to the phases of overhead throwing, most injuries of the elbow occur during the acceleration phase due to the substantial forces generated. In baseball, for example, pitches thrown 80 miles/hour (130 km/hour) generate 64 Newton-meters (Nm) of torque (Fleisig 1995). Half of this force is transmitted to the UCL with reliance on surrounding muscle groups to counteract and dissipate the stresses generated on the elbow in throwing. Cadaveric studies have shown that at 90 degrees of flexion, the greatest contributors to the resistance of valgus stress are the UCL (54%), the osseous component of the joint (36%), and the anterior joint capsule (10%) (Morey 1983, Wo, 2017). Varus strain, on the other hand, is largely counteracted by the osseous component of the elbow at 55% during full extension and 75% at 90 degrees of flexion (Morey 1983, Wo, 2017). When these mechanisms fail, undue stress is placed on the elbow joint and surrounding stabilizers. Here, we discuss the phases of overhand throwing and the associated pathomechanics of the elbow in each phase.
Phases 1 & 2: Windup and Stride
Overhead throwers begin the throwing motion with phases 1 and 2 referred to as windup and stride or early cocking. In these phases, the elbow is flexed and the forearm is pronated. The early stages of overhead throwing have minimal elbow joint involvement and thus elbow pathology is rare in these stages.
Phase 3: Cocking
During phase three, the elbow is flexed between 90 and 120 degrees while the forearm is pronated. The thrower’s arm is overhead as the shoulder is abducted and the arm is externally rotated. The UCL and wrist flexor-pronator complex provide varus forces on the elbow during this phase. Sixty-four N-m of elbow varus torque is generated at the moment just before maximal external rotation (Fleisig et al., 1995).
Phase 4: Acceleration
The acceleration phase of throwing is the most relevant in the discussion of pathomechanics of the elbow and approximately 85% of patients with thrower’s elbow will experience pain during this phase (Cain, 2004). The elbow is rapidly extended at a rate of 600,000 degrees/s (Patel et al., 2014). This rapid extension is necessary to counteract the large valgus forces placed on the joint during the forward motion of the arm in throwing. The anconeus, wrist flexors, and triceps are activated after the point of maximal external rotation of the arm. This activation may impart compressive, stabilizing forces at the joint to counteract the centrifugal forces on the arm that distract the forearm from the elbow joint during acceleration (Werner et al., 1993). Failures of compressive or varus counteraction forces produce much of the pathology seen in the elbow of throwing athletes including medial tension, lateral compression, or posteromedial impingement injury.
Over time, with repeated trauma to the UCL complex, the overhead thrower is prone to developing ligamentous insufficiency. This insufficiency has multiple sequelae including abnormal flexor-extensor muscle activation as well as ulnohumeral osteochondral and ligamentous overload. In an EMG study by Glousman et al., it was found that pitchers with chronic UCL injuries demonstrated increased activity of the extensor carpi radialis brevis and longus and decreased activity of the triceps, flexor carpi radialis, and pronator teres when throwing fastballs. If this latter group of muscles functions as dynamic stabilizers during the acceleration phase, the overhead athlete may be susceptible to further elbow injury. Higher-level athletes with greater muscle mass capable of producing increased torque at the elbow would be at even greater risk (Fleisig et al, 1999).
Another common complication of UCL insufficiency in the overhead athlete is valgus extension overload (VEO). VEO is a constellation of symptoms characterized by reproducible pain at the posteromedial tip of the olecranon process. These injuries occur through a predictable mechanism and present symptomatically in the posterior elbow (Wo, 2017). The pathologic mechanism of VEO is repetitive valgus stress on the elbow during extension which is accentuated in UCL laxity or deficiency. These scenarios can lead to statistically significant decreases in contact surface area and a concomitant increase in contact pressure between the olecranon and posteromedial humeral trochlea (Ahmad CS et al., 2004, Osbahr DC et al., 2010). Olecranon chondromalacia may result from repeated stress under such conditions of altered contact between the two structures.
While the UCL makes the greatest contribution to valgus resistance, the flexor-pronator muscle group also contributes to up to 50% of dynamic valgus resistance (Park, 2004; Wo, 2017). Hypertrophy of the flexor-pronator muscles through repeated contraction can mediate valgus stress on the UCL in throwing athletes (Ciccotti, 2004).
During the acceleration phase of overhead throwing, the ulnar nerve is particularly susceptible to longitudinal traction (Schickendantz, 2002). The nerve may be compressed at a number of sites including the arcade of Struthers and the medial intermuscular septum within the cubital tunnel (Patel et al., 2014). The most common location for ulnar nerve compression is at the cubital tunnel, as elbow flexion may increase compression up to twenty times and subsequently, additionally compromise the vascular supply (Posner, 1998). Ulnar nerve compression may also occur from osteophyte formation, synovitis, nerve subluxation, valgus stress, loose bodies, and muscle hypertrophy (Cain, 2003). Due to the variety of etiologies, careful examination is important in the evaluation of the athlete with suspected ulnar neuropathy.
Phase 5: Deceleration
Phase five of overhead throwing describes a rapid deceleration as much of the kinetic energy generated during acceleration is dissipated. In comparison to the acceleration phase, less than 25% of patients with thrower’s elbow will experience pain during this phase (Cain, 2004). Solomito et al demonstrated the elbow varus moments occurring during the deceleration phase were between 40% and 50% of the magnitude of the peak acceleration phase elbow varus moment. This additional moment occurs at a time when the upper arm muscles are acting eccentrically and may be a potential source of injury for baseball pitchers. During the acceleration phase of the pitching cycle in which the peak elbow varus moment is encountered, there is the greatest potential for injury. However, the varus moment generated during the deceleration phase occurs 24% of the pitch cycle later than the initial peak moment, adding additional stress to the UCL which may place the UCL at a greater risk of fatigue failure (Solmito et al., 2019 – Elbow flexion post ball release is associated with the elbow varus deceleration moments in baseball pitching).
Large compressive forces are also generated during this phase of throwing. During deceleration, the posteromedial portion of the olecranon makes contact with the trochlea and olecranon fossa as the elbow terminally extends and the angular momentum of the forearm dissipates (Paulino 2016). At the medial ulnohumeral articulation, this distribution of force leads to the pathologic shearing, compression, and subsequent microtrauma that induces the formation of reactive bone on both the posterior and medial facets of the olecranon process (Paulino, 2016; Wo, 2017). These bony osteophytes lead to additional impingement in the fossa that may lead to crepitus, catching and pain within this posterior compartment (Kancherla; Wo, 2017). A similar process can be observed with VEO, UCL or flexor-pronator injury, which additionally subjects athletes to the formation of osteophytes (Ahmad, 2004; Paulino, 2017).
Injury to the mechanisms of valgus resistance may also lead to compression of lateral structures, which predisposes to osteochondral defects at the articulation between the radius and capitulum or chondromalacia (Miller, 1994). Stress fractures to the olecranon process are also common to overhead athletes, particularly in skeletally immature patients (Furushima, 2014; Cain, 2003). The most frequently seen fracture is an oblique or transverse fracture in the middle third of the bone caused by repeated microtrauma or excessive tension from the triceps muscle (Cain, 2003; Wo, 2017). Care should be taken with skeletally immature athletes. Stress fractures in this population may cause a Salter-Harris type I and could subsequently impede olecranon apophysis closure, a condition under the term “little leaguer’s elbow” (Furushima, 2014; Cain, 2004). Further research is needed to characterize the importance of this stress in the pathomechanics of the elbow.
Phase 6: Follow-through
During the final stage, the elbow reaches full extension and the throwing motion comes to completion. The olecranon can again come into excessive contact with the posteromedial trochlea as the elbow undergoes terminal extension, particularly with an abbreviated follow-through arc of motion. Osteophytosis of the olecranon may occur in the context of repetitive compressive contact with the humeral trochlea. As such, overhead athletes can often present with loss of throwing accuracy and velocity, elbow impingement symptoms, decreased range of motion, and pain (O’Connell & Field, 2020).
Injury to the UCL has also been observed in a number of non-throwing sports. Gymnasts frequently perform back handsprings that require a 360-degree rotation of the entire body in the sagittal plane, with full-body weight supported by the upper extremities. The reactive forces at the elbow during this maneuver produce approximately 2.37 times the bodyweight force of compression at the elbow, with a valgus force equalling approximately 0.03 x body height x bodyweight (Ramos, 2019; Koh, 1992). In Brazilian jiu-jitsu (BJJ) and mixed martial arts (MMA), the arm may be used in opponent submission techniques. Escape tactics requiring external rotation against a submissive force may subsequently generate a large valgus force at the elbow that can lead to injury of the UCL (Ramos, 2019; Scoggin, 2014). And UCL injuries in professional Americal football also occur through valgus force mechanisms. For example, valgus forces of blocking of defensive and offensive linemen with forward stretched arms account for 50% of injuries and applications of valgus force with the hand planted on the ground counts for 29% of injuries (Ramos, 2019; Kenter, 2000). Injuries to the non-throwing athletes are often the result of contact or traumatic injury whereas throwing athletes incur UCL injury from chronic and repeated stress.
The elbow complex is a biomechanical construct that is important for the general function of all humans. In the area of sport, it is a load transfer mechanism. Its anatomical and biomechanical properties allow it to accept huge compressive, rotary, and tensile forces without breakdown. It is usually a process of poor mechanics or breakdown of other parts of the kinetic chain that leads to damage to this joint. It is also a joint that needs to maintain a delicate balance for homeostasis. When considering surgery or rehabilitation, there should be great effort to minimize secondary damage or scar, while maintaining range of motion and multi-joint mechanics.
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