PITCHING: BIOMECHANICS AND INJURIES
S. Peter Magnusson
Nicholas Institute of Sports Medicine and Athletic Trauma,
Lenox Hill Hospital
New York, N.Y., USA
Correspondence: S. Peter Magnusson
NISMAT-Lenox Hill Hospital
130 East 77 Street
New York, N.Y. 10021
Telephone: (212) 439-2700
Fax: (212) 439-2687
Baseball pitching is an extremely complex neuromuscular task that requires space awareness, precise timing, and coordinated dynamic muscle action. The highly skilled pitcher can in an efficient and speedy manner generate a tremendous propulsive force. However, the pitching motion transpires so rapidly that it is difficult for the naked eye to appreciate the complexity of the movement. With the advent of high speed cinematography and computer assisted data analysis it is now possible to break down the pitching motion into separate phases and carefully analyze them independently. Additionally, electromyographic studies have been able to demonstrate the selective use of muscles in pitching. An improved understanding of the mechanics of pitching can assist in teaching and training. Information about the specific muscle activity of pitching can aid with the development of appropriate conditioning and strengthening programs for purposes of injury prevention. Pitching is an activity that places extreme stress on the throwing arm, however, the entire body is involved in the pitch. The body can be viewed as a series of links, so that in a right handed pitcher the links extend from the left foot, across the trunk, to the right hand. In this series of links forces are transmitted and summated across segments to generate tremendous propulsive force at the distal segment, in this case the right hand. Pitching requires a delicate balance between mobility and stability of the shoulder joint. Mobility of the shoulder joint is paramount to pitching, particularly external rotation. The joint capsule, ligamentous structures and glenoid labrum are static restraints which prevent excessive mobility and consequently injury. The large muscles around the shoulder, and the smaller rotator cuff muscles, provide the fine tuned dynamic restraints. Cumulative trauma is common in pitchers because of the high forces that are generated across the upper extremity and the repetitive nature of baseball pitching. Furthermore, pitching requires a large range of motion in the shoulder joint which requires an increased demand on static and dynamic restraints. Selective muscle weakness, or a lack of timing and coordination may result in increased demand on the dynamic and static structures, which over time will lead to cumulative trauma and occasionally acute trauma. It should be noted that differences exist between pitchers in their throwing mechanics. However, the individual pitcher benefits from throwing in a consistent manner as this may improve efficiency as well as concealing the type of pitch. This chapter will briefly review the common ground of the motion and shoulder muscle activity of pitching using a right handed pitcher as a model. The pitch has been divided into five stages: wind-up, early and late cocking, acceleration, and deceleration (Figure 1). In addition, some common pathology of the upper extremity of throwing athletes will be discussed.
The wind-up phase serves several important functions. It conceals the ball from the hitter and it places the various body segments at their most advantageous position to initiate the propulsive phase of the throw. Additionally, it initiates a rhythm for subsequent coordinated dynamic movements. The wind-up phase begins with the pitcher standing with his pivot foot (right) pointing towards the home plate. He takes a small step back with his stride leg (left) before he pushes off with it. As the stride leg leaves the ground the arms are raised together to conceal the ball. The weight is now entirely on the pivot leg. Simultaneously as the arms are raised either over the head or to chest level the hip and knee of the stride leg are flexed and comes across the body. The stride leg remains flexed and the arms are then dropped while the trunk rotates 90 degrees. The pitchers keeps his eyes on the home plate all along. The wind-up phase ends when the ball leaves the glove.
During the wind-up phase relatively low forces are placed on the throwing arm. Consequently the muscular activity is minimal and as a result very few injuries are incurred in this phase.
The cocking phase is divided into an early and late phase. In the early phase the pivot knee is slightly bent to lower the center of gravity and prepare for the forward lunge. The pivot leg then forcefully lunges forward while the stride leg remains adducted across the trunk in the flexed position. The non-throwing arm also has a forward momentum. Concurrently a segmental rotation takes place towards the home plate starting with the pelvis and continuing with the trunk and finally the shoulder girdle. The stride leg is then abducted, rotated and extended towards the home plate in preparation for foot plant. This stride leg motion assists in the segmental rotation of the trunk which is finally transferred to the shoulder complex. When the foot of the stride leg is planted the velocity of trunk rotation is fastest. The combination of the segmental rotation, the forward motion of the stride leg and non-throwing arm and the backward motion of the pitching arm places the pitcher in a maximally stretched position. The stretched position allows for subsequent recoil in the acceleration phase. When the stride foot is planted the pitching arm should be abducted 90 degrees, elbow flexed to 90 degrees and the humerus should be in neutral rotation with respect to the trunk. The foot plant ends the early cocking phase. In the late cocking phase the arm continues to externally rotated behind the trunk until it reaches maximal external rotation. This external rotation is primarily achieved by the momentum of the rotation of the legs and the trunk leaving the pitching arm behind the trunk. At the end of the late cocking phase the pitching arm can be in as much as 180 degrees of external rotation. There is no forward ball movement in the cocking phase.
Considerable muscle activity is noted in the shoulder girdle complex in the cocking phase in comparison to the wind-up phase. In the early cocking phase when the arm is elevated to 90 degrees the anterior, middle and posterior heads of the deltoid muscle are very active. However, other muscular activity is relatively low. The activity of the deltoid muscles subsequently diminishes during late cocking. In the late cocking, increased muscle activity serves primarily to stabilize the humeral head in the glenoid fossa and to decelerate the external rotation of the arm. The biceps muscle shows moderate activity in this phase. The late cocking phase is terminated by the strong eccentric action of the latissimus and pectoral muscles which serves to protect the joint. Additionally, in this part of the cocking phase the activity of the rotator cuff muscles is dramatically increased demonstrating their integral function in the throwing motion. The supraspinatus muscle is most active at the end of the late cocking phase. In this position the humeral head is most vulnerable to subluxation and therefore the supraspinatus plays a prominent role in stabilizing the humerus. The teres minor and infraspinatus muscles externally rotate the humerus. These muscles display increased activity in the late cocking phase. This activity also serves to reinforce the stability of the humeral head in the glenoid fossa. The subscapularis eccentrically contracts to decelerate the external rotation of the humerus at the end of the late cocking phase. The serratus anterior muscle plays an important role in moving and stabilizing the scapula during late cocking.
It takes approximately 3 to 4 seconds to go through the pitching motion. However, the acceleration phase is an extremely explosive movement which takes a mere fraction of a second. The angular velocity of the internal rotation of the shoulder has been measured as high as 9,000°.sec-1. The acceleration phase begins when the pitching arm has reached terminal external rotation. The arm begins to internally rotate and extend at the elbow. The forearm pronates and finally the wrist flexes from an extended position and the fingers extend to release the ball marking the end of the acceleration phase.
The muscle activity in this phase serves primarily to generate a propulsive force which is ultimately transferred to the ball. The ballistic stretching during the cocking phase preloads all the involved muscles, maximizing the length-tension relationship and ultimately allowing for maximal force production. The subscapularis which reached its maximal activity in the late cocking phase has substantial activity in the acceleration phase as well. Here the subscapularis muscle acts to assist with the internal momentum of the humerus. In addition, the powerful latissimus and pectoral muscles show considerable activity to further assist with the internal rotation moment of the humerus. The triceps show markedly increased activity, which serves to extend the elbow and add to the propulsion prior to ball release.
The deceleration phase begins when the ball leaves the hand and continues until the pitching motion has ceased. The humerus continues to internally rotate and move across the body while the elbow flexes slightly. At this time the body can be described as following the pitching arm as some of the energy is translated to the trunk. Energy transfer causes the trunk to rotate and swing the pivot leg forward positioning the pitcher for a batted ball.
During the previous phases there have been a build up of momentum and energy for propulsion which now has to be dissipated in the relatively brief deceleration phase. Therefore, there is a substantial eccentric demand of several shoulder girdle muscles to absorb the energy and slow down the arm moving across the body. Infraspinatus and teres minor contractions keeps the humeral head in the fossa as the subscapularis continues its activity from the acceleration phase. Trapezius shows increased activity which serves to adduct the scapula. The elbow flexors also eccentrically contract to decelerate the forearm.
Injuries of the shoulder and elbow complexes in pitchers are not uncommon. Fifty percent of baseball pitchers report that at some point injuries kept them from pitching. The forces generated in the pitching motion are considerable and they have to be absorbed by muscles, tendons and bone. Additionally, the repetitive nature of pitching places unique demands on the upper extremity. These demands result in soft tissue adaptations in the shoulder. Pitchers have an exceptional increase in external rotation of the shoulder with a reciprocal decrease in internal rotation. This increased external motion serves to add moment to the acceleration phase. However, because of the enhanced mobility of the shoulder joint it places increased demand on dynamic contribution to joint stability. Although pitching requires an unusual muscular demand pitchers do not develop asymmetric upper extremity strength. The repetitive motion of pitching frequently leads to cumulative trauma. On the contrary, macrotrauma is a rather infrequent end result of pitching.
Tensile stress produced by the valgus moment in the late cocking phase and acceleration phase puts considerable stress on the medial joint capsule and the medial collateral ligaments of the elbow. The anterior portion of the medial collateral ligament is taut throughout flexion and extension of the elbow and is therefore more susceptible to cumulative trauma. Skeletally immature individuals with an open medial growth plate are particularly prone to injuries in this area. Additionally, repetitive stress of the medial joint capsule and ligaments can lead to bone spur formation.
Valgus moment which causes tensile stress on the medial aspect of the elbow simultaneously produces compressive forces of the radial head and capitellum on the lateral aspect of the elbow. Compression may lead to articular cartilage degeneration and subsequent loose body formation in the joint. Again, the epiphysis is particularly prone to damage due to compressive forces in the skeletally immature athlete.
The extension moment of the elbow caused by the acceleration phase causes the olecranon to forcefully make contact with the olecranon fossa. This repetitive compressive force can lead to osteophyte formation and loose body formation. Occasionally it will result in stress fractures of the olecranon.
Wrist extensors originate from the lateral aspect of the elbow. Pathology of these muscle-tendon units is common in racquet sports, but uncommon in the throwing athlete. Wrist flexors and pronator muscles originating from the medial aspect of the elbow are commonly afflicted in pitchers. Late cocking and acceleration phases produces a valgus moment of the medial aspect of the elbow. Wrist flexion attempts to counteract this valgus moment. Repeated tensile stress of these muscle-tendon units can cause cumulative trauma over time resulting in inflammation and pain of the inner aspect of the elbow, which is aggravated with throwing. The posterior and anterior aspect of the elbow house the triceps muscle and elbow flexor muscles respectively. Cumulative trauma of these muscle-tendon units can occur but they are not as commonly involved as those of the medial aspect.
The medial collateral ligaments make up the floor of the cubital tunnel in which the ulnar nerve traverses. Spur formation can elevate the medial collateral ligaments and consequently compress the ulnar nerve. Distally the medial nerve courses anteriorly between the two heads of the pronator teres. Compression stress with repeated pronation can result in neuritis.
The acromioclavicular joint is an important link between the extremity and the trunk. It is stabilized by the coracoclavicular ligaments and the joint contains a fibrocartilage disc. To accomplish abduction and extreme external rotation of the humerus in the cocking and acceleration phase axial rotation occurs at the acromioclavicular joint placing shear forces on the acromioclavicular joint and its fibrocartilage disc. In the deceleration phase when the arm moves across the body this joint is experiencing a compressive load. Repetitive use of the joint in this manner can lead to early degenerative changes.
Repeated anterior shoulder subluxation is a common problem in throwers. In the late cocking phase and acceleration phase the anterior inferior glenohumeral ligaments is stressed and with repeated use may be compromised. When these ligaments do not offer adequate static stabilization there is excessive anterior humeral head translation. The pitcher may complain of a “dead” arm with repeated throwing or that something does not feel right. Often external rotation is limited in these throwers. Anterior shoulder subluxation is also associated with impingement syndrome and labral tears.
The throwing athlete who has an uncorrected anterior instability and continues to throw displays some specific muscular activity patterns. The biceps and supraspinatus muscles show increased activity in the late cocking and acceleration phases. This increased activity may be an attempt to stabilize the humeral head in the glenoid fossa. Decreased activity of the internal rotator muscles during the pitch causes additional humeral external rotation. Lack of a protective eccentric contraction of the internal rotators is thought to contribute to the anterior instability.
The glenoid labrum is a wedge of fibrous tissue surrounding the rim of the glenoid fossa. Glenohumeral ligaments attach to the labrum and the wedge deepens the relatively shallow glenoid fossa to improve glenohumeral stability. Labral tears can result from shear forces which are amplified in the cocking and acceleration phase in the shoulder with increased anterior humeral head translation. The long head of the biceps attaches at the anterosuperior labrum. A forceful contraction of the biceps during the deceleration phase may tear the biceps labral complex. Labral tears are associated with a popping and slipping sensations.
Most injuries associated with throwing result from cumulative trauma. Infrequently the high forces imparted on the throwing arm may result in acute trauma. Spiral humeral fractures have been reported in the literature in the throwing athlete. This fracture of the mid to distal humerus appears to occur as the torque is transmitted to the humerus during the acceleration phase.
Impingement syndrome is not an uncommon problem in the throwing athlete. The subacromial complex is formed by the undersurface of the acromion, the coracoacromial ligament and the soft tissue in the subacromial space; the long head biceps tendon, the subacromial bursa and the rotator cuff tendons. The rotator cuff consists of the supraspinatus, infraspinatus, subscapularis and teres minor muscles. As the arm elevates the humeral head rolls and glides to clear the acromion and coracoacromial ligament. Should adequate clearing not take place the soft tissue in the subacromial space will impinge. This leads to pain and inflammation of the one or more structures in this space. Muscle imbalance in the rotator cuff muscles may result in inadequate humeral compressive forces during elevation causing impingement. The vertical force decreases the subacromial space resulting in impingement of the soft tissues. Additionally, anterior glenohumeral instability can lead to rotator cuff cumulative trauma.
Throwers with impingement syndrome also have specific muscular activity patterns should they continue to throw. In the late cocking phase there is increased deltoid activity which may serve to decrease the demand on the supraspinatus muscle. In the cocking phase there is decreased activity in the internal rotators which increased humeral external rotation. Increased external rotation may result in greater vertical forces on the humerus which compromises the subacromial space.
Thoracic outlet syndrome is a rare entity in throwing athletes. However, as the nerves pass from the cervical spine to the shoulder girdle muscles they may become compressed in the neck region, specifically between the middle and anterior scalene muscles. The athlete may complain of diffuse neck and shoulder pain with tingling, numbness, and weakness in the arm. The suprascapular nerve innervates the infraspinatus and supraspinatus muscles. This nerve courses through the scapular notch and around the neck of the scapular spine where it may become entrapped. Suprascapular nerve entrapment is associated with posterior shoulder pain and atrophy and weakness in the rotator cuff muscles. Because of the weakness it is commonly accompanied by tendinitis and bursitis.
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Figure 1: The five phases of pitching: wind-up, early cocking, late cocking, acceleration and deceleration. (Reproduced with permission from Perry J. and R. Glousman: Biomechanics of throwing. in The Upper Extremity in Sports Medicine E.B. Hershman and J.A. Nicholas (eds), C.V. Mosby Co, St Louis, MO, 1990, p. 740)