Charles Darwin once said that apes “...are quite unable, as I have myself seen, to throw a stone with precision”.1 Yet humans can throw with precision and speed, a skill that likely had significant advantages: throwing can affect change at a distance—something few species can do. Throwing can provide protection against predators and can allow for predation for food resources. Throwing would be important in contesting other hominids for scarce resources. As such, throwing has been critically important in human evolution and likely is a skill that has been promoted through natural selection.2-5
In the orthopedic literature, most published work on throwing will ask proximate questions: “how, what, who, when, and where?” Evolutionary biologists are concerned with ultimate questions6,7: “why?” Asking ultimate questions provides insight into how a behavior might offer advantages under natural selection, which can then improve our understanding of the proximate questions for that behavior.
With regard to the shoulder, a number of mysteries exist that, to date, proximate studies have not been able to solve. This article argues that the human shoulder has evolved for throwing and by using this frame of reference, many of the mysteries surrounding the anatomy of the shoulder can be understood.
Pitching Kinematics
The mechanics of pitching have been analyzed extensively. Fleisig and colleagues8 performed kinematic and electromyographic analyses of pitchers to identify the critical moments of pitching (defined as where the forces are highest and injury is most likely going to occur). They found 2 moments where the forces about the shoulder are highest during pitching: the late cocking phase (defined by the point where the humerus reaches maximal external rotation); and the early deceleration phase (defined by the point when the ball is released). If throwing is important in natural selection of humans, then the shoulder anatomy should be optimized to withstand the forces generated in these positions.
Late Cocking Phase of Throwing
The early phases of throwing are attempting to maximize external rotation of the abducted arm as the velocity of the pitched ball correlates to the amount of external rotation achieved.9-11 In this position, kinetic energy in external rotation is stored and converted into kinetic energy in internal rotation.12 The position of the shoulder during late cocking is 94 ± 21° of thoracohumeral abduction, 11 ± 11° of horizontal adduction, and a remarkable 165 ± 11° of thoracohumeral external rotation (Figure 1).8
Fleisig and colleagues8 estimated the torque and forces about the shoulder, which are quite high for joint compression (480 ± 130 N). They also analyzed the shear forces and while trying to describe the origin of superior labrum anterior to posterior (SLAP) lesions and anterior labral tears, broke down the major shear vector into an anterior force vector (310 ± 100 N) and a superior force (250 ± 80 N).8 Note that the resulting shear vector is in an anterosuperior direction and is approximately 400 N.
Early Deceleration Phase of Throwing
Interestingly, the position of the humerus during this critical moment of throwing is not much different than the position during the late cocking phase of throwing, with 93 ± 10° of thoracohumeral abduction, 6 ± 8° of horizontal adduction.8 The major difference in the position of the arm is found in the amount of thoracohumeral rotation, which is now 64 ± 35° of external rotation (Figure 2).8
The forces in early deceleration are tremendous, with an estimated 1090 ± 110 N joint compression force, and an anteroinferior shear force of approximately 130 N.8
Clearly, if throwing is an important skill in human evolution, adaptations must exist in the shoulder to withstand the high forces in these 2 critical moments of throwing.
Solving Mysteries of Shoulder Anatomy in the Context of Throwing
There are many anatomic features of the shoulder that remain poorly understood. These include the alignment of the glenohumeral joint, the function of the glenohumeral ligaments, the function of the coracoacromial ligament, the depression of the human greater tuberosity, and the nature and function of the very tendinous subscapularis and long head of the biceps. These mysteries of the human shoulder can be solved if one considers the hypothesis that the shoulder has evolved to throw.
Glenohumeral Joint Alignment
The cartilage of the humeral head is thickest at its center, and thinnest at the periphery (Figure 3A).13,14 Conversely, the cartilage of the glenoid is thinnest at the fovea and thickest in the periphery (Figure 3B).14 It seems obvious that in order to maximally distribute high loads across this joint, the center of the humeral head should rest in the center of the glenoid. Interestingly, this does not occur during most positions of the shoulder. When upright, the center of the humeral head is directed above the glenoid in the coronal plane (Figure 3C). In order to align the glenohumeral joint optimally for the distribution of loads across the joint, the humerus must be abducted approximately 60° relative to the scapula. Assuming a 2:1 glenohumeral to scapulothoracic abduction for arm abduction relative to the thorax,15 this equates to approximately 90° of thoracohumeral abduction—the exact kinematic position of the shoulder during both critical moments of throwing (Figure 3D).
