The Thrower’s Shoulder (Part 2): A Few of the Main Biomechanical Factors to Know

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The base to build upon for training, throwing development, and arm care is a deep understanding of the basic anatomy of the shoulder (covered in Part 1). The initial goal should always be to better understand the parts of the shoulder complex and their individual actions. Understanding this basic anatomy can alleviate many of the issues that surround improper loading strategies, incomplete arm care plans, sub-optimal training design, or ineffective rehab approaches. However, in order to optimize the development, risk reduction of injury, and rehabilitation of the throwers’ shoulder we must better understand how each individual part works in synergy with each other to produce fluid, stable, and powerful movements. First we will discuss a few terms that relate to biomechanics and then give a brief synopsis of a few of the most important biomechanical aspects of the shoulder. 

Biomechanics: Biomechanics is the study of the mechanical aspects of living organisms, including the human body. It involves the application of principles from physics and engineering to understand how biological systems move and function.

Kinematics: Kinematics is the branch of mechanics that deals with the description of motion, including displacement, velocity, and acceleration, without considering the forces that cause the motion. 

Kinetics: Kinetics, in contrast to kinematics, involves the study of the forces that cause motion. It explores the relationships between forces, mass, and motion, providing insights into the factors influencing movement.

Arthrokinematics: Arthrokinematics specifically pertain to the study of joint motion. It involves the analysis of how bones move relative to each other at joints. 

Force Couple: Pair of forces, or a multitude of forces that work upon a joint to produce a desired movement. Force couples often provide stability and/or a rotational effect of a joint while preventing translation. Force coupling creates coordinated joint actions. 

Upward Rotation of the Scapula: In order to provide the necessary movement of the acromion (tip top of the scapula) away from humerus, the scapula must upwardly rotate, posterior tilt, and complete some degree of external rotation. The upward rotation is the most significant of these movements and requires a smooth force couple to effectively increase the sub-acromial space. The upward rotators are the upper trap, lower trap, and serratus anterior. Due to the normal dominance of the upper trap and relative weakness of the serratus; we often focus on serratus strength and control and limit the direct strengthening of the upper trap. Poor upward rotation leads to a myriad of dysfunctions but most notably is the ever so common sub-acromial impingement of the supraspinatus or infraspinatus tendons. Poor upward rotation and scapular mobility is also a performance limiter as it prevents terminal or max external rotation.

Downward Rotation of the Scapula: Downward rotation is created through co-activation of rhomboids, levator scapulae, and pectoralis minor. Stiffness or tension present in the rhomboids can prevent smooth upward rotation, additionally tightness in the pec minor can cause tipping or tiling of the scapula forward which limits effective acromion clearing when raising the arm overhead or rotating it externally. 

Clavicular Motion with Scapular Elevation: During scapular elevation, due to the clavicle's intimate attachment to the shoulder blade, the clavicle must upwardly rotate, retract and posteriorly tilt/roll. The clavicle undergoes around 15 degrees of elevation, 15 degrees of retraction and 15-30 degrees of rotation. This is why an AC joint injury of a throwing arm can be so painful due to the motion the ACJ still undergoes under high velocity elevation and rotation of throwing. Additionally, the clavicle requires stability from the coracoclavicular ligament to provide the proper seating and axis for movement of the clavicle. AC joint injuries can become complex in the throwing arm, and exceedingly painful. 

Roll and Glide of the Glenohumeral Joint During 90/90 External Rotation: When the arm is positioned at 90/90 (high-five position) in preparation for throwing, there are unique demands on the shoulder complex. This position is inherently unstable, especially in the front of the shoulder, with virtually no ligament stabilization anteriorly. This anatomical reality enhances the risk of anterior dislocation, but also the stretching of the anterior capsule. As the arm rolls backward into external rotation during the throw, it glides forward toward the front of the shoulder, traditionally this should keep the ball of the humerus centralized in the center of the glenoid (socket). However due to normally increased hypertrophy of the posterior capsule, muscle, and ligaments in throwers; often there is obligate translation where, under high velocity rotation, the ball actually slides and contacts the front of the capsule. This motion creates laxity in the front of the shoulder over time and can lead to anterior instability, posterior shoulder impingement, and/or labral tearing among other pathologies.  

Internal Rotation of Shoulder at Transition and Ball Release: At ball release an athlete produces a rapid amortization phase of external rotation transitioning to internal rotation in order to develop rotational velocity and arm speed. This motion then transitions to the release phase in which internal rotation velocity peaks along with elbow extension, protraction of the shoulder blade, and forearm pronation. In order for a thrower to properly decelerate their arm and decrease these forces as best they can, a thrower requires full scapular protraction  and retraction, strong eccentric posterior cuff strength, efficient co-contraction timing of the rotator cuff, eccentric biceps strength, and a strong/reactive lat to help dissipate forces. Poor stability during this phase can cause SLAP tears due to peel back or traction, bicep tendinopathy, lat strains or posterior cuff strains among other pathologies.

Deltoid vs. Rotator Cuff: The deltoid is a large and powerful muscle that in isolation mainly abducts the arm (lifts to the side). However, if the rotator cuff is insufficient to provide the anchor and stabilization of the humeral head, the deltoid will overpower the motion causing a shrug instead of a smooth lifting of the arm. This is called the shrug sign sometimes seen in rotator cuff tears. In order to prevent the humerus from “riding high” and shrinking the important sub acromial space (where the rotator cuff tendons lie) there needs to be a consistent relationship between the rotator cuff muscles and the deltoid, most importantly that the deltoid does not dominate the strength of the cuff. 

Summary

This blog post is just scratching the surface of isolated biomechanical movements that are important to throwers, as well as possible injury implications of faulty patterns. This list is by no means fully comprehensive, but in light of this blog post these are a few hallmarks that should be in the back of the mind of every performance coach, physical therapist, QB coach, biomechanist, etc who are working with a high level throwers. 

In part 3 of this series, I will expand on a few of the most common dysfunctions and injuries experienced by throwers, how they happen (MOI), and what are a few of the risk reduction strategies someone can implement.