Understanding Shoulder Kinetics for Peak Performance (Part 1, Throwing, Torque and Training)

In my last blog post, I discussed a more kinematic (movement) and integrated approach to understanding the complexities of the shoulder. On the flip side, the first part of this article will take a more kinetic approach with less of an appreciation for kinematics, delving deeper into some of the musculature responsible for controlling motions, as well as an acknowledgment for the forces encountered. Currently, the scientific literature available is relatively limited and unclear in terms of research on a wide spectrum of sports and the demands placed on the shoulder. Therefore, I will be predominantly exploring the baseball pitching literature, relating it to kinematically comparable motions found in other sports that I believe have high transferability. This can hopefully provide a breadth of information to form a foundational understanding of the velocities, forces, and overall demands placed on the glenohumeral joint. However, where applicable, I will try to draw upon research from collision or combat-based sports to build on this discussion.

Research in any field can play a vital role in guiding decisions and shaping program design to ensure individuals are as prepared as possible for the demands and rigour of specific sporting tasks. Effective program design must emphasise task-specificity and implement the most appropriate strategies to train the shoulder, not only for optimal performance but also for resiliency. Therefore, this can be considered a very individualistic approach. Furthermore, a deep understanding of the forces the shoulder must withstand during sporting tasks is essential; without it, training may neglect this joint, potentially limiting the performance ceiling and increasing injury risk. We know that due to the anatomical make-up of the shoulder joint, it is capable of huge ranges of motion to position the hand anywhere in space. This requires the harmonious integration of many muscles to create fluid, efficient, congruent, and stable movement at the joint, reiterating the importance of well-thought-out and balanced training protocols to optimise performance as well as function.

When it comes to appreciating the sorts of velocities and forces the shoulder has to cope with during a sporting action like throwing or pitching, we can investigate peak internal rotation angular velocities and overall distraction forces. Firstly, angular velocity can be defined as the rate of change of angular position of a body segment around a specific axis of rotation over time, measured in degrees per second. Fleisig et al. (1999) and Pappas et al.’s (1985) studies calculated that peak internal rotation angular velocities during the throwing motion ranged between 6,000°/s and 9,000°/s in elite baseball pitchers. This is the fastest in all of human motion, placing the surrounding tissues under extreme stress that the shoulder must be able to repeatedly tolerate. Additionally, distraction forces can be simply described as forces that effectively want to dislocate your shoulder. Austin Einhorn talks at length about the benefits of getting comfortable with hanging and the initial stages of pull-ups or any vertical pulling movements. He takes this view due to our evolutionary backgrounds, as we are a species, along with a few others, that have a rotator cuff designed for brachiation (aka. swinging and suspension!). Not only does hanging encourage strength at length in the rotator cuff, but it also enables us to provide a stimulus great enough to really challenge peak force characteristics of the cuff. This has been backed up by the work of Dr. Sherry Werner in her 2001 study alongside Post and colleagues (2015), who calculated that distraction forces can reach 108-110% of bodyweight in individuals during a baseball pitch.

Furthermore, I believe it would be highly beneficial to gain insight into the specific positions during pitching or serving motions where the shoulder experiences the greatest force, or peak resultant joint torque. Peak resultant torque is the point at which the greatest rotational stress and force is placed upon a specific joint, in the case of throwing and serving motions, the glenohumeral joint. According to a study by Feltner & Dapena (1986) on baseball throwers, peak resultant joint torque occurs in the late cocking stage, very near maximal external rotation (MER), more specifically around 150-180°. This influential paper alludes us to the importance of individuals not only being able to express large magnitudes of force in their internal rotators but crucially near MER. Fully understanding these three kinetic parameters of angular velocity, distraction forces, and peak resultant joint torque will heavily underpin our decisions when it comes to programme design, which will be discussed in the second part of this article.

Up until this point, I have only discussed the shoulder in relation to baseball pitching and other kinematically similar motions. However, Ben Ashworth and colleagues’ (2018) influential work around shoulder rehabilitation and performance introduces us to the ASH (Athletic Shoulder) test, designed to assess peak force and rate of force development (RFD) in three different positions (0°, 90°, and 180° of shoulder abduction). The study used the ASH test to examine how well a group of rugby players transferred forces across the shoulder in those long lever positions, similar to what one may encounter during a collision or tackle in rugby. According to Ashworth et al. (2018), “the ASH test may have value in tackling sports and in other sports, occupations, and tasks that expose the shoulder to longer lever stress or require the ability to transfer forces similarly across the shoulder girdle.” The ASH test has also confirmed the importance of both peak force and RFD capacities in injury rehabilitation and performance settings. Generally speaking, those who displayed poor peak force and RFD capabilities are generally at greater risk of injury. Fundamentally, if we reconsider the testing positions of the ASH test, we can quickly relate them to sporting tasks, reiterating the importance and relevance of such a test in any sports requiring the upper limbs. Although approximate lines in the sand have been drawn in terms of the values of peak force and RFD individuals should aim to reach within the ASH test, I still believe there needs to be a larger pool of data from various sports to back this up.

Moreover, Ben Ashworth introduced me to the muscular imbalance concept of “overpull versus underpull” within the glenohumeral joint. More specifically, overpull of the internal rotators and underpull of the external rotators to ensure optimal joint congruency. Ultimately, we can gain a great deal of insight from the muscle architecture into which muscles should be contributing greater levels of force at certain points during specific tasks. For example, both the pec major and latissimus dorsi muscles both play a big role, if not the biggest, in not only producing huge magnitudes of force within specific tasks, but also in stabilising the glenohumertal joint through optimal length-tension relationships. This is down to their cross sectional areas (CSAs), or size. Muscles with larger CSAs have the ability to produce greater levels of force, which may sound bit obvious! Now you may be thinking, why is muscle balance something worth considering for an individual?

First, we must recognise the SAID principle, which states that our bodies adapt specifically to the demands we place on them. For example, a baseball pitcher throwing huge volumes will develop incredibly strong, robust internal rotators (predominantly pec major, lats, and subscapularis) purely through the task itself. This can become problematic if the other side of the coin is not addressed. The attachments of both the pec major and lat muscles (along with others!) can result in the humeral head gliding anteriorly within the socket, placing large amounts of stress onto the tissues on the anterior aspect of the shoulder. This can cause not only pain but also increase laxity within the joint capsule, creating instability or worsening it, and, in more severe cases, damage to the labrum in the form of SLAP tears. This raises the question of internal versus external rotation strength ratios within the shoulder. In an Instagram post, Steffan Jones again emphasises that “the constant throwing and bowling motion develops task-specific imbalances that cannot be helped. The skill itself is training and overloads the muscles. However, as always, you train off the field what you don’t cover on it. The external rotators become a priority off the field.” Throwing and bowling are just two examples of tasks that can create what Steffan defined as “task-specific imbalances," but these thoughts can be applied to numerous other sporting tasks.

Wilk et al. (1993) and Ellenbecker & Davies (2000) explored these external rotation (ER) to internal rotation (IR) strength ratios. Wilk et al. (1993) suggested that ER/IR strength ratios should range between 61%-65%, meaning the external rotators require 61%-65% of the strength of the internal rotators. In contrast, Ellenbecker & Davies (2000) proposed a slightly higher range of 66%-75% to maintain joint balance. The lower ratio in Wilk et al.’s study may reflect the repetitive nature of baseball pitching, which favours internal rotation in the athletes tested. Regardless, maintaining a ratio within these ranges is crucial for distributing force effectively during high-velocity, repetitive movements, preventing excessive load on a single structure (James, 2004 - “overuse hypothesis”).

Understanding the kinetics of the shoulder, more specifically the glenohumeral joint predominantly through the lens of baseball literature, highlights the complexity of its function across sporting tasks. The forces encountered by the joint - angular velocity, distraction forces, and peak resultant joint torque - place extreme demands on surrounding tissues and musculature. Appreciation of these forces alongside the utilisation of tools like the ASH test and simply assessing an individuals ER/IR strength ratios becomes paramount to enable us to build out a kinetically detailed picture of where an individual is currently limited. Ultimately, this is what underpins and guides our decision making when it comes to programme design which will be discussed in second part of this article.

References / Sources:

Ashworth, B., Hogben, P., Singh, N., Tulloch, L., & Cohen, D. D. (2018). The Athletic Shoulder (ASH) test: reliability of a novel upper body isometric strength test in elite rugby players. BMJ Open Sport & Exercise Medicine, 4(1), e000365.

Austin Einhorn - Founder of Apiros

Ben Ashworth - Founder of Athletic Shoulder & Elite Performance Consultant

Ellenbecker, T. S., & Davies, G. J. (2000). The application of isokinetics in testing and rehabilitation of the shoulder complex. Journal of athletic training, 35(3), 338.

Feltner, M., & Dapena, J. (1986). Dynamics of the Shoulder and Elbow Joints of the Throwing Arm during a Baseball Pitch. International Journal of Sport Biomechanics, 2(4), 235–259.

Fleisig, G. S., Barrentine, S. W., Zheng, N., Escamilla, R. F., & Andrews, J. R. (1999). Kinematic and kinetic comparison of baseball pitching among various levels of development. Journal of biomechanics, 32(12), 1371-1375.

James, C.R. (2004) Considerations of Movement Variability in Biomechanics Research. Innovative Analyses of Human Movement, United Kingdom: Human Kinetics., 29-62.

Pappas, A. M., Zawacki, R. M., & Sullivan, T. J. (1985). Biomechanics of baseball pitching: a preliminary report. The American journal of sports medicine, 13(4), 216-222.

Post, E. G., Laudner, K. G., McLoda, T. A., Wong, R., & Meister, K. (2015). Correlation of shoulder and elbow kinetics with ball velocity in collegiate baseball pitchers. Journal of athletic training, 50(6), 629-633.

Steffan Jones - Founder of PaceLab

Werner, S. L., Gill, T. J., Murray, T. A., Cook, T. D., & Hawkins, R. J. (2001). Relationships between throwing mechanics and shoulder distraction in professional baseball pitchers. The American Journal of Sports Medicine, 29(3), 354-358.

Wilk, K. E., Andrews, J. R., Arrigo, C. A., Keirns, M. A., & Erber, D. J. (1993). The strength characteristics of internal and external rotator muscles in professional baseball pitchers. The American journal of sports medicine, 21(1), 61-66.