Sports Medicine Research: In the Lab & In the Field: Adaptations of Shoulder Joint Stiffness may Lead to Increased Muscle Efficiency and Neuromuscular Control (Sports Med Res)
Monday, October 1, 2012

Adaptations of Shoulder Joint Stiffness may Lead to Increased Muscle Efficiency and Neuromuscular Control

Neuromuscular and stiffness adaptations in division I collegiate baseball players

Thomas SJ., Swanik CB., Higginson JS., Kaminski TW., Swanik KA., Kelly JD. Nazarian LN. Journal of Electromyography and Kinesiology. 2012; ahead of print.

During overhead throwing energy must be absorbed and distributed across soft tissues (i.e., musculotendinous structures, joint capsule/ligaments) within a limited range of motion to minimize the risk of injuries. This is primarily accomplished by the interaction between the nervous and musculoskeletal systems that influences shoulder stiffness. Active stiffness, which combines passive (e.g., capsule, ligament, and tendons) with dynamic components (e.g., muscles co-contraction and reflex responses), is an adaptive process that can help maintain stability and dissipate energy during overhead throwing. Therefore, the objective of this study was to identify if healthy baseball players present with uni-lateral differences in neuromuscular control, posterior capsule thickness, and glenohumeral stiffness regulation, which could be related to repetitive throwing. Electromyography (EMG) activity was recorded from upper, middle, and lower trapezius as well as serratus anterior from 24 healthy baseball players. The investigators also used fine wire EMG to accurately measure activity of 3 rotator cuff muscles (i.e., supraspinatus, infraspinatus, and teres minor). Bilateral measurements for active glenohumeral stiffness were performed with a customized Stiffness and Proprioception Assessment Device that quickly internally rotated the shoulder 20 degrees, from a 90°/90° position with 0° of external rotation. During this quick movement the player performed a standardized contraction of his external rotators. The investigators also assessed bilateral posterior capsule thickness using ultrasound scanning. EMG results showed that serratus anterior preparatory EMG and peak activity were larger on the dominant arm compared to the non-dominant arm. In contrast, preparatory EMG co-contraction (i.e., supraspinatus/infraspinatus ratio and lower trapezius/serratus anterior ratio) and reactive EMG co-contraction (i.e., supraspinatus/teres minor ratio) on the dominant arm were less than the non-dominant arm. In addition to EMG differences, the dominant arm also had significantly greater glenohumeral stiffness compared to the non-dominant arm. Active glenohumeral stiffness was significantly correlated with posterior capsule thickness.

In overhead athletes the posterior capsule and surrounding muscles interact to maximize functional performance and maintain glenohumeral stability. The authors of this study demonstrated that there was altered muscle activity in the dominant arm compared to non-dominant arm. For example, they found greater teres minor activity in the dominant shoulder, which may reflect improved neuromuscular control and stiffness regulation. This finding may suggest that strengthening the shoulder external rotators may help reduce the risk of shoulder injuries, but this should be confirmed in future studies. In addition to altered muscle activation, the authors reported greater active glenohumeral joint stiffness in the dominant arm compared to the nondominant arm, which may be an adaptive response that is intended to enhance energy absorption during the deceleration phase of over hand throwing. Finally, the authors found a correlation between posterior capsule thickness and active glenohumeral stiffness when both arms are combined. It would be interesting to see if the relationship between active stiffness and posterior capsule thickness persists when pitchers with shoulder symptoms are examined. Perhaps the symptomatic population will have increased posterior capsule thickness and abnormally high active stiffness. Future research in this area may help us better understand the role of the posterior capsule in shoulder pathology and provide further rationale for stretches like the sleeperstretch, but we’ll have to wait for these studies. Do you believe maladaptation occurs in the shoulder among athletes with shoulder injuries? Do your rehabilitation programs for posterior shoulder injuries focus on the teres minor and increasing stiffness of the shoulder joint?

Written by: Jane McDevitt MS, ATC, CSCS
Reviewed by: Jeffrey Driban

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Thomas SJ, Swanik CB, Higginson JS, Kaminski TW, Swanik KA, Kelly JD 4th, & Nazarian LN (2012). Neuromuscular and stiffness adaptations in division I collegiate baseball players. Journal of Electromyography and Kinesiology PMID: 22898532

2 comments:

Mark Shires, ATC said...

Additional findings that would have been interesting is a measurement of "reaction time" to force applied.
Sometimes we overlook strength for coordinated and appropriate neuromuscular control.

Stephen Thomas, PhD, ATC said...

Mark that is a great question. We actually did measure EMG onset time for each of the muscles. This was reported in the manuscript. We did not see any bilateral differences in onset time for any of the muscles. These athletes were healthy so it would be interesting examining symptomatic athletes and identify if these EMG onset times are delayed which could leave the shoulder vulnerable to instability during the throwing motion. Thanks for commenting!

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