The Journal of Bone and Joint Surgery (American). 2008;90:154-162.
doi:10.2106/JBJS.F.01133
© 2008 The Journal of Bone and Joint Surgery, Inc.
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Stiffness Regulation and Muscle-Recruitment Strategies of the Shoulder in Response to External Rotation Perturbations

Kellie C. Huxel, PhD, ATC1, C. Buz Swanik, PhD, ATC2, Kathleen A. Swanik, PhD, ATC3, Arthur R. Bartolozzi, MD4, Howard J. Hillstrom, PhD5, Michael R. Sitler, EdD, ATC6 and Dani M. Moffit, MA, ATC7

1 Athletic Training Department, Indiana State University, Terre Haute, IN 47809. E-mail address: khuxel{at}isugw.indstate.edu
2 Human Performance Laboratory, Department of Health, Nutrition & Exercise Sciences, 151 Human Performance Laboratory, 547 South College Avenue, Newark, DE 19716
3 Department of Athletic Training, Neumann College, One Neumann Drive, Aston, PA 19104
4 Pennsylvania Hospital, 880 Spruce Street, Philadelphia, PA 19107
5 Motion Analysis Laboratory, Hospital for Special Surgery, 535 East 70th Street, New York, NY 10021
6 114 Pearson Hall, Kinesiology Department, Temple University, Philadelphia, PA 19122
7 116 Pearson Hall, Kinesiology Department, Temple University, Philadelphia, PA 19122
Investigation performed at the Biokinetics Research Laboratory at Temple University, Philadelphia, Pennsylvania

Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Background: The complex interactions between capsuloligamentous structures and muscle-recruitment strategies that maintain glenohumeral stability remain unclear. The purposes of the present study were to determine whether stiffness regulation and muscle-activation strategies differed under varying joint positions and levels of contraction in the shoulder and to determine the relationship between generalized joint laxity, glenohumeral joint laxity, and joint stiffness.

Methods: Forty healthy, physically active subjects with a mean age (and standard deviation) of 25.2 ± 4.6 years, a mean height of 174.7 ± 6.7 cm, and a mean mass of 73.1 ± 13.8 kg were tested. Shoulder stiffness and the activation of muscles (including the rotator cuff and the anterior deltoid) were measured at two levels of internal rotation torque (0% and 50% of maximum) and two joint positions (0° and 90% of maximum external rotation) before and after a 5° external rotation perturbation. Generalized laxity and glenohumeral joint laxity (in the anterior, posterior, and inferior directions) were also assessed.

Results: Stiffness was 77% greater at 50% of maximum internal rotation torque than at 0% of maximum internal rotation torque (p < 0.001) but was not significantly different between joint positions (p = 0.73). From 0% to 50% of maximum internal rotation torque, preparatory and reactive recruitment of the subscapularis increased significantly more (p < 0.05) than those of the other muscles. Also, subscapularis preparatory activity was 36% greater in 0° of external rotation than in 90% of maximum external rotation (p < 0.01). Generalized joint laxity (as indicated by a score of ≥4) was present in 20% of the subjects. Glenohumeral joint laxity (as indicated by a grade of ≥2) was present in the anterior, posterior, and inferior directions in 13%, 15%, and 15% of the subjects, respectively. No correlation existed between passive stiffness and generalized or glenohumeral laxity (r = –0.12 to 0.29; p = 0.08 to 0.48).

Conclusions: Moderate levels of muscle contraction can significantly increase glenohumeral joint stiffness and stability. Preactivation of the subscapularis appears to be the primary dynamic stabilizer with the arm in 0° of external rotation. However, with the arm in 90% of maximum external rotation (the apprehension position), less subscapularis activity is observed and the maintenance of stability may shift toward other musculoskeletal structures because joint stiffness does not change. A relationship between generalized joint laxity, glenohumeral laxity, and stiffness was not observed in healthy subjects.

Clinical Relevance: Clinically, healthy populations have comparable levels of stiffness, or joint stability, in 0° of external rotation and in 90% of maximum external rotation (the apprehension position).


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