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Timothy J. Henry, Scott M. Lephart, Jorge Giraldo, David Stone and Freddie H. Fu

Context:

Muscle fatigue is an important concept in regard to the muscle function of the shoulder joint. Its effect on the muscle force couples of the glenohumeral joint has not been fully identified.

Objective:

To examine the effects of muscle fatigue on muscle force-couple activation in the normal shoulder.

Design:

Pretest, posttest.

Patients:

Ten male subjects, age 18–30 years, with no previous history of shoulder problems.

Main Outcome Measures:

EMG (area) values were assessed for the anterior and middle deltoid, subscapularis, and infraspinatus muscles during 4 dynamic stabilizing exercises before and after muscle fatigue. The exercises examined were a push-up, horizontal abduction, segmental stabilization, and rotational movement on a slide board.

Results:

No significant differences were observed for any of the muscles tested.

Conclusions:

The results of our study indicate that force-couple coactivation of the glenohumeral joint is not significantly altered after muscle fatigue.

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Justin M. Stanek, Todd A. McLoda, Val J. Csiszer and A.J. Hansen

Context:

Selected muscles in the kinetic chain may help explain the body’s ability to avert injury during unexpected perturbation.

Objective:

To determine the activation of the ipsilateral rectus femoris (RF), gluteus maximus (MA), gluteus medius (ME), and contralateral external obliques (EO) during normal and perturbed gait.

Design:

Single-factor, repeated measures.

Setting:

University research laboratory.

Participants:

32 physically active, college-age subjects.

Intervention:

Subjects walked a total of 20 trials the length of a 6.1-m custom runway capable of releasing either side into 30° of unexpected inversion. During 5 trials, the platform released into inversion.

Main Outcome Measures:

Average, peak, and time to peak EMG were analyzed across the 4 muscles, and comparisons were made between the walking trials and perturbed trials.

Results:

Significantly higher average and peak muscle activity were noted for the perturbed condition for RF, MA, and EO. Time to peak muscle activity was faster during the perturbed condition for the EO.

Conclusion:

Rapid contractions of selected postural muscles in the kinetic chain help explain the body’s reaction to unexpected perturbation.

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Jonathon R. Staples, Kevin A. Schafer, Matthew V. Smith, John Motley, Mark Halstead, Andrew Blackman, Amanda Haas, Karen Steger-May, Matthew J. Matava, Rick W. Wright and Robert H. Brophy

Context: Patients with anterior cruciate ligament (ACL) tears are likely to have deficient dynamic postural stability compared with healthy sex- and age-matched controls. Objectives: To test the hypothesis that patients undergoing ACL reconstruction have decreased dynamic postural stability compared with matched healthy controls. Design: Prospective case-control study. Setting: Orthopedic sports medicine and physical therapy clinics. Patients or Other Participants: Patients aged 20 years and younger with an ACL tear scheduled for reconstruction were enrolled prospectively. Controls were recruited from local high schools and colleges via flyers. Interventions: Patients underwent double-stance dynamic postural stability testing prior to surgery, recording time to failure and dynamic motion analysis (DMA) scores. Patients were then matched with healthy controls. Main Outcome Measures: Demographics, time to failure, and DMA scores were compared between groups. Results: A total of 19 females and 12 males with ACL tears were matched with controls. Individuals with ACL tears were more active (Marx activity score: 15.7 [1.0] vs 10.8 [4.9], P < .001); had shorter times until test failure (84.4 [15.8] vs 99.5 [14.5] s, P < .001); and had higher (worse) DMA scores (627 [147] vs 481 [132], P < .001), indicating less dynamic postural stability. Six patients with ACL deficiency (1 male and 5 females) demonstrated lower (better) DMA scores than their controls, and another 7 (4 males and 3 females) were within 20% of controls. Conclusions: Patients undergoing ACL reconstruction had worse global dynamic postural stability compared with well-matched controls. This may represent the effect of the ACL injury or preexisting deficits that contributed to the injury itself. These differences should be studied further to evaluate their relevance to ACL injury risk, rehabilitation, and return to play.

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W. Steven Tucker, Charles W. Armstrong, Erik E. Swartz, Brian M. Campbell and James M. Rankin

Context:

Closed kinetic chain exercises are reported to provide a more functional rehabilitation outcome.

Objective:

To determine the amount of muscle activity in 4 shoulder muscles during exercise on the Cuff Link.

Design:

Repeated measures.

Setting:

Laboratory.

Subjects:

10 men and 10 women, age 18–50.

Intervention:

Subjects performed 3 sets of 5 revolutions on the Cuff Link in non-weight-bearing, partial-weight-bearing, and full-weight-bearing positions.

Main Outcome Measures:

Electromyography data were collected from the upper trapezius, anterior deltoid, serratus anterior, and pectoralis major and were expressed as percentage of maximal isometric contractions.

Results:

Significant differences were found across the weight-bearing conditions for all 4 muscles. Exercise on the Cuff Link required minimal to significant amounts of muscle recruitment.

Conclusions:

Muscle recruitment increases as weight bearing increases during use of the Cuff Link, suggesting an increase in dynamic stabilization of the glenohumeral joint.

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C. Buz Swanik, Scott M. Lephart, Frank P. Giannantonio and Freddie H. Fu

Anterior cruciate ligament (ACL) injury disrupts static and dynamic knee restraints, compromising functional stability. Deafferentation of ACL mechan-oreceptors alters the spinal reflex pathways to motor nerves and muscle spindles in addition to the cortical pathways for conscious and unconscious appreciation of proprioception and kinesthesia. These pathways are required by the feed-forward and feedback neuromuscular control systems to dynamically stabilize joints. Feed-forward motor control is responsible for preparatory muscle activity, while feedback motor control regulates reactive muscle activity. The level of muscle activation, preparatory or reactive, influences muscular stiffness, thereby providing dynamic restraint for the ACL-deficient athlete. Rehabilitation protocols should incorporate activities that enhance muscle stiffness while encouraging adaptations to peripheral afferents, spinal reflexes, and cortical motor patterns. Four elements crucial for reestablishing neuromuscular control and functional stability are proprioceptive and kinesthetic awareness, dynamic stability, preparatory and reactive muscle characteristics, and conscious and unconscious functional motor patterns.

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Mary E. Ubinger, William E. Prentice and Kevin M. Guskiewicz

When the upper extremity is injured, open kinetic chain (OKC) exercises are primarily used to increase strength and restore functional ability—the goals of rehabilitation. It is also imperative, however, that the receptors responsible for static and dynamic stabilization of the joint be trained. This can be done with closed kinetic chain (CKC) exercises. The purposes of this study were to investigate the effect of a 4-week CKC training program on the neuromuscular control of the upper extremity and to determine whether there was a significant difference between skill-dominant limb and nondominant limb stability indices. Thirty-two physically active participants (14 men, 18 women) were tested on the FASTEX 4 weeks apart. The training group's scores significantly improved, whereas the control group's scores remained the same. It was concluded that the CKC training significantly improved the training group's ability to remain stable. The results suggest that CKC training can increase the accuracy of joint position sense because of increased stimulation of the mechanoreceptors.

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Brian W. Wiese, Kevin Miller and Eduardo Godoy

greater dynamic stabilization, muscular strength, and neuromuscular control of the glenohumeral joint. Without these beneficial physical presentations, athletes may be at greater risk for traumatic or chronic injuries. Because our athlete participated in a collision sport, we were more concerned about

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Erica M. Willadsen, Andrea B. Zahn and Chris J. Durall

on increased coordination, strength, and stability of lumbopelvic musculature) (4) The control group performed their normal daily activities. Plyometric group: maximum effort plyometric jumping and cutting maneuvers. Balance group: dynamic stabilization and balance exercises to strengthen lower

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Bin Chen, Yichao Zhao, Xianxin Cao, Guojiong Hu, Lincoln B. Chen and Wenxin Niu

static (labrum, ligament, and capsule) and dynamic mechanisms (musculature). 11 , 12 The shoulder rotators play a crucial role in the dynamic stabilization of the shoulder joint due to contraction of the muscles causing translation of the humeral head to the center of the glenoid and compression of the

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Chadwick Debison-Larabie, Bernadette A. Murphy and Michael W.R. Holmes

. Spine . 1995 ; 20 ( 8 suppl ): 1S – 73S . PubMed ID: 7604354 7604354 6. Tierney RT , Sitler MR , Swanik CB , Swanik KA , Higgins M , Torg J . Gender differences in head-neck segment dynamic stabilization during head acceleration . Med Sci Sports Exerc . 2005 ; 37 ( 2 ): 272 – 279