title: Shoulder-Muscle Activation in Individuals With Previous Shoulder Injuries.
abstract: Understanding how muscles activate in a population with a previous glenohumeral-joint (GH) injury may help clinicians understand how to build a conservative treatment plan to strengthen or activate the specific muscles in an attempt to reduce recurrent shoulder injury and development of GH laxity.
  
  
  
  To investigate muscle-activation differences between the previously injured limb of individuals with a history of GH-joint injury and healthy matched controls during functional isometric contractions.
  
  
  
  Case control.
  
  
  
  University research laboratory.
  
  
  
  17 individuals (8 women, 9 men; age 22.3 ± 2.6 y, height 172.4 ± 8.8 cm, mass 75.4 ± 16.5 kg) with previous unilateral shoulder pain and 17 (8 women, 9 men; age 22.9 ± 3.9 y, height 170.9 ± 11.3 cm, mass 73.6 ± 22.9 kg) with no history of shoulder pain or injury.
  
  
  
  Diagnostic ultrasound measurements of the supraspinatus were completed in both resting and contracted states to assess changes in muscle thickness. Manual muscle tests (anterior deltoid, upper trapezius, infraspinatus, lower trapezius, serratus anterior) and functional isometric contractions (forward flexion, scaption, abduction) were measured using electromyography.
  
  
  
  Peak, normalized activation of each muscle and supraspinatus thickness activation ratio were compared between groups and bilaterally within groups using separate ANOVAs.
  
  
  
  The anterior deltoid was significantly less activated during all functional isometric tasks in previously injured subjects than in healthy subjects (P = .024). In previously injured subjects, the involved limb-lower trapezius was significantly less activated during scaption and abduction tasks than the contralateral side (P = .022 and P = .031, respectively).
  
  
  
  There were decreases in muscle activation in the anterior deltoid between previously injured and healthy people, as well as in the lower trapezius, in previously injured subjects. Understanding the source of muscle-activation deficits can help clinicians focus rehabilitation exercises on specific muscles.
authors.affiliation: Dept of Kinesiology, University of Virginia, Charlottesville, VA.
authors.forename: Alyssa
authors.initials: A
authors.lastname: Muething
authors.forename: Shellie
authors.initials: S
authors.lastname: Acocello
authors.forename: Kimberly A
authors.initials: KA
authors.lastname: Pritchard
authors.forename: Stephen F
authors.initials: SF
authors.lastname: Brockmeier
authors.forename: Susan A
authors.initials: SA
authors.lastname: Saliba
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Hart
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D004576:Electromyography; D005081:Exercise Therapy / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000502:physiology / Q000503:physiopathology*; D016059:Range of Motion, Articular / Q000502:physiology*; D012782:Shoulder / Q000000981:diagnostic imaging / Q000503:physiopathology; D000070599:Shoulder Injuries; D014463:Ultrasonography; D055815:Young Adult