title: Experimental pain inhibits infraspinatus activation during isometric external rotation.
abstract: BACKGROUND
  The effect of pain on muscle activation is poorly understood. This study examined the effects of acute experimental pain on rotator cuff muscle force and voluntary activation (VA). We hypothesized that acute subacromial pain would cause inhibition of infraspinatus VA with a corresponding decrease in external rotation force.
  
  
  METHODS
  Seventeen healthy adults with no known shoulder pathology were tested. Isolated external rotation force was tested on a dynamometer. Participants performed 2 baseline maximum voluntary isometric contractions of external rotation, during which maximal electrical stimulation was used to assess VA. To elicit pain, 1.5 mL 5% hypertonic saline was injected into the subacromial space, and testing of maximum voluntary isometric contractions force and VA was repeated 3 times at 5-minute intervals.
  
  
  RESULTS
  Mean ± standard deviation initial pain from the injection was 6.6 ± 1.3 points of 10 possible and produced a 32.8% decline in force and a 22.7% decline in VA (P < .05). Pain diminished over a 10-minute period. As pain resolved, force and VA improved (P < .0125). There was a strong relationship between force and VA (r(2) = 0.78, P < .05) and a moderate relationship between pain and VA (r(2) = 0.31, P < .05).
  
  
  CONCLUSIONS
  Experimental subacromial pain elicits a decline in force and VA of the infraspinatus. Although this study only examines acute experimental pain, it supports the concept that pain affects rotator cuff muscle recruitment and function, which may contribute to abnormal shoulder mechanics in patients with rotator cuff pathology.
authors.affiliation: Department of Physical Therapy, Arcadia University, 450 S Easton Rd, Glenside, PA 19038, USA.stackhouse@arcadia.edu
authors.forename: Scott K
authors.initials: SK
authors.lastname: Stackhouse
authors.forename: Ashley
authors.initials: A
authors.lastname: Eisennagel
authors.forename: Jennifer
authors.initials: J
authors.lastname: Eisennagel
authors.forename: Hallie
authors.initials: H
authors.lastname: Lenker
authors.forename: Brett A
authors.initials: BA
authors.lastname: Sweitzer
authors.forename: Philip W
authors.initials: PW
authors.lastname: McClure
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D010146:Pain / Q000503:physiopathology*; D012399:Rotation; D055815:Young Adult