title: Physiological response to submaximal isometric contractions of the paravertebral muscles.
abstract: METHODS
  Brief (30-second) isometric trunk extensions at 5%, 20%, 40%, 60%, and 80% of maximal voluntary contraction (MVC) and 3 minutes of prolonged trunk extension (20% MVC) in erect position were studied in nine healthy male subjects.
  
  
  OBJECTIVE
  To investigate the intercorrelation between intramuscular pressure and tissue oxygenation of the paravertebral muscles during submaximal isometric contractions and further, to evaluate paravertebral electromyogram and intramuscular pressure as indicators of force development.
  
  
  BACKGROUND
  Local physiologic responses to muscle contraction are incompletely understood.
  
  
  METHODS
  Relative oxygenation was monitored with noninvasive near-infrared spectroscopy, intramuscular pressure was measured with a transducer-tipped catheter, and surface electromyogram was monitored at three recording sites.
  
  
  RESULTS
  The root mean square amplitudes of the paravertebral electromyogram (L4, left and right; T12, right) and intramuscular pressure measured in the lumbar multifidus muscle at L4 increased with greater force development in a curvilinear manner. A significant decrease in the oxygenation of the lumbar paravertebral muscle in response to muscle contraction was found at an initial contraction level of 20% MVC. This corresponded to a paravertebral intramuscular pressure of 30-40 mm Hg. However, during prolonged trunk extension, no further decrease in tissue oxygenation was found compared with the tissue oxygenation level at the end of the brief contractions, indicating that homeostatic adjustments (mean blood pressure and heart rate) over time were sufficient to maintain paravertebral muscle oxygen levels.
  
  
  CONCLUSIONS
  At a threshold intramuscular pressure of 30-40 mm Hg during muscle contraction, oxygenation in the paravertebral muscles is significantly reduced. The effect of further increase in intramuscular pressure on tissue oxygenation over time may be compensated for by an increase in blood pressure and heart rate. Surface electromyogram amplitudes and intramuscular pressure can be used as indicators of paravertebral muscle force.
authors.affiliation: Department of Physiology, National Institute of Occupational Health, Copenhagen, Denmark. brj@ami.dk
authors.forename: B R
authors.initials: BR
authors.lastname: Jensen
authors.forename: K
authors.initials: K
authors.lastname: Jørgensen
authors.forename: A R
authors.initials: AR
authors.lastname: Hargens
authors.forename: P K
authors.initials: PK
authors.lastname: Nielsen
authors.forename: T
authors.initials: T
authors.lastname: Nicolaisen
mesh_terms: D000328:Adult; D004576:Electromyography; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008161:Lumbosacral Region; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D010101:Oxygen Consumption / Q000502:physiology; D011312:Pressure; D019265:Spectroscopy, Near-Infrared; D014160:Transducers, Pressure