title: The role of preload forces in spinal manipulation: experimental investigation of kinematic and electromyographic responses in healthy adults.
abstract: OBJECTIVE
  Previous studies have identified preload forces and an important feature of skillful execution of spinal manipulative therapy (SMT) as performed by manual therapists (eg, doctors of chiropractic and osteopathy). It has been suggested that applying a gradual force before the thrust increases the spinal unit stiffness, minimizing displacement during the thrust. Therefore, the main objective of this study was to assess the vertebral unit biomechanical and neuromuscular responses to a graded increase of preload forces.
  
  
  METHODS
  Twenty-three participants underwent 4 different SMT force-time profiles delivered by a servo-controlled linear actuator motor and varying in their preload forces, respectively, set to 5, 50, 95, and 140N in 1 experimental session. Kinematic markers were place on T6, T7, and T8 and electromyographic electrodes were applied over paraspinal muscles on both sides of the spine.
  
  
  RESULTS
  Increasing preload forces led to an increase in neuromuscular responses of thoracic paraspinal muscles and vertebral segmental displacements during the preload phase of SMT. Increasing the preload force also yielded a significant decrease in sagittal vertebral displacement and paraspinal muscle activity during and immediately after the thrust phase of spinal manipulation. Changes observed during the SMT thrust phase could be explained by the proportional increase in preload force or the related changes in rate of force application. Although only healthy participants were tested in this study, preload forces may be an important parameter underlying SMT mechanism of action. Future studies should investigate the clinical implications of varying SMT dosages.
  
  
  CONCLUSIONS
  The present results suggest that neuromuscular and biomechanical responses to SMT may be modulated by preload through changes in the rate of force application. Overall, the present results suggest that preload and rate of force application may be important parameters underlying SMT mechanism of action.
authors.affiliation: Post-doctoral fellowship, Département de Chiropratique, Université du Québec à Trois-Rivières, Trois-Rivières, QC G9A 5H7, Canada.
authors.forename: François
authors.initials: F
authors.lastname: Nougarou
authors.affiliation: Full professor, Département des sciences de l'activité physique, Université du Québec à Trois-Rivières, Trois-Rivières, QC G9A 5H7, Canada.
authors.forename: Claude
authors.initials: C
authors.lastname: Dugas
authors.affiliation: Undergraduate student, Département de Chiropratique, Université du Québec à Trois-Rivières, Trois-Rivières, QC G9A 5H7, Canada.
authors.forename: Michel
authors.initials: M
authors.lastname: Loranger
authors.affiliation: MSc student, Département des sciences de l'activité physique, Université du Québec à Trois-Rivières, Trois-Rivières, QC G9A 5H7, Canada.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Pagé
authors.affiliation: Full professor, Département des sciences de l'activité physique, Université du Québec à Trois-Rivières, Trois-Rivières, QC G9A 5H7, Canada. Electronic address: Martin.descarreaux@uqtr.ca.
authors.forename: Martin
authors.initials: M
authors.lastname: Descarreaux
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology; D004576:Electromyography; D006801:Humans; D020393:Manipulation, Spinal / Q000379:methods*; D064170:Paraspinal Muscles / Q000502:physiology*; D013904:Thoracic Vertebrae / Q000502:physiology; D055815:Young Adult
keywords: Chiropractic; Dose Response Relationship; Electromyography; Force; Kinematics, Manipulation; Spinal Manipulation