Repeated spinal flexion modulates the flexion-relaxation phenomenon.
OBJECTIVE
To determine if repeated spinal flexion and loading modulate the deactivation of lumbar muscles near full flexion (flexion-relaxation).


METHODS
Repeated measures experimental study of the effect of repetitive trunk flexion and added mass on the flexion-relaxation phenomenon.


BACKGROUND
Repeated flexion causes muscular fatigue, creep of passive tissues and diminished protective reflexes. However, flexion-relaxation has not been studied in repeated trunk flexion, and could be related to the increased risk of low-back disorders.


METHODS
Thirty healthy young subjects performed 100 trunk flexion movements between standing and full flexion. Erector spinae electromyography and lumbar spine flexion were measured during cycles 1-10 (no load), 11-20 (performed holding a mass in the hands), 81-90 (mass in the hands) and 91-100 (no load). The spinal flexion angle at myoelectric silence and full flexion were extracted from each movement cycle.


RESULTS
Twenty-three of the 30 subjects showed flexion-relaxation throughout the repeated trunk flexion. The flexion-relaxation and maximum flexion angles increased at the end of the experiment; the flexion-relaxation angle relative to the maximum flexion angle also increased. This effect depended on the load condition; the flexion-relaxation and maximum flexion angles showed a greater increase in the unloaded than loaded condition.


CONCLUSIONS
The flexion-relaxation phenomenon was changed due to repeated trunk flexion. The increases in flexion-relaxation angle likely involve changes to the neuromuscular control system.


CONCLUSIONS
The deactivation of the erector muscles near full flexion occurs at a greater spinal flexion angle and a greater proportion of maximum spinal flexion following repeated spinal flexion. This may be related to the increased risk of injury associated with repeated flexion.
Dickey
James P
JP
Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ont., Canada N1G 2W1. jdickey@uoguelph.ca
McNorton
Sara
S
Potvin
James R
JR
D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D004576:Electromyography; D005260:Female; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology*; D008297:Male; D009068:Movement / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D012399:Rotation; D013131:Spine / Q000502:physiology; D016474:Weight-Bearing / Q000502:physiology*