Multifidus EMG and tension-relaxation recovery after prolonged static lumbar flexion.
METHODS
The electromyogram (EMG) from the in vivo feline L1 to the L7 multifidus was recorded during the application of a 20-minute static lumbar flexion and after 7 hours of rest.


OBJECTIVE
To determine the recovery of tension-relaxation and laxity in the lumbar viscoelastic structures as well as the recovery of reflexive EMG activity in the multifidus after prolonged static flexion.


BACKGROUND
It has been established that prolonged static flexion of the spine induces creep or tension-relaxation in its viscoelastic structures as well as a sharp decrease in the reflexive activity of the dorsal musculature and initiation of spasms. Epidemiologic studies have pointed out that such static flexion is associated with unusually high rates of low back disorders. The rate and pattern of recovery of reflexive muscular activity with rest after static flexion is still unknown.


METHODS
The lumbar spines of seven in vivo feline preparations were subjected to 20 minutes of passive anterior flexion followed by 7 hours of rest while monitoring flexion tension, EMG from the L1-L7 multifidus muscles, and the strain of the L4/L5 supraspinal ligament. A model describing the pattern of recovery of muscular activity and viscoelastic tension was developed.


RESULTS
Twenty minutes of lumbar flexion was associated with an initial sharp decrease of multifidus EMG activity followed by spasms. During rest, EMG activity demonstrated an initial hyperexcitability on flexion, followed by an exponential recovery of muscle activity. Full recovery of residual strain in the L4/L5 supraspinous ligament and multifidus activity was not obtained after 7 hours of rest.


CONCLUSIONS
Static flexion of the lumbar spine is an extremely imposing function on its viscoelastic tissues, resulting in spasms and requiring long periods of rest before normal functions are re-established.
Jackson
M
M
Bioengineering Laboratory, Department of Orthopaedic Surgery, Louisiana State University Medical Center, New Orleans 70112, USA.
Solomonow
M
M
Zhou
B
B
Baratta
R V
RV
Harris
M
M
D000818:Animals; D002415:Cats; D004576:Electromyography / Q000379:methods; D008159:Lumbar Vertebrae / Q000737:chemistry / Q000473:pathology; D023421:Models, Animal; D009119:Muscle Contraction / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology*; D018482:Muscle, Skeletal / Q000473:pathology*; D020127:Recovery of Function / Q000502:physiology*