Multifidus EMG and tension-relaxation recovery after prolonged static lumbar flexion

Design. The electromyogram (EMG) from the in vivo feline L1 to the L7 multi fidus was recorded during the application of a 20-minute static lumbar flex ion and after 7 hours of rest. Objective. To determine the recovery of tension-relaxation and laxity in th e lumbar viscoelastic structures as well as the recovery of reflexive EMG a ctivity in the multifidus after prolonged static flexion. Summary of Background. It has been established that prolonged static flexio n of the spine induces creep or tension-relaxation in its viscoelastic stru ctures as well as a sharp decrease in the reflexive activity of the dorsal musculature and initiation of spasms. Epidemiologic studies have pointed ou t: that such static flexion is associated with unusually high rates of low back disorders. The rate and pattern of recovery of reflexive muscular acti vity with rest after static flexion is still unknown. Methods. The lumbar spines of seven in vivo feline preparations were subjec ted to 20 minutes of passive anterior flexion followed by 7 hours of rest w hile monitoring flexion tension, EMG from the L1-L7 multifidus muscles, and the strain of the L4/L5 supraspinal ligament. A model describing the patte rn of recovery of muscular activity and viscoelastic tension was developed. Results. Twenty minutes of lumbar flexion was associated with an initial sh arp decrease of multifidus EMG activity followed by spasms. During rest, EM G activity demonstrated an initial hyperexcitability on flexion, followed b y an exponential recovery of muscle activity. Full recovery of residual str ain in the L4/L5 supraspinous ligament and multifidus activity was not obta ined after 7 hours of rest. Conclusions. Static flexion of the lumbar spine is an extremely imposing fu nction on its viscoelastic tissues, resulting in spasms and requiring long periods of rest before normal functions are re-established.