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Study Design: The reflexive EMG from the L3-L4 to L5-L6 multifidus of the in vivo feline was recorded during application of single passive flexion-extension cycle of the lumbar spine., Objective: To determine the effect of viscoelastic hysteresis associated with a single-cycle flexion-extension and of increasing cycle frequency on the initiation and cessation displacement and tension thresholds of reflexive EMG from the multifidus muscles., Summary of Background Data: It is known that reflexive EMG can be recorded from some paraspinal muscles as a result of mechanical stimulation of lumbar ligaments and other viscoelastic structures. It is also known that mechanical neutral zones exist in the spine, that viscoelastic hysteresis is associated with a stretch-release cycle, and that the rate of stretch and release has a profound impact on viscoelastic tissue responses. It is unknown what are the neurologic neutral zones of the spine within which reflexive EMG does not exist, as well as the dependence of such neurologic neutral zones on viscoelastic hysteresis and increasing frequency of a flexion-extension cycle., Methods: Single passive flexion-extension cycles of frequencies ranging from 0.1 to 1.0 Hz were applied to the lumbar spine of the feline while recording intramuscular EMG from the L3-L4 to L5-L6 multifidus. The displacement and tension thresholds associated with the initiation and cessation of EMG activity during the cycle were analyzed with respect to the cycles' viscoelastic hysteresis and frequency. The peak EMG discharge was tested for relationships with cycle frequency., Results: The displacement and tension thresholds during the flexion phase of the cycle were significantly lower than the corresponding thresholds in the extension phase of the cycle. As the cycle frequency increased, EMG was triggered significantly earlier (lower displacement and tension thresholds) in the flexion phase and terminated earlier (higher displacement and tension thresholds) in the extension phase. The peak EMG was significantly larger as cycle frequency increased., Conclusions: Reflexive muscle forces are triggered at lower displacement or tension during flexion but diminish early during extension, leaving the spine unprotected for a substantial part of the extension movement. The muscle forces are recruited earlier and with larger intensity as the velocity of the movement increases, lending more protection to the spine. Faster extension movement, however, creates a larger window during which the spine is exposed to instability and injury because of lack of muscle forces.