Neuromuscular neutral zones response to static lumbar flexion: muscular stability compensator.

BACKGROUND: The impact of six sequential static loading and rest of the lumbar spine on the changes in the neuromuscular neutral zones and thereby on spine stability was assessed.

METHODS: Six 10 min sessions of static load of a moderate level each spaced by 10 min rest were applied to the in vivo feline model. Test cycles of 0.25 Hz and at the same moderate peak load were applied before and every hour after the static loading sequence up to 7h. Load, displacement and electromyographic activity of the lumbar multifidi muscles were recorded throughout.

FINDINGS: Displacement and tension neuromuscular neutral zones were defined as the displacement or tension, in the increase and decrease phases of each cycle, when the electromyogram initiated and ceased activity, respectively. Displacement neuromuscular neutral zones demonstrated significant (P<0.001) increase immediately post-static loading, followed by an exponential decrease to pre-loading baseline by the 7th hour. Tension neuromuscular neutral zones, however, demonstrated significant (P<0.001) increase in the 2h immediately after the static loading and a significant decrease (P<0.001) thereafter. Peak electromyogram decreased in the first 3h post-loading, but significantly (P<0.001) increased thereafter to the 7th hour.

INTERPRETATION: It was concluded that the first 2-3h post-static loading finds the spine with significant laxity in the viscoelastic tissues concurrently with deficient muscular activation and therefore exposed to the risk of instability. It is also evident that a neural control compensation mechanism exists where it enhances the activation of the musculature to earlier and at higher activation magnitude, 2-3h post-loading, increasing lumbar stability while the viscoelastic tissues are still lax.