Electromyographic activity of erectores spinae exhibits points of abrupt change during trunk flexion from the erect position and return extension. This study examined the positions at which the myoelectric activity suddenly disappeared and later reappeared. Forty adults were investigated to define accurately the inclinations of the trunk, pelvis, and vertebral column at these positions. The positions at the commencement and cessation of the period of electrical silence both occurred at two-thirds of maximum trunk flexion (x= 80[degrees] +/- 13[degrees] SD). At these positions, all flexion measurements were significantly less than their maxima (P <0.001). Hip flexion at the commencement of electrical silence was slightly above one-half its maximum range, and similar to the position at the recommencement of electrical activity (x= 40[degrees] +/- 12[degrees] SD). The most reproducible measurement (r=0.88) in both positions was vertebral flexion (89% Max.; x= 48[degrees] +/- 6[degrees] SD). Eleven of the male subjects repeated the experimental task holding 10.1 kg in their hands. The effect of this was to produce inhibition and reactivation of erectores spinae at a greater degree of vertebral flexion.
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