PASSIVE TISSUES HELP THE BACK MUSCLES TO GENERATE EXTENSOR MOMENTS DURING LIFTING

We examined the possibility that passive tissues can help the erector spinae to generate large extensor moments during lifting. One hundred and forty-nine healthy men and women participated in the study. Subjec ts pulled upwards with steadily increasing force on a floor-mounted lo ad cell, while EMG activity was recorded from electrodes overlying the erector spinae at L3 and T10. Extensor moment was calculated from the load cell data, and was plotted against the full-wave rectified and a veraged EMG signal. The relationship was linear with an intercept on t he extensor moment axis (I) which indicated the flexion moment resiste d by 'passive' (electrically silent) tissues. The dependence of I on l umbar flexion angle was studied by repeating the isometric pulls betwe en 6 and 12 times, with the subject positioned in varying amounts of f lexion, as measured by the '3-Space Isotrak'. Subjects then lifted wei ghts of up to 20 kg from the floor, using 'stoop', 'squat' and 'freest yle' techniques, while lumbar flexion and EMG activity were recorded a t 28 Hz. The isometric pulls showed that, on average, I increased from 25 Nm in lordotic postures to 120 Nm (for men) and 77 Nm (for women), in full flexion. During the lifts, peak extensor moment was generated with the lumbar spine flexed by 78-97% of the range between erect sta nding and full flexion. Comparisons with the static calibrations showe d that between 16 and 31% of the peak extensor moment generated during lifting was unrelated to EMG activity in the erector spinae. Comparis ons with cadaveric data suggested that less than a quarter of this 'pa ssive' extensor moment was due to the intervertebral discs and ligamen ts.