Quantitative studies of the flexion-relaxation phenomenon in the back muscles.

In quiet standing positions involving substantial trunk flexion, myoelectric activity in the back muscles diminishes to low levels. Aspects of that "flexion-relaxation" phenomenon were explored through measurements of myoelectric activities in 11 young men during performance of 19 isometric tasks in flexed positions. Biomechanical model analyses were used to predict the internal loads imposed on the lumbar trunk structures during those performances. Flexion-relaxation consistently occurred in quiet flexed standing, but marked increases in myoelectric activity were found on imposition of external loads in flexed positions. Increases in myoelectric activity per unit increase in back muscle contraction force increase were nearly the same as those found in upright postures. Whether or not flexion-relaxation occurs, large trunk flexions load the spine heavily.