Decrease in trunk muscular response to perturbation with preactivation of lumbar spinal musculature

Study Design. An experimental study of healthy subjects' trunk muscle respo nses to force perturbations at differing angles and steady state efforts. Objectives. To determine whether increased preactivation of muscles was ass ociated with decreased likelihood of muscular activation in response to a t ransient force perturbation. Summary of Background Data. Trunk stability (ability to return to equilibri um position after a perturbation) requires the stiffness of appropriately a ctivated muscles to prevent buckling and consequent "self-injury." Therefor e, greater trunk muscle preactivation might decrease the likelihood of refl ex muscle responses to small perturbations. Methods. Each of 13 subjects stood in an apparatus with the pelvis immobili zed. A harness around the thorax provided a preload and a force perturbatio n by a horizontal cable and a movable pulley attached to one of five anchor age points on a wall track surrounding the subject at angles of 0 degrees, 45 degrees, 90 degrees, 135 degrees, and 180 degrees to the forward directi on. Subjects first equilibrated with a preload effort of nominally 20% or 4 0% of their maximum extension effort. Then a single full sine-wave force pe rturbation pulse of nominal amplitude, 7.5% or 15% of maximum effort, durat ion 80 milliseconds or 300 milliseconds, was applied at a random time, with three repeated trials of each test condition. The applied force (via a loa d cell) and the electromyographic activity of six right and left pairs of t runk muscles were recorded. Muscle responses were detected by two methods. 1) Shewhart method: electromyographic signal greater than "baseline" values by more than three standard deviations, and 2) Mean Electromyographic Diff erence method: mean electromyographic signal in a time window 25 to 150 mil liseconds after the force perturbation greater than that in a 25- to 150-mi llisecond window before the perturbation. Results. Lower preload efforts were associated with more muscle responses ( overall mean response detection rate = 33% at low preload and 25% at high p reload). Using the Shewhart method, there were significant differences by e ffort (P < 0.05) for all abdominal muscles and for all left dorsal muscles except multifidus. Using the Mean Electromyographic Difference method, ther e were significant differences by effort (P < 0.05) for the same dorsal mus cles, but only for one of the abdominal muscles. Conclusions. Findings are consistent with the hypothesis that the spine can be stabilized by the stiffness of activated muscles, obviating the need fo r active muscle responses to perturbations.