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

Abstract：

STUDY DESIGN:An experimental study of healthy subjects' trunk muscle responses to force perturbations at differing angles and steady state efforts. OBJECTIVES:To determine whether increased preactivation of muscles was associated with decreased likelihood of muscular activation in response to a transient force perturbation. SUMMARY OF BACKGROUND DATA:Trunk stability (ability to return to equilibrium position after a perturbation) requires the stiffness of appropriately activated muscles to prevent buckling and consequent "self-injury." Therefore, greater trunk muscle preactivation might decrease the likelihood of reflex muscle responses to small perturbations. METHODS:Each of 13 subjects stood in an apparatus with the pelvis immobilized. A harness around the thorax provided a preload and a force perturbation by a horizontal cable and a movable pulley attached to one of five anchorage 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 direction. Subjects first equilibrated with a preload effort of nominally 20% or 40% of their maximum extension effort. Then a single full sine-wave force perturbation pulse of nominal amplitude, 7.5% or 15% of maximum effort, duration 80 milliseconds or 300 milliseconds, was applied at a random time, with three repeated trials of each test condition. The applied force (via a load cell) and the electromyographic activity of six right and left pairs of trunk 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 Difference method: mean electromyographic signal in a time window 25 to 150 milliseconds after the force perturbation greater than that in a 25- to 150-millisecond 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 preload). Using the Shewhart method, there were significant differences by effort (P<0.05) for all abdominal muscles and for all left dorsal muscles except multifidus. Using the Mean Electromyographic Difference method, there were significant differences by effort (P<0.05) for the same dorsal muscles, 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 for active muscle responses to perturbations.

journal_name

Spine (Phila Pa 1976)

journal_title

Spine

authors

Stokes IA,Gardner-Morse M,Henry SM,Badger GJ

doi

10.1097/00007632-200008010-00015

keywords:

subject

Has Abstract

pub_date

2000-08-01 00:00:00

pages

1957-64

issue

eissn

0362-2436

issn

1528-1159

journal_volume

pub_type

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