The purpose of this study was to examining electromyographically the effects of trunk flexion to the left and right on the cervical muscle response to increasing low-velocity posterolateral impacts. Twenty healthy volunteers were subjected to rear impacts of 4.5, 8.2, 10.6, and 13.9 m/s2 acceleration, offset by 45 degrees to the subject's right with trunk flexed to right and left. At a peak acceleration of 13.9 m/s2, with the trunk flexed to the right, the right trapezius generated 26% and left trapezius 29% of the maximal voluntary contraction electromyogram, while all other muscles generated 20% or less of this variable. When the trunk is flexed to the left, under these same conditions, the right trapezius generated 25% of its maximal voluntary contraction electromyogram, with 30% of the maximal voluntary contraction for the right trapezius, and less than 12% for the remaining muscles. Electromyographic and kinematic variables were significantly affected by the levels of acceleration (P less than 0.05). The time to onset and time to peak electromyogram for all muscles progressively decreased with increasing levels of acceleration, for both trunk flexion conditions. Being "out-of-position" at the time of impact greatly reduces the EMG response of the cervical muscles compared to what is the known response for impacts with neutral posture. Muscle injury seems less likely under these conditions in low-velocity impacts. (Author/publisher)
Abstract