Double-Sided Mechanical Shocks Provoke Larger Seated Postural Reactions Compared With Single-Sided Mechanical Shocks.

Abstract

Human volunteers were exposed experimentally to single-sided mechanical shocks (SSMS) and double-sided mechanical shocks (DSMS) while seated. The aim of this study was to describe and contrast seated postural reactions due to SSMS or DSMS in healthy male adults. Mechanical shocks to the body, caused when driving on irregular terrain, are suggested to be hazardous to the spine and may be associated with the reported musculoskeletal pain of the back and neck among professional drivers. However, very little is known about the characteristics of seated postural reactions and the biomechanical effects caused by mechanical shocks. Twenty healthy male subjects (18-43 years old) were exposed while seated to 5 SSMS and 15 DSMS in lateral directions. The second acceleration in the DSMS was in the opposite direction to the first acceleration and was fast, medium, or slow depending on the speed of direction change. Surface electromyography (EMG) was recorded in muscles of the upper neck, trapezius, erector spinae, and external oblique, while kinematics were recorded with inertial sensors placed at the neck, trunk, and pelvis. Muscle activity was normalized to maximum voluntary contractions (MVCs). The EMG amplitudes were significantly higher (0.6-1%; P < 0.001) for the fast DSMS than all other shocks. Range of motion (ROM) of the neck and trunk was greater during the DSMS than the SSMS. Evoked muscle activity was less than 2% MVC in the trapezius, less than 10% MVC in the erector spinae and upper neck, while the activity exceeded 10% MVC in the external oblique muscles. Fast DSMS in lateral directions appear more demanding than SSMS, demonstrating augmented seated postural reactions. However, the present mechanical shocks employed did not seem to induce postural reactions with regard to ROM or muscle activity of a magnitude likely to cause musculoskeletal overload. 4.