Double-sided mechanical shocks provoke larger seated postural reactions compared to single-sided mechanical shocks

STUDY DESIGN:Human volunteers were exposed experimentally to single-sidedmechanical shocks (SSMS) and double-sided mechanical shocks (DSMS) while seated. OBJECTIVE:To describe and contrast seated postural reactions due to SSMSor DSMS in healthy male adults. SUMMARY OF BACKGROUND DATA: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. METHODS: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 DSMSwas in the opposite direction to the first acceleration and was eitherfast, 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 (MVC). RESULTS:The EMG amplitudes were significantly higher (0.6-1%; p