TRUNK KINEMATICS AND TRUNK MUSCLE-ACTIVITY DURING A RAPIDLY APPLIED LOAD

This study investigated the trunk kinematics and electromyographic (EM G) activity of eight trunk muscles when ''expected'' and ''unexpected' ' loads were applied directly to the torso. Twenty individuals (mean a ge: 25.1 yr; range 20-33 yr) participated in this mixed model study in which gender was the between-subjects factor, and expectancy and symm etry of the applied load were within-subject factors. The sudden load was delivered to the subject via a cable attached to a thoracic harnes s and motion was restricted to the lumbar spine by strapping the pelvi s to a rigid fixation apparatus. Surface EMG was recorded bilaterally from the longissimus thoracis (LGT), erector spinae (ERS), rectus abdo minis (RAB) and the external obliques (EXO). Trunk kinematics were mea sured with a Lumbar Motion Monitor(TM). During expected loading condit ions, the peak muscle activity was reduced for the RAB and EXO bilater ally, and for the ERS(R) (p < 0.01) relative to the unexpected conditi ons. Conversely, the normalized area of EMG activity prior to the onse t of load was increased for the ERS and EXO bilaterally, and for the R AB(R) (p < 0.05) during an expected loading event. Trunk motion in the sagittal and frontal planes was reduced during expected loading. Acti vation of the trunk muscles just prior to a rapid loading event increa ses trunk stiffness, decreasing trunk displacement and peak muscle act ivity. (C) 1998 Elsevier Science Ltd. All rights reserved.