Neuromuscular Control and Active Trunk Stiffness during Isometric Flexion and Extension

Abstract

Pushing and pulling tasks account for 20% of low-back injury claims. Torso flexion necessary for pushing exertions requires different muscle recruitment than for extension exertions typical of lifting tasks. These differences in recruitment and control may influence spinal stability and associated risk of injury. Active muscle stiffness is considered the primary stabilizing mechanism for spinal stability. Therefore, active trunk stiffness was recorded while subjects generated upright isometric trunk flexion and extension exertions against an isotonic preload. Small pseudo-random force disturbances were superimposed on the preloads causing small amplitude trunk movements. Trunk stiffness was computed from systems identification of the measured force and trunk motion data. Results demonstrated significantly greater stiffness during flexion exertions as compared to extension exertions. EMG data suggest this difference was due to increased co-contraction during the flexion exertions. These behaviours were attributed to the need to augment neuromuscular control of spinal stability during pushing tasks. Keywords: Spine; Co-contraction; Push; Manual Materials Handling; Biomechanics