Identifying intrinsic and reflexive properties of the low-back by inertial loading

Abnormal neuromuscular control (NMC) has been suggested as a cause or effect of a-specific low back pain. This study aimed to gain more insight into the stabilization of the low back during postural control by developing a method to study the intrinsic and reflexive components of NMC. Inertial loading of the trunk by perturbation of subjects on a movable platform was used to induce force disturbances during posture maintenance without modulation due to manipulator contact to the upper body and pre-loading. Experimental conditions included variations in perturbation frequency content (low, mid and high bandwidth), visual feedback (eyes open/eyes closed) and task instructions (relax/resist). Frequency response functions (FRFs) were obtained for internal low back displacement (bending) and reflexes. Task instruction resulted in 49% (p=0.018) lower internal displacement gains below 1Hz during resist tasks, indicating good task compliance. Reflexive FRFs were not significantly influenced. Decreasing the perturbation frequency content from high to low bandwidth resulted in a 25% lower internal displacement gain (p=0.044) below 1 Hz. The presence of visual feedback reduced displacement gains with 9% (p=0.040) and the reflexive gain decreased by 64% (P=0.036) in the eyes open condition, EMG RMS increased indicating increased co-contraction. A physiologically-based model including intrinsic damping and stiffness as well as muscle spindle (position and velocity) and Golgi tendon organ (force) feedback was fitted to the full bandwidth data without visual feedback. This model accounted for 72% of the variance in displacement and 42% of the variance in reflexes. Analysis using this model showed that co-contraction as well as reflexes were modulated in reaction to experimental task. Analysis of the contribution of different model parts to the system behavior during resist tasks showed that the reflexive model part increased low back displacement. This shows that reflexes in this case decrease low back stiffness, due to inhibitory GTO reflexes.BMEBioMechanical EngineeringMechanical, Maritime and Materials Engineerin