Trunk dynamics, including stiffness, mass and damping were quantified during trunk extension exertions with and without voluntary recruitment of antagonistic co-contraction. The objective of this study was to empirically evaluate the influence of co-activation on trunk stiffness. Muscle activity associated with voluntary co-contraction has been shown to increase joint stiffness in the ankle and elbow. Although biomechanical models assume co-active recruitment causes increase trunk stiffness it has never been empirically demonstrated. Small trunk displacements invoked by pseudorandom force disturbances during trunk extension exertions were recorded from 17 subjects at two co-contraction conditions (minimal and maximal voluntary co-contraction recruitment). EMG data were recorded from eight trunk muscles as a baseline measure of co-activation. Increased EMG activity confirms that muscle recruitment patterns were different between the two co-contraction conditions. Trunk stiffness was determined from analyses of impulse response functions (IRFs) of trunk dynamics wherein the kinematics were represented as a second-order behavior. Trunk stiffness increased 37.8% (p < 0.004) from minimal to maximal co-activation. Results support the assumption used in published models of spine biomechanics that recruitment of trunk muscle co-contraction increases trunk stiffness thereby supporting conclusions from those models that co-contraction may contribute to spinal stability.

Background.-Static trunk flexion working postures and disturbed trunk muscle reflexes are related to increased risk of low-back pain. Animal studies conclude that these factors may be related; passive tissue strain in spinal ligaments causes subsequent short-term changes in reflex. Although studies have documented changes in the myoelectric onset angle of flexion-relaxation following prolonged static flexion and cyclic flexion we could find no published evidence related to the human reflex response of the trunk extensor muscles following a period of static flexion-relaxation loading.Methods.-Eighteen subjects maintained static lumbar flexion for 15 min. Paraspinal muscle reflexes were elicited both before and after the flexion-relaxation protocol using pseudorandom stochastic force disturbances while recording EMG. Reflex gain was computed from the peak value of the impulse response function relating input force perturbation to EMG response using timedomain deconvolution analyses.Findings.-Reflexes showed a trend toward increased gain after the period of flexion-relaxation (P < 0.055) and were increased with trunk extension exertion (P < 0.021). Significant gender differences in reflex gain were observed (P < 0.01).Interpretations.-Occupational activities requiring extended periods of trunk flexion contribute to changes in reflex behavior of the paraspinal muscles. Results suggest potential mechanisms by which flexed posture work may contribute to low-back pain. Significant gender differences indicate risk analyses should consider personal factors when considering neuromuscular behavior.

Abstract-This research compares transtibial prosthetic sockets made by central fabrication facilities with their corresponding American Academy of Orthotists and Prosthetists (AAOP) electronic shape files and assesses the central fabrication process. We ordered three different socket shapes from each of 10 manufacturers. Then we digitized the sockets using a very accurate custom mechanical digitizer. Results showed that quality varied considerably among the different manufacturers. Four of the companies consistently made sockets within +/-1.1% volume (approximately 1 sock ply) of the AAOP electronic shape file, while six other companies did not. Six of the companies showed consistent undersizing or oversizing in their sockets, which suggests a consistent calibration or manufacturing error. Other companies showed inconsistent sizing or shape distortion, a difficult problem that represents a most challenging limitation for central fabrication facilities.

Study Design.-Repeated measures experimental study of the effect of flexion-relaxation, recovery, and gender on paraspinal reflex dynamics.Objective.-To determine the effect of prolonged flexion-relaxation and recovery time on reflex behavior in human subjects.Summary of Background Data.-Prolonged spinal flexion has been shown to disturb the paraspinal reflex activity in both animals and human beings. Laxity in passive tissues of the spine from flexion strain may contribute to desensitization of mechanoreceptors. Animal studies indicate that recovery of reflexes may take up to several hours. Little is known about human paraspinal reflex behavior following flexion tasks or the recovery of reflex behavior following the flexion tasks.Methods.-A total of 25 subjects performed static flexionrelaxation tasks. Paraspinal muscle reflexes were recorded before and immediately after flexion-relaxation and after a recovery period. Reflexes were quantified from systems identification analyses of electromyographic response in relation to pseudorandom force disturbances applied to the trunk.Results.-Trunk angle measured during flexion-relaxation postures was significantly higher following static flexion-relaxation tasks (P < 0.001), indicating creep deformation of passive supporting structures in the trunk. Reflex response was diminished following flexion-relaxation (P < 0.029) and failed to recover to baseline levels during 16 minutes of recovery.Conclusion.-Reduced reflex may indicate that the spine is less stable following prolonged flexion-relaxation and, therefore, susceptible to injury. The absence of recovery in reflex after a substantial time indicates that increased low back pain risk from flexion-relaxation may persist after the end of the flexion task.Keywords flexion-relaxation; reflex; low back; spine; electromyogram; stability Address correspondence and reprint requests to Kevin P. Granata, PhD, Musculoskeletal Biomechanics Laboratories, Department of Engineering Science and Mechanics, School of Biomedical Engineering and Science, VA Polytechnic Institute and State University, 219 Norris Hall (0219), Blacksburg, VA 24061; E-mail: E-mail: Granata@VT.edu. Federal funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript. This research was supported by grants R01 AR46111 from NIAMS of the National Institute of Health and R01 OH07352-01 from NIOSH of the Centers of Disease Control. NIH Public AccessAuthor Manuscript Spine (Phila Pa 1976). Author manuscript; available in PMC 2007 March 2. Published in final edited form as:Spine (Phila Pa 1976 One of the underlying mechanisms of reduced paraspinal reflexes associated with prolonged spinal flexion is linked to the function of the viscoelastic tissues in the spine. 9 Mechanoreceptors in the spinal ligaments reflexively activate the paraspinal musculature. 10-13 This reflex regulates continuous spinal movement, and acts to control stabi...

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