Persons with unilateral lower-limb amputation have altered and asymmetric trunk mechanical and neuromuscular behaviors estimated using multidirectional trunk perturbations

Among persons with unilateral lower-limb amputation (LLA), proximal compensations and preferential use of the sound limb during gait and movement may lead to chronic alterations and/or asymmetries in trunk mechanical and neuromuscular behaviors. Trunk stiffness, the magnitude and timing of maximum reflex force, and EMG reflex delays of superficial trunk muscles, were estimated here using multidirectional (anteriorly- and laterally-directed) position-controlled horizontal trunk perturbations (±5mm, applied at T8) with the pelvis immobilized. Alterations and asymmetries in these trunk behaviors were quantified and compared among eight males with unilateral LLA, and eight male non-amputation controls. During anteriorly-directed perturbations, trunk stiffness and maximum reflex force were 24% and 23% lower, respectively, among participants with LLA compared to non-amputation controls, and the timing of maximum reflex force was 8% later. During lateral perturbations, trunk stiffness and maximum reflex force were also significantly lower among participants with LLA, by 22% and 27%, respectively. Bilateral asymmetries were present in trunk stiffness and the timing of maximum reflex force among persons with LLA. Specifically, trunk stiffness was 20% lower and timing of maximum reflex force was 9% later during perturbations involving spinal tissues and muscles ipsilateral to the side of amputation. Reduced and asymmetric trunk mechanical and neuromuscular behaviors may suggest a condition of reduced trunk stability among individuals with LLA, which could be due to repeated exposure to altered and asymmetric gait and movement and/or compensatory muscle recruitment in response to lost or altered musculature subsequent to LLA.

Does prolonged seated deskwork alter the lumbar flexion relaxation phenomenon?

Sustained maximum lumbar spine flexion can increase the angle at which the low back flexion relaxation phenomenon (FRP) is observed. This adaptation has been hypothesized to have implications for the control of lumbar spine stability and increase the potential for low back injury. The objective of this study was to investigate if similar changes in the FRP would occur from sub-maximal spine flexion induced by an extended continuous duration of seated office deskwork. Twenty-three participants (12 male and 11 female) performed three bouts of full forward spine flexion interspersed with two 1-h periods of seated deskwork. Lumbar spine angular kinematics and electromyographic activity from the lumbar erector spinae were obtained throughout all trials. The angles at which myoelectric silence occurred (FRP onset) were documented. Lumbar flexion at FRP onset increased by 1.3±1.5° after 1-h of sitting (p<0.05) with no further increase after 2-h. However, when the angle at the FRP onset was normalized to the total range of flexion, there was no difference in the FRP onset. These results suggest that the seated posture may induce residual deformation in the viscoelastic passive tissues of the low back; this could increase the challenge of controlling spine motion and reduce the load-bearing capacity of the lumbar spine system during activities performed following extended bouts of sitting.