Purpose: The goal of this study was to assess differences in low back stabilization and underlying mechanisms between patients with low back pain (LBP) and healthy controls. It has been hypothesized that inadequate trunk stabilization could contribute to LBP through high tissue strains and/or impingement. Evidence to support this is inconsistent, and not all methods that have been used to study trunk stabilization are equally suitable. We have recently developed a method to assess intrinsic and reflexive contributions to trunk stabilization, which aims to circumvent the limitations of previous studies. Methods: Forty-nine participants suffering from chronic LBP and a control group of fifty healthy subjects participated in this study. Trunk stabilization was measured using force-controlled perturbations directly applied to the trunk. The actuator displacement and contact force between the actuator and subject were measured as well as electromyography (EMG) of the M. Longissimus. Underlying mechanisms were characterized using system identification. Results: LBP patients showed lower admittance, i.e., less displacement per unit of force applied, mainly due to higher position, velocity and acceleration feedback gains. Among patients, lower trunk admittance and higher reflex gains were associated with more negative pain-related cognitions. Conclusion: Trunk stabilization differs between LBP patients and controls, with the same perturbations causing less trunk movement in patients, due to stronger reflexes. We interpret these changes as reflecting protective behavior. Graphic abstract: These slides can be retrieved under Electronic Supplementary Material.[Figure not available: see fulltext.].

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The human head-neck system requires continuous muscular stabilization in the presence of gravity and trunk motion. This chapter presents experimental and modeling efforts, applying mechanical perturbations to seated subjects, evaluating trunk and head motion, to investigate postural stabilization.

A detailed multisegment neck model has been developed including vestibular/visual and muscular feedback loops and cocontraction. Dynamic validation is presented in the frequency domain in all six motion directions. The neck model captures primary motion responses and interaction terms such as head rotation in response to seat translation. Results show major contributions of vestibular/visual feedback stabilizing the head in space while muscular feedback stabilizes the head on the torso. In addition, muscular feedback is essential to stabilize the individual vertebral joints and prevent neck buckling. The contribution of cocontraction is estimated to be minor in the neck. Validation in impact conditions shows that postural control parameters estimated that fitting the model to small-amplitude experimental data can predict postural responses in high-amplitude loading conditions reasonably well.

This manuscript focuses on the neck but also includes experiments with combined stabilization of the complete spine, measuring trunk and head motion, with a perspective toward full spine and full-body modeling. Lumbar stabilization has been captured using a simplified model by assuming a virtual pivot around L4/L5. The model uniquely separates stabilizing contributions of intrinsic stiffness and damping (including muscle cocontraction) and muscle feedback (length, velocity, and acceleration). The model parameters allowed us to estimate the relative contributions of intrinsic and reflexive stabilization and showed intrinsic contributions, similar to or larger than reflexive contributions in lumbar stabilization with horizontal perturbations to the trunk or pelvis. Experiments with a rotating pelvis showed relevant contributions of vestibular and visual feedback, which are more effective to minimize head than trunk rotation.

A full-body human model with multisegment spine was previously validated for impact and vertical vibration. Integrating the new detailed neck model in the full-body human model will enable simulation of full-body vibration and impact scenarios with realistic compliant seat models. Further experiments and modeling efforts will aim to capture sensory integration of visual and vestibular motion perceptions in relation to posture maintenance and motion sickness.@en

Trunk stabilization is required to control posture and movement during daily activities. Various sensory modalities, such as muscle spindles, Golgi tendon organs and the vestibular system, might contribute to trunk stabilization and our aim was to assess the contribution of these modalities to trunk stabilization. In 35 healthy subjects, upper-body sway was evoked by continuous unpredictable, force-controlled perturbations to the trunk in the anterior direction. Subjects were instructed to either ‘maximally resist the perturbation’ or to ‘relax but remain upright’ with eyes closed. Frequency response functions (FRFs) of admittance, the amount of movement per unit of force applied, and reflexes, the modulation of trunk extensor activity per unit of trunk displacement, were obtained. To these FRFs, we fitted physiological models, to estimate intrinsic trunk stiffness and damping, as well as feedback gains and delays. The different model versions were compared to assess which feedback loops contribute to trunk stabilization. Intrinsic stiffness and damping and muscle spindle (short-delay) feedback alone were sufficient to accurately describe trunk stabilization, but only with unrealistically low reflex delays. Addition of muscle spindle acceleration feedback or inhibitory Golgi tendon organ feedback yielded realistic delays and improved the model fit, with a significantly better model fit with acceleration feedback. Addition of vestibular feedback did not improve the model fit. In conclusion, muscle spindle feedback and intrinsic mechanical properties are sufficient to describe trunk stabilization in the sagittal plane under small mechanical perturbations, provided that muscle spindles encode acceleration in addition to velocity and position information.@en

The goal of this study was to investigate the human ability to stabilize the trunk in space during pelvic tilt. Upper body sway was evoked in kneeling-seated healthy subjects by angular platform perturbations with a rotation around a virtual lowback pivot point between the L4 and L5 vertebrae. To investigate motor control modulation, variations in task instruction (balance naturally or minimize trunk sway), vision (eyes open or closed), and perturbation bandwidth (from 0.2 up to 1, 3, or 10 Hz) were applied. Cocontraction and proprioceptive muscle spindle feedback were associated with minimizing low-back flexion/extension (trunk-on-pelvis stabilization), while vestibular and visual feedback were supposed to contribute to trunk-in-space stabilization. Trunk-in-space stabilization was only observed with the minimize trunk sway task instruction, while the task instruction to balance naturally led to trunk-on-pelvis stabilization with trunk rotations even exceeding the perturbations. This indicates that vestibular feedback is used when minimizing trunk sway but has only a minor contribution during natural trunk stabilization in the sagittal plane. The eyes open condition resulted in reduced global trunk rotations and increased global trunk reflexive responses, demonstrating effective visual contributions to trunk-inspace stabilization. On the other hand, increasing perturbation bandwidth caused a decreased feedback contribution leading to deteriorated trunk-in-space stabilization.

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