Motor control deficits have been suggested as potential cause and/or effect of a-specific chronic low-back pain and its recurrent behavior. Therefore, the goal of this study is to identify motor control in low-back stabilization by simultaneously quantifying the intrinsic and reflexive contributions. Upper body sway was evoked using continuous force perturbations at the trunk, while subjects performed a resist or relax task. Frequency response functions (FRFs) and coherences of the admittance (kinematics) and reflexes (sEMG) were obtained. In comparison with the relax task, the resist task resulted in a 61% decrease in admittance and a 73% increase in reflex gain below 1.1Hz. Intrinsic and reflexive contributions were captured by a physiologically-based, neuromuscular model, including proprioceptive feedback from muscle spindles (position and velocity) and Golgi tendon organs (force). This model described on average 90% of the variance in kinematics and 39% of the variance in sEMG, while resulting parameter values were consistent over subjects.

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.

The goal of this study is to assess how reflexes and intrinsic properties contribute to low-back stabilization and modulate with conditions. Upper body sway was evoked by anterior-posterior platform translations, while subjects were seated with a restrained pelvis and free upper body. Kinematic analysis of trunk translations and rotations illustrated that a fixed rotation point between the vertebrae L4 and L5 adequately captures lumbar bending up to 5 Hz. To investigate the motor control modulation, the conditions varied in vision (eyes open or closed), task instruction (Balance naturally or Resist perturbations by minimizing low-back motions), and perturbation bandwidth (from 0.2 up to 1, 3 or 10 Hz). Frequency response functions and physiological modeling parameters showed substantial modulation between all conditions. The eyes-open condition led to trunk-in-space behavior with additional long-latency visual feedback and decreased proprioceptive feedback. The task instruction to resist led to trunk-on-pelvis stabilization behavior, which was achieved by higher co-contraction levels and increased reflexive velocity feedback. Perturbations below the low-back natural frequency (~1 Hz) led to trunk-on-pelvis stabilization behavior, mainly attributed to increased intrinsic damping. This indicates that bandwidth effects should not be ignored and that experiments with high-bandwidth perturbations do not fully represent the intrinsic and reflexive behavior during most (low-bandwidth) daily life activities. The neck stabilized the head orientation effectively (head rotation amplitudes 2 % of trunk), but did not effectively stabilize the head in space (global head translations exceeded trunk translations by 20 %). This indicates that low-back motor control is involved in head-in-space stabilization and could explain the low-back motor control modulations due to vision.

The goal of this study was to determine the effects of vision and lumbar posture on trunk neuromuscular control. Torso perturbations were applied with a pushing device while the subjects were restrained at the pelvis in a kneeling-seated position. Torso kinematics and the muscle activity of the lumbar part of the M. Longissimus were recorded for 14 healthy subjects. Four conditions were included: a flexion, extension and neutral lumbar posture with eyes closed and the neutral posture with eyes open. Frequency response functions of the admittance and reflexes showed that there was no significant difference between the eyes open and eyes closed conditions, thereby confirming that vision does not play a role in the stabilization of the trunk during small-amplitude trunk perturbations. In contrast, manipulating posture did lead to significant differences. In particular, the flexed condition led to a lower admittance and lower reflex contribution compared to the neutral condition. Furthermore, the muscle pre-activation (prior to the onset of the perturbation) was significantly lower in the flexed posture compared to neutral. This confirms that flexing the lumbar spine increases the passive tissue stiffness and decreases the contribution of reflex activity to trunk control.

van Drunen P, van der Helm FC, van Dieën JH, Happee R. Trunk stabilization during sagittal pelvic tilt: from trunk-on-pelvis to trunk-in-space due to vestibular and visual feedback.