Decreased variability in postural control strategies in young people with non-specific low back pain is associated with altered proprioceptive reweighting

Precision control of trunk movement in low back pain patients

Motor control is challenged in tasks with high precision demands. In such tasks, signal-dependent neuromuscular noise causes errors and proprioceptive feedback is required for optimal performance. Pain may affect proprioception, muscle activation patterns and resulting kinematics. Therefore, we investigated precision control of trunk movement in 18 low back pain (LBP) patients and 13 healthy control subjects. The subjects performed a spiral-tracking task requiring precise trunk movements, in conditions with and without disturbance of proprioception by lumbar muscle vibration. Tracking task performance and trunk muscle electromyography were recorded. In conditions without lumbar muscle vibration, tracking errors were 27.1% larger in LBP patients compared to healthy controls. Vibration caused tracking errors to increase by 10.5% in healthy controls, but not in LBP patients. These results suggest that reduced precision in LBP patients might be explained by proprioceptive deficits. Ratios of antagonistic over agonistic muscle activation were similar between groups. Tracking errors increased trunk inclination, but no significant relation between tracking error and agonistic muscle activation was found. Tracking errors did not decrease when antagonistic muscle activation increased, so, neither healthy subjects nor LBP patients appear to counteract trunk movement errors by increasing co-contraction.

Using vertebral movement and intact paraspinal muscles to determine the distribution of intrafusal fiber innervation of muscle spindle afferents in the anesthetized cat

Increasing our knowledge regarding intrafusal fiber distribution and physiology of paraspinal proprioceptors may provide key insights regarding proprioceptive deficits in trunk control associated with low back pain and lead to more effective clinical intervention. The use of vertebral movement as a means to reliably stretch paraspinal muscles would greatly facilitate physiological study of paraspinal muscle proprioceptors where muscle tendon isolation is either very difficult or impossible. The effects of succinylcholine (SCh) on 194 muscle spindle afferents from lumbar longissimus or multifidus muscles in response to computer-controlled, ramp-and-hold movements of the L(6) vertebra were investigated in anesthetized cats. Paraspinal muscles were stretched by moving the L(6) vertebra 1.5-1.7 mm in the dorsal-ventral direction. Initial frequency (IF), dynamic difference (DD), their changes (∆) following SCh injection (100-400 μg kg(-1)), and post-SCh dynamic difference (SChDD) were measured. Muscle spindle intrafusal fiber terminations were classified as primary or secondary fibers as well as bag(1) (b(1)c), bag(2) (b(2)c), b(1)b(2)c, or chain (c) fibers. Intrafusal fiber subpopulations were distinguished using logarithmic transformation of SChDD and ∆IF distributions as established by previous investigators. Increases in DD indicate strength of b(1)c influence while increases in IF indicate strength of b(2)c influence. Out of 194 afferents, 46.9 % of afferents terminated on b(2)c fibers, 46.4 % on b(1)b(2)c fibers, 1 % on b(1)c fibers, and 5.7 % terminated on c fibers. Based on these intrafusal fiber subpopulation distributions, controlled vertebral movement can effectively substitute for direct tendon stretch and allow further investigation of paraspinal proprioceptors in this anatomically complex body region.

Children with ADHD show no deficits in plantar foot sensitivity and static balance compared to healthy controls

The goal of this study was to investigate plantar foot sensitivity and balance control of ADHD (n=21) impaired children compared to age-matched healthy controls (n=25). Thresholds were measured at 200 Hz at three anatomical locations of the plantar foot area of both feet (hallux, first metatarsal head (METI) and heel). Body balance was quantified using the length, area and velocity described by the center of pressure (COP) during two-legged as well as one-legged stand (right and left legs). The comparison of vibration thresholds showed no differences between ADHD and healthy children at all anatomical locations of both feet. Whereas COP excursion and area were significantly lower in ADHD subjects compared to the healthy controls during two-legged stand, no differences were found in those variables when balancing on one leg. No differences in COP velocity between ADHD and healthy children were found in any analyzed conditions. The results indicate that the unusual and simple test situation may have increased the perception of vibration stimuli by the ADHD children. Furthermore, ADHD subjects seem to be less variable when performing simple tasks than healthy controls.

Precision control of an upright trunk posture in low back pain patients

BACKGROUND

Low back pain appears to be associated with impaired trunk postural control, which could be caused by proprioceptive deficits. We assessed control of trunk posture in conditions requiring high and low precision, with and without disturbance of proprioception by lumbar muscle vibration.

METHODS

Twenty a-specific low back pain patients and 13 healthy controls maintained a self-chosen upright trunk posture. Initial frontal and sagittal plane angles of an opto-electronic marker on the 12th thoracic spinous process defined the center of a target area on a monitor. Subjects were instructed to stay within that target and visual feedback was provided when they left the target. The precision demand was manipulated by changing target size. The standard deviation of trunk angle quantified precision and mean Euclidian distance to target center quantified accuracy. Ratios of antagonistic co-activation were calculated from trunk muscle electromyography recordings.

FINDINGS

With the small target, visual feedback was present intermittently and patients controlled their trunk as accurately and precisely as healthy controls. For the large target, subjects mostly stayed within the target, and patients were on average 0.18° (31%) less accurate than healthy controls (P=0.025), due to a larger postural drift. Lumbar muscle vibration deteriorated control over trunk posture in both groups and ratios of antagonistic co-activation did not differ between groups or conditions.

INTERPRETATION

These results indicate that the weighting of proprioceptive feedback from lumbar muscle spindles did not differ between groups and that low back pain patients were less able to detect low frequency drift in posture.

Individuals with non-specific low back pain use a trunk stiffening strategy to maintain upright posture

There is increasing evidence that individuals with non-specific low back pain (LBP) have altered movement coordination. However, the relationship of this neuromotor impairment to recurrent pain episodes is unknown. To assess coordination while minimizing the confounding influences of pain we characterized automatic postural responses to multi-directional support surface translations in individuals with a history of LBP who were not in an active episode of their pain. Twenty subjects with and 21 subjects without non-specific LBP stood on a platform that was translated unexpectedly in 12 directions. Net joint torques of the ankles, knees, hips, and trunk in the frontal and sagittal planes as well as surface electromyographs of 12 lower leg and trunk muscles were compared across perturbation directions to determine if individuals with LBP responded using a trunk stiffening strategy. Individuals with LBP demonstrated reduced peak trunk torques, and enhanced activation of the trunk and ankle muscle responses following perturbations. These results suggest that individuals with LBP use a strategy of trunk stiffening achieved through co-activation of trunk musculature, aided by enhanced distal responses, to respond to unexpected support surface perturbations. Notably, these neuromotor alterations persisted between active pain periods and could represent either movement patterns that have developed in response to pain or could reflect underlying impairments that may contribute to recurrent episodes of LBP.

A history of low back pain associates with altered electromyographic activation patterns in response to perturbations of standing balance

People with a history of low back pain (LBP) exhibit altered responses to postural perturbations, and the central neural control underlying these changes in postural responses remains unclear. To characterize more thoroughly the change in muscle activation patterns of people with LBP in response to a perturbation of standing balance, and to gain insight into the influence of early- vs. late-phase postural responses (differentiated by estimates of voluntary reaction times), this study evaluated the intermuscular patterns of electromyographic (EMG) activations from 24 people with and 21 people without a history of chronic, recurrent LBP in response to 12 directions of support surface translations. Two-factor general linear models examined differences between the 2 subject groups and 12 recorded muscles of the trunk and lower leg in the percentage of trials with bursts of EMG activation as well as the amplitudes of integrated EMG activation for each perturbation direction. The subjects with LBP exhibited 1) higher baseline EMG amplitudes of the erector spinae muscles before perturbation onset, 2) fewer early-phase activations at the internal oblique and gastrocnemius muscles, 3) fewer late-phase activations at the erector spinae, internal and external oblique, rectus abdominae, and tibialis anterior muscles, and 4) higher EMG amplitudes of the gastrocnemius muscle following the perturbation. The results indicate that a history of LBP associates with higher baseline muscle activation and that EMG responses are modulated from this activated state, rather than exhibiting acute burst activity from a quiescent state, perhaps to circumvent trunk displacements.