Spine stability: lessons from balancing a stick

Using continuous relative phase and modified vector coding analyses to quantify spinal coordination and coordinative variability for healthy and chronic low back pain patients: An exploratory comparative analysis

Differences in coordination and coordinative variability are common in people with low back pain. While differences may relate to the different analyses used to quantify these metrics, the preferred approach remains unclear. We aimed to compare coordination and coordinative variability, in people with and without low back pain performing a lifting/lowering task, using continuous relative phase and vector coding procedures, and to identify which technique better detects group differences. Upper lumbar (T12-L3), lower lumbar (L3-S1), and hip angular kinematics were measured using electromagnetic motion capture during 10 crate lifting/lowering repetitions from adults with (n = 47) and without (n = 17) low back pain. Coordination and coordinative variability for the Hip-Lower Lumbar and Lower Lumbar-Upper Lumbar joint pairs were quantified using mean absolute relative phase and deviation phase (continuous relative phase), and coupling angle and coupling angle variability (vector coding), respectively. T-tests examined group differences in coordination and variability. Cohen's d bootstrapping analyses identified the more sensitive technique for detecting group differences. Less in-phase and more variable behavior was observed in the low back pain group, mostly independent of joint pair and analytical technique (P < 0.05, Cohen's d range = 0.61 to 1.33). Qualitatively, the low back group limited motion at the lower lumbar spine during lifting/lowering. Continuous relative phase was more sensitive in detecting group differences in coordinative variability, while vector coding was more sensitive towards differences in coordination. These procedures convey distinct information and have their respective merits. Researchers should consider the choice of analytical techniques based on their study objectives.

Kinematic changes of the trunk and lower limbs during voluntary lateral sway postural control in adults with low back pain

Introduction: Voluntary lateral weight shifting is essential for gait initiation. However, kinematic changes during voluntary lateral weight shifting remain unknown in people with low back pain (LBP). This study aims to explore the differences in kinematics and muscle activation when performing a voluntary lateral weight shifting task between patients with LBP and asymptomatic controls without pain. Methods: Twenty-eight participants volunteered in this study (14 in both the LBP group and the control group). The Sway Discrimination Apparatus (SwayDA) was used to generate a postural sway control task, mimicking lateral weight shifting movements when initiating gait. Kinematic parameters, including range of motion (ROM) and standard deviation of ROM (Std-ROM) of the lumbar spine, pelvis, and lower limb joints, were recorded using a motion capture system during lateral weight shifting. The electroactivity of the trunk and lower limb muscles was measured through surface electromyography using root mean square (RMS). The significant level was 0.05. An independent t-test was employed to compare kinematic parameters, and muscle activation between the LBP group and the control group. A paired-sample t-test, adjusted with Bonferroni correction (significant level of 0.025), was utilized to examine differences between the ipsilateral weight shifting towards side (dominant side) and the contralateral side. Results: The results of kinematic parameters showed significantly decreased ROM and std-ROM of the ipsilateral hip in the transverse plane (tROM = -2.059, p = 0.050; tstd-ROM = -2.670, p = 0.013), as well as decreased ROM of the ipsilateral knee in the coronal plane (t = -2.148, p = 0.042), in the LBP group compared to the control group. For the asymptomatic controls, significantly larger ROM and ROM-std were observed in the hip and knee joints on the ipsilateral side in contrast to the contralateral side (3.287 ≤ t ≤ 4.500, 0.001 ≤ p≤ 0.006), but no significant differences were found between the two sides in the LBP group. In addition, the LBP group showed significantly lower RMS of the biceps femoris than the control group (tRMS = -2.186, p = 0.044). Discussion: Patients with LBP showed a conservative postural control pattern, characterized by reduced ROM of ipsilateral joints and diminished activation of the biceps femoris. These findings suggested the importance of voluntary postural control assessment and intervention to maximize recovery.

Voluntary postural sway control and mobility in adults with low back pain

Introduction

Low back pain (LBP) is associated with altered somatosensory perception, which is involved in both involuntary and voluntary control of posture. Currently, there is a lack of methods and tools for assessing somatosensory acuity in patients with LBP. The purpose of this study was (1) to assess the reliability of the sway discrimination apparatus (SwayDA) (2) to evaluate the differences in somatosensory acuity between patients with LBP and pain-free individuals, and (3) to examine relationships between somatosensory acuity, severity of LBP, and mobility in patients with LBP.

Methods

Twenty participants (10 patients with LBP and 10 matched asymptomatic controls) were recruited in a test-retest reliability test. Another 56 participants were recruited for this study with 28 individuals presenting with LBP and a further twenty-eight being asymptomatic. The SwayDA was custom-built to measure somatosensory perception during voluntary anterior-posterior (SwayDA-AP), medial-lateral to the dominant side (SwayDA-ML-D), and non-dominant side (SwayDA-ML-ND) postural sway control. Participants also completed mobility tests, including 10 times and 1-min sit-to-stand tests (10-STS, 1 m-STS). The area under the receiver operating characteristic curve (AUC) was calculated to quantify somatosensory acuity in discriminating different voluntary postural sway extents.

Results

The ICC (2.1) for the SwayDA-AP, SwayDA-ML-D, and SwayDA-ML-ND were 0.741, 0.717, and 0.805 with MDC95 0.071, 0.043, and 0.050. Patients with LBP demonstrated significantly lower SwayDA scores (tSwayDA-AP = -2.142, p = 0.037; tSwayDA-ML-D = -2.266, p = 0.027) than asymptomatic controls. The AUC values of the SwayDA-AP test were significantly correlated with ODI (rSwayDA-AP-ODI = -0.391, p = 0.039). Performances on the 1 m-STS and the 10-STS were significantly correlated with the AUC scores from all the SwayDA tests (-0.513 ≤ r ≤ 0.441, all p < 0.05).

Discussion

The SwayDA tests evaluated showed acceptable reliability in assessing somatosensory acuity during voluntary postural sway. Somatosensory acuity was diminished in patients with LBP compared to asymptomatic controls. In patients with LBP, lower somatosensory acuity was associated with increased LBP-related disability. Future research could focus on investigating the factors contributing to the decreased somatosensory perception and mobility in individuals with LBP.

Widespread Proprioceptive Acuity Impairment in Chronic Back Pain: A Cross-sectional Study

OBJECTIVE

To investigate whether proprioceptive accuracy measured with the Joint Position Sense (JPS) in patients with chronic neck and low back pain is impaired exclusively in affected areas or also in distant areas, not affected by pain.

DESIGN

Cross-sectional study.

SETTING

Interdisciplinary outpatient rehabilitation clinic for back and neck pain.

PARTICIPANTS

Patients with chronic neck pain (n=30), patients with chronic low back pain (n=30), and age- and sex-matched asymptomatic control subjects (n=30; N=90).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Patients and asymptomatic control subjects completed a test procedure for the JPS of the cervical spine, lumbar spine, and ankle in a randomized order. Between group differences were analyzed with the univariate analysis of variance and associations of the JPS with clinical features using the Pearson's correlation coefficient.

RESULTS

Both patients with chronic neck pain (P<.001) and patients with chronic low back pain (P<.01) differed significantly from asymptomatic controls in the JPS of the cervical spine, lumbar spine and ankle joint, regardless of the painful area. No difference was shown between patient groups (P>.05). An association of the JPS with clinical characteristics, however, could not be shown.

CONCLUSION

These results suggest widespread impairment of proprioceptive accuracy in patients with chronic and low back pain and a role for central sensorimotor processes in musculoskeletal pain conditions.

Comparison of proprioceptive postural control strategies between prolonged standing induced low back pain developers and non-low back pain developers

BACKGROUND

Proprioception deficit has been suggested as a possible mechanism contributing for the impaired postural control in low back pain (LBP) patients. Whether proprioception deficit is a result of or a cause of LBP has not been investigated.

OBJECTIVE

The purpose of this study was to compare proprioceptive postural control strategies between prolonged standing induced low back pain developers (PDs) and non-pain developers (NPDs).

METHOD

Thirty-two healthy subjects performed 1-h prolonged standing and their ratings of perceived LBP have been recorded. Eight quiet standing trials for 60 s performed immediately before and after the prolonged standing. Postural control was challenged by muscle vibration and different postural conditions during quiet standing. Data were recorded using a force platform.

RESULTS

Forty percentage of participants is classified as PD. Before the prolonged standing, relative proprioceptive weighting was greater in the PD compared to NPD group (P = .029). Main effect of postural condition (F_1,24 = 5.21, P = .032) and interaction of time by group (F_1,24 = 8.08, P = .009) were significant for COP displacement in anteroposterior direction. Interaction of postural condition by group (F_1,26 = 7.82, P = .010) and time by group (F_1,26 = 9.71, P = .004) were significant for COP displacement in mediolateral direction. Main effect of postural condition (F_1,26 = 6.31, P = .018) and interaction of postural condition by group (F_1,26 = 7.07, P = .013) were significant for mean velocity in mediolateral direction.

CONCLUSION

The PD group has altered proprioceptive postural control strategies before and after prolonged standing. Proprioception deficit should not be considered to be solely an adaptive response and may be causal for LBP development.

The Effect of Lumbar Belts with Different Extensibilities on Kinematic, Kinetic, and Muscle Activity of Sit-to-Stand Motions in Patients with Nonspecific Low Back Pain

Although lumbar belts can be used for the treatment and prevention of low back pain, the role of the lumbar belt remains unclear without clear guidelines. This study aimed to investigate the effect of lumbar belts with different extensibilities on the kinematics, kinetics, and muscle activity of sit-to-stand motions in terms of motor control in patients with nonspecific low back pain. A total of 30 subjects participated in the study: 15 patients with nonspecific low back pain and 15 healthy adults. Participants performed the sit-to-stand motion in random order of three conditions: no lumbar belt, wearing an extensible lumbar belt, and wearing a non-extensible lumbar belt. The sit-to-stand motion's kinematic, kinetic, and muscle activity variables in each condition were measured using a three-dimensional motion analysis device, force plate, and surface electromyography. An interaction effect was found for the time taken, anterior pelvic tilt angle, and muscle activity of the vastus lateralis and biceps femoris. The two lumbar belts with different extensibilities had a positive effect on motor control in patients with nonspecific low back pain. Therefore, both types of extensible lumbar belts can be useful in the sit-to-stand motion, which is an important functional activity for patients with nonspecific low back pain.

Effects of different modalities of afferent stimuli of the lumbo-sacral area on control of lumbar paravertebral muscles

Somatosensory feedback to the central nervous system is essential to plan, perform and refine spine motor control. However, the influence of somatosensory afferent input from the trunk on the motor output to trunk muscles has received little attention. The objective was to compare the effects of distinct modalities of afferent stimulation on the net motoneuron and corticomotor excitability of paravertebral muscles. Fourteen individuals were recruited. Modulation of corticospinal excitability (motor-evoked potential [MEP]) of paravertebral muscles was measured when afferent stimuli (cutaneous noxious and non-noxious, muscle contraction) were delivered to the trunk at 10 intervals prior to transcranial magnetic stimulation. Each peripheral stimulation was applied alone, and subsequent EMG modulation was measured to control for net motoneuron excitability. MEP modulation and MEP/EMG ratio were used as measures of corticospinal excitability with and without control of net motoneuron excitability, respectively. MEP and EMG modulation were smaller after evoked muscle contraction than after cutaneous noxious and non-noxious stimuli. MEP/EMG ratio was not different between stimulation types. Both MEP and EMG amplitudes were reduced after evoked muscle contraction, but not when expressed as MEP/EMG ratio. Noxious and non-noxious stimulation had limited impact on all variables. Distinct modalities of peripheral afferent stimulation of the lumbo-sacral area differently modulated responses of paravertebral muscles, but without an influence on corticospinal excitability with control of net motoneuron excitability. Muscle stimulation reduced paravertebral activity and was best explained by spinal mechanisms. The impact of afferent stimulation on back muscles differs from the effects reported for limb muscles.

Effect of Six-Week Resistance and Sensorimotor Training on Trunk Strength and Stability in Elite Adolescent Athletes: A Randomized Controlled Pilot Trial

Intervention in the form of core-specific stability exercises is evident to improve trunk stability. The purpose was to assess the effect of an additional 6 weeks sensorimotor or resistance training on maximum isokinetic trunk strength and response to sudden dynamic trunk loading (STL) in highly trained adolescent athletes. The study was conducted as a single-blind, 3-armed randomized controlled trial. Twenty-four adolescent athletes (14f/10 m, 16 ± 1 yrs.;178 ± 10 cm; 67 ± 11 kg; training sessions/week 15 ± 5; training h/week 22 ± 8) were randomized into resistance training (RT; n = 7), sensorimotor training (SMT; n = 10), and control group (CG; n = 7). Athletes were instructed to perform standardized, center-based training for 6 weeks, two times per week, with a duration of 1 h each session. SMT consisted of four different core-specific sensorimotor exercises using instable surfaces. RT consisted of four trunk strength exercises using strength training machines, as well as an isokinetic dynamometer. All participants in the CG received an unspecific heart frequency controlled, ergometer-based endurance training (50 min at max. heart frequency of 130HF). For each athlete, each training session was documented in an individual training diary (e.g., level of SMT exercise; 1RM for strength exercise, pain). At baseline (M1) and after 6 weeks of intervention (M2), participants’ maximum strength in trunk rotation (ROM:63°) and flexion/extension (ROM:55°) was tested on an isokinetic dynamometer (concentric/eccentric 30°/s). STL was assessed in eccentric (30°/s) mode with additional dynamometer-induced perturbation as a marker of core stability. Peak torque [Nm] was calculated as the main outcome. The primary outcome measurements (trunk rotation/extension peak torque: con, ecc, STL) were statistically analyzed by means of the two-factor repeated measures analysis of variance (α = 0.05). Out of 12 possible sessions, athletes participated between 8 and 9 sessions (SMT: 9 ± 3; RT: 8 ± 3; CG: 8 ± 4). Regarding main outcomes of trunk performance, experimental groups showed no significant pre–post difference for maximum trunk strength testing as well as for perturbation compensation (p &gt; 0.05). It is concluded, that future interventions should exceed 6 weeks duration with at least 2 sessions per week to induce enhanced trunk strength or compensatory response to sudden, high-intensity trunk loading in already highly trained adolescent athletes, regardless of training regime.

Evaluating stability of human spine in static tasks: a combined in vivo-computational study

Various interpretations and parameters have been proposed to assess spinal stability such as antagonist muscle coactivity, trunk stiffness and spinal buckling load; however, the correlation between these parameters remains unknown. We evaluated spinal stability during different tasks while changing the external moment and load height and investigated likely relationships between different EMG- and model-based parameters (e.g., EMG coactivity ratio, trunk stiffness, force coactivity ratio) and stability margins. EMG and kinematics of 40 young healthy subjects were recorded during various quasi-static tasks. Muscle forces, trunk stiffness and stability margins were calculated by a nonlinear subject-specific EMG-assisted-optimization musculoskeletal model of the trunk. The load elevation and external moment increased muscle activities and trunk stiffness while all stability margins (i.e., buckling loads) decreased. The force coactivity ratio was strongly correlated with the hand-load stability margin (i.e., additional weight in hands to initiate instability; R² = 0.54) demonstrating the stabilizing role of abdominal muscles. The total trunk stiffness (Pearson's r = 0.96) and the sum of EMGs of back muscles (Pearson's r = 0.65) contributed the most to the T1 stability margin (i.e., additional required load at T1 for instability/buckling). Force coactivity ratio and trunk stiffness can be used as alternative spinal stability metrics.

Spinal movement variability associated with low back pain: A scoping review

OBJECTIVE

To identify suggestions for future research on spinal movement variability (SMV) in individuals with low back pain (LBP) by investigating (1) the methodologies and statistical tools used to assess SMV; (2) characteristics that influence the direction of change in SMV; (3) the methodological quality and potential biases in the published studies; and (4) strategies for optimizing SMV in LBP patients.

METHODS

We searched literature databases (CENTRAL, Medline, PubMed, Embase, and CINAHL) and comprehensively reviewed the relevant papers up to 5 May 2020. Eligibility criteria included studies investigating SMV in LBP subjects by measuring trunk angle using motion capture devices during voluntary repeated trunk movements in any plane. The Newcastle-Ottawa risk of bias tool was used for data quality assessment. Results were reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews.

RESULTS

Eighteen studies were included: 14 cross-sectional and 4 prospective studies. Seven linear and non-linear statistical tools were used. Common movement tasks included trunk forward bending and backward return, and object lifting. Study results on SMV changes associated with LBP were inconsistent. Two of the three interventional studies reported changes in SMV, one of which was a randomized controlled trial (RCT) involving neuromuscular exercise interventions. Many studies did not account for the potential risk of selection bias in the LBP population.

CONCLUSION

Designers of future studies should recognize that each of the two types of statistical tools assesses functionally different aspects of SMV. Future studies should also consider dividing participants into subgroups according to LBP characteristics, as three potential subgroups with different SMV characteristics were proposed in our study. Different task demands also produced different effects. We found preliminary evidence in a RCT that neuromuscular exercises could modify SMV, suggesting a rationale for well-designed RCTs involving neuromuscular exercise interventions in future studies.

Novel assessment of the variation in cervical inter-vertebral motor control in a healthy pain-free population

Spinal control at intervertebral levels is dependent on interactions between the active, passive and neural control elements. However, this has never been quantifiable, and has therefore been outside the reach of clinical assessments and research. This study used fluoroscopy during repeated unconstrained flexion and return neck movements to calculate intersegmental motor control (MC), defined as the difference and variation in repeated continuous angular motion from its average path. The study aimed to determine control values for MC at individual levels and its variability. Twenty male volunteers aged 19–29 received fluoroscopic screening of their cervical spines during 4 repetitions of neutral to full flexion and return motion. Moving vertebral images from C0–C1 to C6–C7 were tracked using cross-correlation codes written in Matlab. MC for each level was defined as the mean of the absolute differences between each repetition’s angular path and their mean and its variability as represented by the SD. 1-way ANOVA and Tukey multiple comparisons were used to identify significant contrasts between levels. The mean MC differences and SDs were highest at C1-2, suggesting that this level has the least control and the most variability. Results at this level alone were highly significant (F-ratio 10.88 and 9.79 P < 0.0001). Significant contrasts were only found between C1-C2 and all other levels. The mean MC difference for summed C1-6 levels was 3.4° (0.7–6.1). This study is the first to quantify intervertebral MC in the cervical spine in asymptomatic people. Studies of neck pain patients are now merited.

Neuromuscular organisation and robustness of postural control in the presence of perturbations

Perturbation-based exercise interventions challenge balance and improve reactive motor control. Our purpose was to investigate the modular organisation during a standing balance task in both stable and unstable conditions to provide new insights into the neuromuscular control mechanisms needed to cope with perturbations. Fifteen participants performed 54 cycles of a specific task (i.e. pass from a double- to a single-leg standing) on stable ground and an unstable oscillating platform (Posturomed). Muscle synergies were extracted from the electromyographic activity of thirteen lower limb muscles. The maximum Lyapunov exponents of different body segments were calculated using kinematic data. We found two synergies functionally associated with the single- and double-leg stance in both stable and unstable conditions. Nonetheless, in the unstable condition participants needed an extra muscle synergy also functionally related to the single stance. Although a simple organisation of the neuromuscular system was sufficient to maintain the postural control in both conditions, the increased challenge in the oscillating platform was solved by adding one extra synergy. The addition of a new synergy with complementary function highlighted an increased motor output's robustness (i.e. ability to cope with errors) in the presence of perturbations.

Anterior Cruciate Ligament Injury Mechanisms and the Kinetic Chain Linkage: The Effect of Proximal Joint Stiffness on Distal Knee Control During Bilateral Landings

BACKGROUND

Neuromuscular deficits at the trunk and hip may contribute to dynamic knee valgus and anterior cruciate ligament injury mechanisms. However, comprehensive examination of neuromuscular patterns and their mechanical influence is lacking.

OBJECTIVES

To investigate the influence of lumbar spine joint rotational stiffness (JRS) and the gluteal musculature contribution to hip JRS on dynamic knee valgus.

METHODS

In this cross-sectional study, 18 university-aged women completed a drop vertical jump while we measured kinematics, kinetics, and 24 channels of electromyography (EMG) spanning the trunk and hip musculature. We classified each limb as high or low valgus, based on frontal plane knee displacement magnitude. We used anatomically detailed, EMG-driven biomechanical models to quantify lumbar spine JRS and muscle contributions to hip JRS.

RESULTS

Low-valgus limbs generated greater gluteus medius frontal JRS (P = .002; effect size, 1.3) and gluteus maximus transverse JRS (P = .003; effect size, 1.2) compared to high-valgus limbs. Participants with bilateral high-valgus collapse had substantially reduced lumbar spine sagittal JRS compared to the group with low valgus on both limbs (P = .05; effect size, 5.1). Those with low valgus on both limbs also had a peak lumbar spine flexion angle of 24° ± 4°, compared to the bilateral high-valgus group's angle of 38° ± 10° (P = .09; effect size, 1.8).

CONCLUSION

Participants who avoided high medial knee displacement had greater proximal JRS. Increased JRS at the lumbar spine and greater JRS contributions from the gluteal musculature are linked with preventing high medial knee displacement. J Orthop Sports Phys Ther 2019;49(8):601-610. Epub 26 May 2019. doi:10.2519/jospt.2019.8248.

Movement variability in adults with low back pain during sit-to-stand-to-sit

BACKGROUND

Differences in movement variability may be related to a guarded response to pain or a less robust movement pattern, indicating a potential dysfunction in motor control. The study objective was to compare patterns of lumbo-pelvic coordinative variability, during repeated sit-to-stand-to-sit, in individuals with low back pain and healthy adults.

METHODS

Participants were adults with low back pain (n = 16) and healthy controls (n = 21). Kinematics for the T12-L3, L3-S1, and hip segments were measured using electromagnetic motion capture during 10 sit-to-stand-to-sit trials. Continuous relative phase analysis using the Hilbert transform method determined coordination and variability of the Hip-L3S1, and L3S1-T12L3 segments, deconstructed into 4 periods (start/up/down/end). T-tests compared coordination and variability of the full task between groups, and a mixed ANOVA compared the effects of group and period for the two segments.

FINDINGS

Across the full task, the low back pain group demonstrated more variable (mean difference = -6.95, 95% CI = -12.3 to -1.59) and greater out-of-phase behavior (mean difference = -22.6, 95% CI = -39.1 to -6.03) in the LHip-L3S1 segment. Group-period interaction effects revealed greater variability in the start period (mean difference = -0.325, 95% CI = -0.493 to -0.156) and more out-of-phase behavior in the start (mean difference = -0.350, 95% CI = -0.549 to -0.150) and end (mean difference = -0.354, 95% CI = -0.602 to -0.105) periods for the LHip-L3S1 segment.

INTERPRETATION

Excessive variability may relate to reports of poor spinal proprioception in low back pain; however, based on our sample characteristics (low pain and disability) and lack of symptoms during the task, classifying our findings as dysfunctional may not be fully warranted.

Relationships between muscle electrical activity and the control of inter-vertebral motion during a forward bending task

Muscle strengthening exercises are commonly used in primary care for the treatment of chronic, non-specific low back pain (CNSLBP) as it has been theorised that increased muscle activity contributes to the stabilisation of inter-vertebral motion segments during bending and other spinal movements, however this has never been demonstrated in vivo. This study used contemporaneous quantitative fluoroscopy (QF) and surface electromyography (sEMG) to investigate relationships between continuous inter-vertebral motion variables and muscle electrical activity in the lumbar multifidus (LMU), lumbar and thoracic erector spinae (LES and TES) during standardised lumbar flexion and return in 18 healthy male human subjects. Our results demonstrated that the variability in the sharing of angular motion (i.e. Motion Share Variability MSV) and motion segment laxity during a bending task were significantly (p < 0.05) negatively correlated (Spearman) with muscle electrical activity throughout the participant bend for both locally and globally acting muscle groups. MSV was also strongly correlated with L2-3 laxity. The former suggests a damping mechanism reducing irregular displacements (i.e. less variability in the sharing of segmental motion) during bending and an action of spinal stabilisation by muscles at segmental levels, and the latter a synergy between laxity at L2-3 and MSV. While this has previously been theorised, it has never been shown in vivo at the inter-vertebral level. These assessments may be considered for use in validation studies of exercise programs for CNSLBP, however further replication is required.

Assessing delay and lag in sagittal trunk control using a tracking task

Slower trunk muscle responses are linked to back pain and injury. Unfortunately, clinical assessments of spine function do not objectively evaluate this important attribute, which reflects speed of trunk control. Speed of trunk control can be parsed into two components: (1) delay, the time it takes to initiate a movement, and (2) lag, the time it takes to execute a movement once initiated. The goal of this study is to demonstrate a new approach to assess delay and lag in trunk control using a simple tracking task. Ten healthy subjects performed four blocks of six trials of trunk tracking in the sagittal plane. Delay and lag were estimated by modeling trunk control for predictable and unpredictable (control mode) trunk movements in flexion and extension (control direction) at movement amplitudes of 2°, 4°, and 6° (control amplitude). The main effect of control mode, direction, and amplitude of movement were compared between trial blocks to assess secondary influencers (e.g., fatigue). Only control mode was consistent across trial blocks with predictable movements being faster than unpredictable for both delay and lag. Control direction and amplitude effects on delay and lag were consistent across the first two trial blocks and less consistent in later blocks. Given the heterogeneity in the presentation of back pain, clinical assessment of trunk control should include different control modes, directions, and amplitudes. To reduce testing time and the influence of fatigue, we recommend six trials to assess trunk control.

Do Older Adults and Those Recovered from Low Back Injury Share Common Muscle Activation Adaptations?

Theoretical models suggest trunk muscle activation compensates for spinal systems impairments. The purpose of this study was to determine if two populations (older adults and those recovered from a lower back injury (rLBI)) with spinal system impairments have similar muscle activation patterns to each other, but differ from controls. Trunk electromyograms collected from 12 older adults, 16 rLBI, and 19 controls during two dynamic tasks showed that older adults and rLBI had higher activation amplitudes, sustained temporal and more synergistic activation relative to controls. However, differences found between older adults and rLBI suggest that spinal system impairments differed between groups or that recent pain (rLBI) uniquely influenced muscle activation. This sheds light on our understanding of the relationship between spinal system impairments and muscle activation.

Multi-objective analysis for assessing simultaneous changes in regional spinal curvatures under backpack carriage in young adults

Change in sagittal spinal curvature from the neutral upright stance is an important measure of the heaviness and correctness of backpack use. As current recommendations, with respect to spinal profile, of backpack load thresholds were based on the significant curvature change in individual spinal region only, this study investigated the most critical backpack load by assessing simultaneously the spinal curvature changes along the whole spine. A motion analysis system was used to measure the curvature changes in cervical, upper thoracic, lower thoracic and lumbar regions with backpack load at 0, 5, 10, 15 and 20% of body weight. A multi-objective goal programming model was adopted to determine the global critical load of maximum curvature change of the whole spine in accordance with the maximum curvature changes of the four spinal regions. Results suggested that the most critical backpack load was 13% of body weight for healthy male college students. Practitioner Summary: As current recommendations of backpack load thresholds were based on the significant curvature change in individual spinal region only, this study investigated the backpack load by considering simultaneously the spinal curvature changes along the whole spine. The recommendation, in terms of the global critical load, was 13% of body weight for healthy male college students.

Is There a Relationship Between Lumbar Proprioception and Low Back Pain? A Systematic Review With Meta-Analysis

OBJECTIVE

To systematically review the relationship between lumbar proprioception and low back pain (LBP).

DATA SOURCES

Four electronic databases (PubMed, EMBASE, CINAHL, SPORTDiscus) and reference lists of relevant articles were searched from inception to March-April 2014.

STUDY SELECTION

Studies compared lumbar proprioception in patients with LBP with controls or prospectively evaluated the relationship between proprioception and LBP. Two reviewers independently screened articles and determined inclusion through consensus.

DATA EXTRACTION

Data extraction and methodologic quality assessment were independently performed using standardized checklists.

DATA SYNTHESIS

Twenty-two studies (1203 participants) were included. Studies measured lumbar proprioception via active or passive joint repositioning sense (JRS) or threshold to detection of passive motion (TTDPM). Data from 17 studies were pooled for meta-analyses to compare patients with controls. Otherwise, descriptive syntheses were performed. Data were analyzed according to measurement method and LBP subgroup. Active JRS was worse in patients compared with controls when measured in sitting (standard mean difference, .97; 95% confidence interval [CI], .31-1.64). There were no differences between groups measured via active JRS in standing (standard mean difference, .41; 95% CI, -.07 to .89) or passive JRS in sitting (standard mean difference, .38; 95% CI, -.83 to 1.58). Patients in the O'Sullivan flexion impairment subgroup had worse proprioception than the total LBP cohort. The TTDPM was significantly worse in patients than controls. One prospective study found no link between lumbar proprioception and LBP.

CONCLUSIONS

Patients with LBP have impaired lumbar proprioception compared with controls when measured actively in sitting positions (particularly those in the O'Sullivan flexion impairment subgroup) or via TTDPM. Clinicians should consider the relationship between sitting and proprioception in LBP and subgroup patients to guide management. Further studies focusing on subgroups, longitudinal assessment, and improving proprioception measurement are needed.

A new method for sudden mechanical perturbation with axial load, to assess postural control in sitting and standing

Sudden application of load along a sagittal or coronal axis has been used to study trunk stiffness, but not axial (vertical) load. This study introduces a new method for sudden-release axial load perturbation. Prima facie validity was supported by comparison with standard mechanical systems. We report the response of the human body to axial perturbation in sitting and standing and within-day repeatability of measures. Load of 20% of body weight was released from light contact onto the shoulders of 22 healthy participants (10 males). Force input was measured via force transducers at shoulders, output via a force plate below the participant, and kinematics via 3-D motion capture. System identification was used to fit data from the time of load release to time of peak load-displacement, fitting with a 2nd-order mass-spring-damper system with a delay term. At peak load-displacement, the mean (SD) effective stiffness measured with this device for participants in sitting was 12.0(3.4)N/mm, and in standing was 13.3(4.2)N/mm. Peak force output exceeded input by 44.8 (10.0)% in sitting and by 30.4(7.9)% in standing. Intra-class correlation coefficients for within-day repeatability of axial stiffness were 0.58 (CI: -0.03 to 0.83) in sitting and 0.82(0.57-0.93) in standing. Despite greater degrees of freedom in standing than sitting, standing involved lesser time, downward displacement, peak output force and was more repeatable in defending upright postural control against the same axial loads. This method provides a foundation for future studies of neuromuscular control with axial perturbation.

Trunk Dynamics Are Impaired in Ballet Dancers with Back Pain but Improve with Imagery

PURPOSE

Trunk control is essential in ballet and may be compromised in dancers with a history of low back pain (LBP) by associated changes in motor control. This study aimed to compare trunk mechanical properties between professional ballet dancers with and without a history of LBP. As a secondary aim, we assessed whether asking dancers to use motor imagery to respond in a "fluid" manner could change the mechanical properties of the trunk and whether this was possible for both groups.

METHODS

Trunk mechanical properties of stiffness and damping were estimated with a linear second-order system, from trunk movement in response to perturbations, in professional ballet dancers with (n = 22) and without (n = 8) a history of LBP. The second-order model adequately described trunk movement in response to the perturbations. Trials were performed with and without motor imagery to respond in a fluid manner to the perturbation.

RESULTS

Dancers with a history of LBP had lower damping than dancers without LBP during the standard condition (P = 0.002) but had greater damping during the "fluid" condition (P < 0.001), with values similar to dancers without LBP (P = 0.226). Damping in the dancers without LBP was similar between the conditions (P > 0.99). Stiffness was not different between the dancers with and those without a history of LBP (P = 0.252) but was less during the fluid condition than the standard condition (P < 0.001).

CONCLUSION

Although dancers with a history of LBP have less trunk damping than those without LBP, they have the capacity to modulate the trunk's mechanical properties to match that of pain-free dancers by increasing damping with motor imagery. These observations have potential relevance for LBP recurrence and rehabilitation.

Less precise motor control leads to increased agonist-antagonist muscle activation during stick balancing

Human motor control has constraints in terms of its responsiveness, which limit its ability to successfully perform tasks. In a previous study, it was shown that the ability to balance an upright stick became progressively more challenging as the natural frequency (angular velocity without control) of the stick increased. Furthermore, forearm and trunk agonist and antagonist muscle activation increased as the natural frequency of the stick increased, providing evidence that the central nervous system produces agonist-antagonist muscle activation to match task dynamics. In the present study, visual feedback of the stick position was influenced by changing where subject focused on the stick during stick balancing. It was hypothesized that a lower focal height would degrade motor control (more uncertainty in tracking stick position), thus making balancing more challenging. The probability of successfully balancing the stick at four different focal heights was determined along with the average angular velocity of the stick. Electromyographic signals from forearm and trunk muscles were also recorded. As expected, the probability of successfully balancing the stick decreased and the average angular velocity of the stick increased as subjects focused lower on the stick. In addition, changes in the level of agonist and antagonist muscle activation in the forearm and trunk was linearly related to changes in the angular velocity of the stick during balancing. One possible explanation for this is that the central nervous system increases muscle activation to account for less precise motor control, possibly to improve the responsiveness of human motor control.

The influence of simulated transversus abdominis muscle force on sacroiliac joint flexibility during asymmetric moment application to the pelvis

BACKGROUND

The role of so-called local muscle system in motor control of the lower back and pelvis is a subject of ongoing debate. Prevailing beliefs in stabilizing function of this system were recently challenged. This study investigated the impact of in vitro simulated force of transversely oriented fibres of the transversus abdominis muscle (a part of the local system) on flexibility of the sacroiliac joint during asymmetric moment application to the pelvis.

METHODS

In 8 embalmed specimens an incremental moment was applied in the sagittal plane to one innominate with respect to the fixed contralateral innominate. Ranges of motion of the sacroiliac joint were recorded using the Vicon Motion Capture System. Load-deformation curves were plotted and flexibility of the sacroiliac joint was calculated separately for anterior and posterior rotations of the innominate, with and without simulated muscle force.

FINDINGS

Flexibility of the sacroiliac joint was significantly bigger during anterior rotation of the innominate, as compared to posterior rotation (Anova P<0.05). After application of simulated force of transversus abdominis, flexibility of the joint did not change both during anterior and posterior rotations of the innominate.

INTERPRETATION

A lack of a stiffening effect of simulated transversus abdominis force on the sacroiliac joint was demonstrated. Earlier hypotheses suggesting a stiffening influence of this muscle on the pelvis cannot be confirmed. Consistent with previous findings smaller flexibility of the joint recorded during posterior rotation of the innominate may be of clinical importance for physio- and manual therapists. However, major limitations of the study should be acknowledged: in vitro conditions and simulation of only solitary muscle force.

Coupling relationship between the central pattern generator and the cerebral cortex with time delay

Brain activity is a cooperative process among neurons and involves the coupling relationship, which is crucial to perform operational tasks in various specialized areas of the nervous system. A finite signal transmission speed along the axons results in a space-dependent time delay. The central pattern generator (CPG) can in principle produce basic locomotor rhythm in the absence of inputs from higher brain centers and peripheral sensory feedback. To study the dynamic performance of CPG with time delay and its coupling relationship with the cerebral cortex, a new CPG model with time delay and a model of the neural mass model (NMM) and the CPG are developed. The coupling model is based on biological experimental results. Bifurcation theories and maximal Lyapunov exponent are used to analyze the dynamic performance. From the results, some CPGs are suggested to be embedded in limbs and composed of the parameters space which corresponds to the one of the cerebral cortex. This embodiment of humans can reduce the burden of the brain and simplify the control of the locomotion. The results also show that the phase diagram of the CPG cannot keep the limit cycle, and that the state of the NMM becomes increasingly chaotic as time delay increases. This finding implies that a person with slow reaction can easily lose the stability of his or her locomotion.

Task-specificity of bilateral anticipatory activation of the deep abdominal muscles in healthy and chronic low back pain populations

STUDY DESIGN

Cross-sectional study of lumbopelvic muscle activation during rapid limb movements in chronic low back pain (CLBP) patients and healthy controls.

INTRODUCTION

Controversy exists over whether bilateral anticipatory activation of the deep abdominal muscles represents a normal motor control strategy prior to all rapid limb movements, or if this is simply a task-specific strategy appropriate for only certain movement conditions.

OBJECTIVE

To assess the onset timing of the transversus abdominis/internal oblique muscles (TrA/IO) during two rapid limb movement tasks with different postural demands - bilateral shoulder flexion in standing, unilateral hip extension in prone lying - as well as differences between CLBP and controls.

METHODS

Twelve CLBP and 13 controls performed the two tasks in response to an auditory cue. Surface EMG was acquired bilaterally from five muscles, including TrA/IO.

RESULTS

In both groups, 50% of bilateral shoulder flexion trials showed bilateral anticipatory TrA/IO activation. This was rare, however, in unilateral hip extension for which only the TrA/IO contralateral to the moving leg showed anticipatory activation. The only significant difference in lumbo-pelvic muscle onset timing between CLBP and controls was a delay in semitendinosus activation during bilateral shoulder flexion in standing.

CONCLUSION

Our data suggest that bilateral anticipatory TrA/IO activation is a task-specific motor control strategy, appropriate for only certain rapid limb movement conditions. Furthermore, the presence of altered semitendinosus onset timing in the CLBP group during bilateral shoulder flexion may be reflective of other possible lumbo-pelvic motor control alterations among this population.

Limits in motor control bandwidth during stick balancing

Why can we balance a yardstick but not a pencil on the tip of our finger? As with other physical systems, human motor control has constraints, referred to as bandwidth, which restricts the range of frequency over which the system can operate within some tolerated level of error. To investigate control bandwidth, the natural frequency of a stick used during a stick-balancing task was modified by adjusting the height of a mass attached to the stick. The ability to successfully balance the stick with the mass positioned at four different heights was determined. In addition, electromyographic signals from forearm and trunk muscles were recorded during the trials. We hypothesized that 1) the probability of successfully balancing would decrease as mass height decreased; and 2) the level of muscle activation in both agonist and antagonist would increase as the natural frequency of the stick increased. Results showed that as the mass height decreased the probability of successfully balancing the stick decreased. Changes in the probability of success with respect to mass height showed a threshold effect, suggesting that limits in human control bandwidth were approached at the lowest mass height. Also, the level of muscle activation in both the agonist and antagonist of the forearm and trunk increased linearly as the natural frequency of the stick increased. These changes in muscle activation suggest that the central nervous system adapts muscle activation to task dynamics, possibly to improve control bandwidth.

Learning about gravity: segmental assessment of upright control as infants develop independent sitting

The question of how infants attain upright sitting is at the core of understanding the development of most functional abilities. Our simple, practical method of securing the hips and different trunk segments while evaluating the infant's ability to vertically align and stabilize the trunk in space contributes a useful method and new insights into the development of upright control. Previous studies have considered the trunk to develop as a single segment. The goal of the present study was to examine how postural control changes across multiple trunk segments during typical development (TD) of sitting balance. For this purpose, electromyography (EMG) and kinematic data were collected at four levels of trunk support (axillae, midribs, waist, hips), in a longitudinal study of eight TD infants (3-9 mo of age). We found that developmental changes in stability were specific to the region of the trunk being investigated, changes in antagonistic muscle activity differed for the anterior-posterior versus the medial-lateral axis, and the relationship between muscle activation and movement changed from erratic attempts to gain upright position to anticipatory graded responses as infants developed upright control through a four-stage behavioral process. This information can be used by researchers to further refine hypotheses regarding this developmental process and by clinicians who wish to develop and test more specific treatment programs for children with postural dysfunction.

Frontal plane hip and ankle sensorimotor function, not age, predicts unipedal stance time

INTRODUCTION

Changes occur in muscles and nerves with aging. In this study we explore the relationship between unipedal stance time (UST) and frontal plane hip and ankle sensorimotor function in subjects with diabetic neuropathy.

METHODS

UST, quantitative measures of frontal plane ankle proprioceptive thresholds, and ankle and hip motor function were tested in 41 subjects with a spectrum of lower limb sensorimotor function ranging from healthy to moderately severe diabetic neuropathy.

RESULTS

Frontal plane hip and ankle sensorimotor function demonstrated significant relationships with UST. Multivariate analysis identified only composite hip strength, ankle proprioceptive threshold, and age to be significant predictors of UST (R(2) = 0.73), explaining 46%, 24%, and 3% of the variance, respectively.

CONCLUSIONS

Frontal plane hip strength was the single best predictor of UST and appeared to compensate for less precise ankle proprioceptive thresholds. This finding is clinically relevant given the possibility of strengthening the hip, even in patients with significant peripheral neuropathy.