Identification of intrinsic and reflexive contributions to trunk stabilization in patients with low back pain: a case-control study.

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. These slides can be retrieved under Electronic Supplementary Material.

Modulation of intrinsic and reflexive contributions to low-back stabilization due to vision, task instruction, and perturbation bandwidth.

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.

Why do drivers maintain short headways in fog? A driving-simulator study evaluating feeling of risk and lateral control during automated and manual car following.

Drivers in fog tend to maintain short headways, but the reasons behind this phenomenon are not well understood. This study evaluated the effect of headway on lateral control and feeling of risk in both foggy and clear conditions. Twenty-seven participants completed four sessions in a driving simulator: clear automated (CA), clear manual (CM), fog automated (FA) and fog manual (FM). In CM and FM, the drivers used the steering wheel, throttle and brake pedals. In CA and FA, a controller regulated the distance to the lead car, and the driver only had to steer. Drivers indicated how much risk they felt on a touchscreen. Consistent with our hypothesis, feeling of risk and steering activity were elevated when the lead car was not visible. These results might explain why drivers adopt short headways in fog. Practitioner Summary: Fog poses a serious road safety hazard. Our driving-simulator study provides the first experimental evidence to explain the role of risk-feeling and lateral control in headway reduction. These results are valuable for devising effective driver assistance and support systems.

Dystonic neck muscles show a shift in relative autospectral power during isometric contractions.

OBJECTIVE: To identify effects of a deviant motor drive in the autospectral power of dystonic muscles during voluntary contraction in cervical dystonia patients. METHODS: Submaximal (20%) isometric head-neck tasks were performed with the head fixed, measuring surface EMG of the sternocleidomastoid, splenius capitis and semispinalis capitis in CD patients and controls. Autospectral power of muscle activity, and head forces was analyzed using cumulative distribution functions (CDF). A downward shift between the theta/low alpha-band (3-10Hz) and the high alpha/beta-band (10-30Hz) was detected using the CDF10, defined as the cumulative power from 3 to 10Hz relative to power from 3 to 30Hz. RESULTS: CDF10 was increased in dystonic muscles compared to controls and patient muscles unaffected by dystonia, due to a 3-10Hz power increase and a 10-30Hz decrease. CDF10 also increased in patient head forces. CONCLUSIONS: Submaximal isometric contractions with the head fixed provided a well-defined test condition minimizing effects of reflexive feedback and tremor. We associate shifts in autospectral power with prokinetic sensorimotor control. SIGNIFICANCE: Analysis of autospectral power in isometric tasks with the head fixed is a promising approach in research and diagnostics of cervical dystonia.

Spectral EMG Changes in Cervical Dystonia Patients and the Influence of Botulinum Toxin Treatment.

Botulinum toxin (BoNT) injections in the dystonic muscles is the preferred treatment for Cervical Dystonia (CD), but the proper identification of the dystonic muscles remains a challenge. Previous studies showed decreased 8-14 Hz autospectral power in the electromyography (EMG) of splenius muscles in CD patients. Cumulative distribution functions (CDF's) of dystonic muscles showed increased CDF10 values, representing increased autospectral powers between 3 and 10 Hz, relative to power between 3 and 32 Hz. In this study, we evaluated both methods and investigated the effects of botulinum toxin. Intramuscular EMG recordings were obtained from the splenius, semispinalis, and sternocleidomastoid muscles during standardized isometric tasks in 4 BoNT-naïve CD patients, 12 BoNT-treated patients, and 8 healthy controls. BoNT-treated patients were measured 4-7 weeks after their last BoNT injections and again after 11-15 weeks. We found significantly decreased 8-14 Hz autospectral power in splenius muscles, but not in the semispinalis and sternocleidomastoid muscles of CD patients when compared to healthy controls. CDF10 analysis was superior in demonstrating subtle autospectral changes, and showed increased CDF10 values in all studied muscles of CD patients. These results did not change significantly after BoNT injections. Further studies are needed to investigate the origin of these autospectral changes in dystonia patients, and to assess their potential in muscle selection for BoNT treatment.

Improved identification of dystonic cervical muscles via abnormal muscle activity during isometric contractions.

BACKGROUND: The preferred treatment for cervical dystonia (CD) is injection of botulinum toxin in the dystonic muscles. Unfortunately, in the absence of reliable diagnostic methods it can be difficult to discriminate dystonic muscles from healthy muscles acting in compensation. We investigated if dystonic muscle activation patterns could be identified in cervical dystonia patients during a harmonized isometric contraction task. Furthermore, we investigated whether dystonia worsens at higher levels of voluntary contraction, which might further improve the identification of dystonic muscle activity. METHODS: An isometric device was used to investigate muscle activation during voluntary contraction tasks in 10 controls and 10 CD patients. Surface electromyography (EMG) of the sternocleidomastoidus, splenius capitis, and semispinalis capitis muscles was evaluated during a rest task and when performing submaximal (20%) and maximal voluntary contractions for eight head transversal force directions and for head twist. Two measures were developed to identify dystonic activation: 1) Muscle activity in the contraction direction in which the contribution of the muscle was lowest (Minimum EMG), and 2) the average muscle activity over all contraction directions (Total Mean EMG). RESULTS: Patients showed increased dystonic activity in the rest task and during submaximal contractions relative to controls, but not during maximal contractions. Increases in Minimum EMG indicated an inability of patients to deactivate dystonic muscles counteracting the task. Increases in Total Mean EMG indicated dystonic activity in all task directions. During maximal contractions these effects were absent in dystonic muscles. Dystonia is therefore found not to worsen at higher levels of isometric voluntary contraction. The activity of dystonic muscles modulated with different loading directions similar to controls. Using Minimum EMG 54% of the muscles clinically diagnosed as dystonic and 91% of non-dystonic muscles were predicted correctly. CONCLUSIONS: Dystonic muscle activity was found in cervical dystonia patients during submaximal contractions in all task directions using a harmonized isometric task, but no differences were found during maximal contractions. With some adaptation this method may prove useful to identify dystonic muscles.

Inverse dynamic optimization including muscular dynamics, a new simulation method applied to goal directed movements.

This paper presents a new method for estimating muscular force and activation from experimental kinematic data. The method combines conventional inverse dynamics with optimization utilizing a dynamic muscle model. The method uses only very limited computational power, which makes it a useful tool especially for complex systems like the shoulder or the locomotor system. The net torques/forces are calculated by using conventional inverse dynamics. A solution of the load sharing problem is determined by minimization of the weighted sum of squared muscle forces. The load sharing problem is solved with a dynamic constraint reflecting physiological muscle properties. This constraint takes into account the nonlinear dynamics of the contractile element (CE) and the series elastic element (SE), active state dynamics and neural excitation dynamics. This physiological constraint is determined with an inverse muscle model. With this model, muscular states and neural inputs are also estimated. The method of inverse dynamics requires position, velocity and acceleration signals as input. A method to prepare such signals from noisy measured data is presented.

The control of shoulder muscles during goal directed movements, an inverse dynamic analysis.

Fast goal directed arm movements in the sagittal plane were analyzed with a three-dimensional shoulder model with 95 muscle elements. Dynamics of the muscle elements were described by a third-order nonlinear muscle model. Muscle forces and activation were estimated using the method of inverse muscular dynamics, an optimization scheme which uses only very limited computational power. Most model results were similar to the EMG but some differences between model results and EMG were found in muscles where the EMG activity was subject dependent. For the movement studied, the thoracoscapular muscles were shown to deliver about 40% of the energy required for the acceleration of the arm during anteflexion and about 22% during retroflexion. Activity of thoracoscapular muscles was also required to ensure contact between the thorax and the scapula which is important for the mechanical stability of the shoulder. The rotator cuff muscles were found to deliver about 19% of the energy required for the acceleration of the arm during anteflexion and about 8% during retroflexion.

Goal-directed arm movements: I. Analysis of EMG records in shoulder and elbow muscles.

The coordinated control of shoulder and elbow muscles during goaldirected arm movements has been studied. Timing and amplitude of the electromyographic activity of 13 muscles and muscle parts have been analyzed. Triphasic alternating agonist-antagonist activity commonly associated with acceleration and deceleration of the limb has been found in eight shoulder muscles or muscle parts. Of these muscles, the pectoralis major (pars clavicularis), the deltoideus (pars anterior and posterior), and the latissimus dorsi act on the humerus and can be regarded as prime movers of the upper arm. The serratus anterior and the trapezius (pars descendens, transversalis, and ascendens) act on the scapula. In these scapular muscles, agonist activity similar to that in the prime movers has been found. This indicates an important role of scapular muscles during acceleration of the arm. Significant diferences in timing between synergist muscles have been demonstrated: Activity of the latissimus dorsi precedes the deltoideus pars posterior by up to 62 ms. Only minor differences in timing were observed between scapular muscles and prime movers. In biarticular (shoulder and elbow) muscles and in muscles acting only on the elbow, continued activity has been observed throughout the movement. This activity yielded coactivation of functional antagonist muscles. The movement direction in which the largest activity occurred is consistent with the function of overcoming a resistance in the elbow resulting from the cocontraction found.

Goal-directed arm movements. II: A kinematic model and its relation to EMG records.

In a previous study the EMG records of shoulder and elbow muscles during goal-directed arm movements were discussed. Here an analysis of the kinematic signals is presented and the relation between EMG and kinematic signals is assessed in a correlation analysis. The displacement as a function of time (movement trace) is analyzed. The movement trace is described by a model with five parameters that are estimated using a least squares criterion. Four parameters describe the timing of a triphasic muscular input and a fifth parameter describes neuromuscular dynamics. The parameters provide a means to compare the shape of movement traces recorded under different experimental conditions. After scaling, the shapes of movement traces with different step sizes and velocities/durations can be compared. The scaled parameters reveal a significant dependence on the movement direction of maximally fast movements. Furthermore, the scaled parameters depend significantly on the maximum velocity obtained by the subjects. This dependence can be interpreted in terms of neural inputs and muscular dynamics, as the parameters have been defined in these terms. The parameters estimated from the kinematics have a high correlation with EMG timing. A further analysis of these correlations indicates that the movement obtained is closely coupled to activity of the four prime movers of the upper arm and of three out of four scapular muscles.

Goal-directed arm movements. III: Feedback and adaptation in response to inertia perturbations.

Goal-directed shoulder-elbow movements with a maximal and with a submaximal velocity have been studied. At the movement onset the inertial load to be displaced was changed unexpectedly. The adaptation of movement and muscular activity have been described with a moving average model. Significant adaptation effects were demonstrated in the first two or three movements after a change of mass. Adaptation only partly compensated the mass effects: A higher mass led to a persistent reduction of movement velocity. Amplitudes of muscular activity showed no adaptation of muscular effort, but activation durations were strongly modified. Thus the hypothesis that adaptation pursues a certain movement trajectory as a function of time had to be rejected. However, after scaling towards peak velocity, a shape invariance was demonstrated in the movement trajectory. In the first moyements after a change of mass, effective and substantial modifications of muscular activity appeared about 90 ms after movement onset. Earlier modifications suggest a force feedback leading to a yielding towards the disturbance instead of a compensation. Such force feedback may, however, increase system bandwidth as it will allow increased position/velocity feedback gains.

EMG coherence and spectral analysis in cervical dystonia: discriminative tools to identify dystonic muscles?

OBJECTIVE: Botulinum toxin injections in the dystonic muscles are the preferred treatment for cervical dystonia (CD), but proper selection of the dystonic muscles remains a challenge. We investigated the use of EMG coherence and autospectral analysis as discriminative tools to identify dystonic muscles in CD patients. METHODS: We compared the occurrence of 8-14 Hz autospectral peaks and 4-7 Hz intermuscular coherences between 10 CD patients and 10 healthy controls. Secondly, we compared the muscles with significant 4-7 Hz coherences with the muscles that were selected clinically for botulinum toxin treatment. RESULTS: Autospectral peaks between 8 and 14 Hz were significantly more often absent in the splenius capitis (SPL) muscles of CD patients compared to controls (p<0.01). Contrary to previous findings, there was no significant difference in the occurrence of 4-7 Hz intermuscular coherences between patients and controls and the diagnostic accuracy of coherence analysis to identify the clinically dystonic muscles was low. CONCLUSION: Intermuscular EMG coherence analysis cannot reliably discriminate patients from controls. Autospectral changes in the SPL muscles are a more discriminative feature of CD. In patients, coherence analysis does not seem to be a reliable method to identify dystonic muscles. The clinical relevance and the origin of the autospectral changes need further study.

Identifying intrinsic and reflexive contributions to low-back stabilization.

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.

Time optimality in the control of human movements.

In a simulation study the control of maximally fast goal directed movements has been analyzed. For a simple linear model it is shown that the presence of a third input block reduces the movement duration. The time optimal size of the third block depends on the ratio of a neuromuscular time constant (first-order lag) and movement time. As a second step a non-linear muscle model was simulated. By an optimization of input parameters it was found that the time optimal input, as expected, switches between maximal agonist and maximal antagonist activation. As for the linear model, a third phase was required for an optimal movement. It was found that the third phase serves to compensate the slowly decaying antagonist force. Also an input similar to experimentally found activation patterns was simulated. This input contains a silent period between the first two bursts and the second and the third burst have submaximal amplitudes. This input led to a near time optimal movement with a duration 9% larger than the minimal duration but with largely reduced muscle forces. This suggests that a criterion is minimized which also takes into account the effort spent. Including gravity in the model indicates optimality of a silent period between the third phase and a final agonist activity to resist gravity. When assuming different dynamics for agonist and antagonist, the optimal switch times for agonist and antagonist no longer coincide, also after the three block pattern some extra activity is required to obtain a cancellation of the slowly decaying force in agonist and antagonist.

The use of internal representation in fast gold-directed movements: a modeling approach.

This study investigates the role of the human central nervous system (CNS) in the control of fast goal-directed movements. The main problem is that the latencies inherent in the transmission of physiological signals cause a delayed feedback of sensory information. Therefore, the muscle command signals cannot be explained by a simple servo-loop, so a more sophisticated control structure is required. Our hypothesis is that the CNS employs an internal representation of the controlled system in order to circumvent the drawbacks of the physiological loop delay. To test this hypothesis a mathematical model based on an internal representation and an internal state feedback has been developed. Computer simulations of double-step stimuli (control behaviour), tendon vibration and torque disturbances (disturbance behaviour) and load perturbations (adaptation behaviour) proved to agree remarkably well with experimental observations. The proposed control model can explain the open-loop and closed-loop aspects of human motor control. Hence, the use of an internal representation in generating the muscle command signals is very plausible.