Limit cycle behavior in spasticity: analysis and evaluation

H-reflex conditioning during locomotion in people with spinal cord injury

KEY POINTS

In people or animals with incomplete spinal cord injury (SCI), changing a spinal reflex through an operant conditioning protocol can improve locomotion. All previous studies conditioned the reflex during steady-state maintenance of a specific posture. By contrast, the present study down-conditioned the reflex during the swing-phase of locomotion in people with hyperreflexia as a result of chronic incomplete SCI. The aim was to modify the functioning of the reflex in a specific phase of a dynamic movement. This novel swing-phase conditioning protocol decreased the reflex much faster and farther than did the steady-state protocol in people or animals with or without SCI, and it also improved locomotion. The reflex decrease persisted for at least 6 months after conditioning ended. The results suggest that conditioning reflex function in a specific phase of a dynamic movement offers a new approach to enhancing and/or accelerating recovery after SCI or in other disorders.

ABSTRACT

In animals and people with incomplete spinal cord injury, appropriate operant conditioning of a spinal reflex can improve impaired locomotion. In all previous conditioning studies, the reflex was conditioned during steady-state maintenance of a stable posture; this steady-state protocol aimed to change the excitability of the targeted reflex pathway; reflex size gradually changed over 8-10 weeks. The present study introduces a new protocol, comprising a dynamic protocol that aims to change the functioning of the reflex pathway during a specific phase of a complex movement. Specifically, we down-conditioned the soleus H-reflex during the swing-phase of locomotion in people with hyperreflexia as a result of chronic incomplete SCI. The swing-phase H-reflex, which is absent or very small in neurologically normal individuals, is abnormally large in this patient population. The results were clear. With swing-phase down-conditioning, the H-reflex decreased much faster and farther than did the H-reflex in all previous animal or human studies with the steady-state protocol, and the decrease persisted for at least 6 months after conditioning ended. The H-reflex decrease was accompanied by improvements in walking speed and in the modulation of locomotor electromyograph activity in proximal and distal muscles of both legs. These results provide new insight into the factors controlling spinal reflex conditioning; they suggest that the conditioning protocols targeting reflex function in a specific movement phase provide a promising new opportunity to enhance functional recovery after SCI or in other disorders.

Volume-constrained microcontainers enable myoepithelial functional differentiation in highly parallel mammary organoid culture

A long-standing constraint on organoid culture is the need to add exogenous substances to provide hydrogel matrix, which limits the study of fully human or fully native organoids. This paper introduces an approach to culture reconstituted mammary organoids without the impediment of exogenous matrix. We enclose organoids in nanoliter-scale, topologically enclosed, fluid compartments surrounded by agar. Organoids cultured in these “microcontainers” appear to secrete enough extracellular matrix to yield a self-sufficient microenvironment without exogenous supplements. In microcontainers, mammary organoids exhibit contractility and a high-level, physiological, myoepithelial (MEP) behavior that has not been previously reported in reconstituted organoids. The presence of contractility suggests that microcontainers elicit MEP functional differentiation, an important milestone. Microcontainers yield thousands of substantially identical and individually trackable organoids within a single culture vessel, enabling longitudinal studies and statistically powerful experiments, such as the evaluation of small effect sizes. Microcontainers open new doors for researchers who rely on organoid models.

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Microcontainers are volume-constrained microwells with hydrogel lids

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Microcontainers enable statistically robust experimental design with organoids

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Organoids produce their own extracellular matrix within microcontainers

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Myoepithelial cells in mammary organoids achieve fully functional differentiation

Temporal Indices of Ankle Clonus and Relationship to Electrophysiologic and Clinical Measures in Persons With Spinal Cord Injury

BACKGROUND AND PURPOSE

Clonus arising from plantar flexor hyperreflexia is a phenomenon that is commonly observed in persons with spastic hypertonia. We assessed the temporal components of a biomechanical measure to quantify ankle clonus, and validated these in persons with spasticity due to spinal cord injury.

METHODS

In 40 individuals with chronic (>1 year) spinal cord injury, we elicited ankle clonus using a standardized mechanical perturbation (drop test). We examined reliability and construct validity of 2 components of the drop test: clonus duration (timed with a stopwatch) and number of oscillations in the first 10-second interval (measured via optical motion capture). We compared these measures to the Spinal Cord Assessment Tool for Spastic reflexes (SCATS) clonus score and H-reflex/M-wave (H/M) ratio, a clinical and electrophysiologic measure, respectively.

RESULTS

Intra- and interrater reliability of clonus duration measurement was good [intraclass correlation coefficient, ICC (2, 1) = 1.00]; test-retest reliability was good both at 1 hour [ICC (2, 2) = 0.99] and at 1 week [ICC (2, 2) = 0.99]. Clonus duration was moderately correlated with SCATS clonus score (r = 0.58). Number of oscillations had good within-session test-retest reliability [ICC (2, 1) > 0.90] and strong correlations with SCATS clonus score (r = 0.86) and soleus H/M ratio (r = 0.77).

DISCUSSION AND CONCLUSIONS

Clonus duration and number of oscillations as measured with a standardized test are reliable and valid measures of plantar flexor hyperreflexia that are accessible for clinical use. Tools for objective measurement of ankle clonus are valuable for assessing effectiveness of interventions directed at normalizing reflex activity associated with spasticity.Video Abstract available for more insights from the authors (see Supplemental Digital Content 1, http://links.lww.com/JNPT/A179).

Orbital stability analysis in biomechanics: a systematic review of a nonlinear technique to detect instability of motor tasks

Falls represent a heavy economic and clinical burden on society. The identification of individual chronic characteristics associated with falling is of fundamental importance for the clinicians; in particular, the stability of daily motor tasks is one of the main factors that the clinicians look for during assessment procedures. Various methods for the assessment of stability in human movement are present in literature, and methods coming from stability analysis of nonlinear dynamic systems applied to biomechanics recently showed promise. One of these techniques is orbital stability analysis via Floquet multipliers. This method allows to measure orbital stability of periodic nonlinear dynamic systems and it seems a promising approach for the definition of a reliable motor stability index, taking into account for the whole task cycle dynamics. Despite the premises, its use in the assessment of fall risk has been deemed controversial. The aim of this systematic review was therefore to provide a critical evaluation of the literature on the topic of applications of orbital stability analysis in biomechanics, with particular focus to methodologic aspects. Four electronic databases have been searched for articles relative to the topic; 23 articles were selected for review. Quality of the studies present in literature has been assessed with a customised quality assessment tool. Overall quality of the literature in the field was found to be high. The most critical aspect was found to be the lack of uniformity in the implementation of the analysis to biomechanical time series, particularly in the choice of state space and number of cycles to include in the analysis.

Cutaneous inputs from the back abolish locomotor-like activity and reduce spastic-like activity in the adult cat following complete spinal cord injury

Spasticity is a condition that can include increased muscle tone, clonus, spasms, and hyperreflexia. In this study, we report the effect of manually stimulating the dorsal lumbosacral skin on spontaneous locomotor-like activity and on a variety of reflex responses in 5 decerebrate chronic spinal cats treated with clonidine. Cats were spinalized 1 month before the terminal experiment. Stretch reflexes were evoked by stretching the left triceps surae muscles. Crossed reflexes were elicited by electrically stimulating the right tibial or superficial peroneal nerves. Wind-up of reflex responses was evoked by electrically stimulating the left tibial or superficial peroneal nerves. We found that pinching the skin of the back abolished spontaneous locomotor-like activity. We also found that back pinch abolished the rhythmic activity observed during reflex testing without eliminating the reflex responses. Some of the rhythmic episodes of activity observed during reflex testing were consistent with clonus with an oscillation frequency greater than 3 Hz. Pinching the skin of the back effectively abolished rhythmic activity occurring spontaneously or evoked during reflex testing, irrespective of oscillation frequency. The results are consistent with the hypothesis that locomotion and clonus are produced by common central pattern-generators. Stimulating the skin of the back could prove helpful in managing undesired rhythmic activity in spinal cord-injured humans.

Characteristics of lower extremity clonus after human cervical spinal cord injury

Clonus can interfere with self-care and rehabilitation of people with spinal cord injury. Our aim was to characterize clonus and to evaluate factors that influence clonus duration in muscles paralyzed chronically by spinal cord injury. Electromyographic activity was recorded from soleus and 7 other limb muscles (5 ipsilateral, 2 contralateral) during clonus. In 14 subjects, clonus frequency in soleus averaged 5.4±0.9 Hz and was slower when the reflex path was longer. Contraction frequency slowed at the beginning and end of clonus (sometimes by 2 Hz). The magnitude of one cycle changed the timing and magnitude of the next cycle. These data suggest that afferent input influences the frequency and maintenance of clonus. Recording from many muscles revealed that clonus was prolonged (>40 sec) when only ipsilateral triceps surae or triceps surae and tibialis anterior were involved. Therefore, localized inputs to spinal circuits were important to sustain clonus. Clonus was intermediate (median: 21 sec) with activation of three or four ipsilateral muscles and these contractions were associated with greater activation of ipsilateral flexors. Clonus was short (<5 sec) when ipsilateral and contralateral muscles were activated (five or six muscles). Activation of extraneous afferent input, particularly contralateral muscles, may provide a way to shorten clonus after spinal cord injury.

Clonus is explained from increased reflex gain and enlarged tissue viscoelasticity

Upper motor neuron diseases (UMND), such as stroke and spinal cord injury (SCI), are assumed to produce alterations in muscle tissue in association with neural damage. Distinguishing between these two factors is of clinical importance in choosing appropriate therapy. We studied the effect of changes in the gain of the Ia reflex pathway and tissue viscoelasticity on the emergence, frequency, and persistence of ankle clonus: a clinically significant, involuntary oscillatory movement disorder. Monte Carlo simulations were performed to explain our experimental observations in patients with stroke (n = 3) and SCI (n = 4) using a nonlinear antagonistic muscle model of the human ankle joint. Ia reflex gain was varied by changing motor unit pool threshold and gain, and passive tissue viscosity and elasticity were varied by changing optimal muscle length. Tissue viscoelasticity appeared to have a strong effect on the emergence and persistence of clonus. Observed frequencies of ankle movement, prior to and after the experimental intervention of a sudden damper, was predicted by the model. The simulations revealed that reflex gains were largest in patients with the largest tissue viscoelasticity. We conclude that ankle clonus in stroke and SCI is the result of a combination of, and suggests a relation between, (i) a decrease in threshold and an increase in gain of the motor unit pool and (ii) a decrease in optimal muscle length.

The relation between neuromechanical parameters and Ashworth score in stroke patients

Background

Quantifying increased joint resistance into its contributing factors i.e. stiffness and viscosity ("hypertonia") and stretch reflexes ("hyperreflexia") is important in stroke rehabilitation. Existing clinical tests, such as the Ashworth Score, do not permit discrimination between underlying tissue and reflexive (neural) properties. We propose an instrumented identification paradigm for early and tailor made interventions.

Methods

Ramp-and-Hold ankle dorsiflexion rotations of various durations were imposed using a manipulator. A one second rotation over the Range of Motion similar to the Ashworth condition was included. Tissue stiffness and viscosity and reflexive torque were estimated using a nonlinear model and compared to the Ashworth Score of nineteen stroke patients and seven controls.

Results

Ankle viscosity moderately increased, stiffness was indifferent and reflexive torque decreased with movement duration. Compared to controls, patients with an Ashworth Score of 1 and 2+ were significantly stiffer and had higher viscosity and patients with an Ashworth Score of 2+ showed higher reflexive torque. For the one second movement, stiffness correlated to Ashworth Score (r² = 0.51, F = 32.7, p < 0.001) with minor uncorrelated reflexive torque. Reflexive torque correlated to Ashworth Score at shorter movement durations (r² = 0.25, F = 11, p = 0.002).

Conclusion

Stroke patients were distinguished from controls by tissue stiffness and viscosity and to a lesser extent by reflexive torque from the soleus muscle. These parameters were also sensitive to discriminate patients, clinically graded by the Ashworth Score. Movement duration affected viscosity and reflexive torque which are clinically relevant parameters. Full evaluation of pathological joint resistance therefore requires instrumented tests at various movement conditions.

Quantification of reflex activity in stroke survivors during an imposed multi-joint leg extension movement

The goal of this study was to compare short- and long-latency reflex responses in eight major lower-extremity muscle groups following an imposed multi-joint leg movement between a group of 14 chronic (>1 year) stroke survivors and 10 healthy age-matched controls, and to investigate the influence of joint velocities and muscle excitation levels on these reflex responses in each respective group. Subjects were seated with their foot anchored to a sliding footplate that could extend their leg. Prior to the leg being moved, subjects were instructed to pre-activate hip and knee flexors and extensors. Feedback of joint torque was used to help subjects activate muscles over a range of excitation levels. Following pre-activation, the subject's leg was passively extended so the knee or hip joint rotated at one of three different speeds (30, 60, and 120 degrees /s). In general, it was found that the magnitude of stroke survivors' reflex response was greater compared to controls' in certain biarticular muscles, notably the gastrocnemius and medial hamstring, and the uniarticular adductor longus, and that the long-latency reflex component (between 40 and 150 ms post-movement) accounted for most of the observed differences. Furthermore, while reflex response amplitudes increased in both groups with increasing movement speed, the rate of increase was significantly larger in stroke subjects than in controls. Clinically, these findings may help explain why stroke survivors walk slowly since it is under these conditions that reflex responses better emulate those of their able-bodied counterparts.

Spastic Reflexes Triggered by Ankle Load Release in Human Spinal Cord Injury

The rapid decrease in firing of load-sensitive group Ib muscle afferents during unloading may be particularly important in triggering the swing phase of gait. However, it still remains unclear whether load-sensitive muscle afferents modulate reflex activity in human spinal cord injury (SCI), as suggested by studies in the cat. The right hip of 12 individuals with chronic SCI was subjected to ramp (60°/s) and hold (10 s) movements over a range from 40° flexion to 0–10°extension using a custom servomotor system. An ankle dorsiflexion load was imposed and released after the hip reached a targeted position using a custom-designed pneumatic motor system. Isometric joint torques of the hip and knee, reaction torque of the ankle, and surface electromyograms (EMGs) from eight muscles of the leg were recorded following the imposed hip movement and ankle load release. Reflexes, characterized by hip flexion torque, knee extension, and coactivation of ankle flexors and extensors, were triggered by ankle load release when the hip was in an extended position. The ankle load release was observed to enhance the reflexes triggered by hip extension itself, suggesting that ankle load afferents play an important role in spastic reflexes in human SCI and that the reflex pathways associated with ankle load afferents have important implications in the spinal reflex regulation of human movement. Such muscle behaviors emphasize the role of ankle load afferents and hip proprioceptors on locomotion. This knowledge may be especially helpful in the treatment of spasms and in identifying rehabilitation strategies for producing functional movements in human SCI.

Effectiveness of automated locomotor training in patients with chronic incomplete spinal cord injury: A multicenter trial

OBJECTIVE

To determine whether automated locomotor training with a driven-gait orthosis (DGO) can increase functional mobility in people with chronic, motor incomplete spinal cord injury (SCI).

DESIGN

Repeated assessment of the same patients or single-case experimental A-B design.

SETTING

Research units of rehabilitation hospitals in Chicago; Heidelberg, Germany; and Basel and Zurich, Switzerland.

PARTICIPANTS

Twenty patients with a chronic (>2 y postinjury), motor incomplete SCI, classified by the American Spinal Injury Association (ASIA) Impairment Scale with ASIA grades C (n=9) and D (n=11) injury. Most patients (n=16) were ambulatory before locomotor training.

INTERVENTION

Locomotor training was provided using robotic-assisted, body-weight-supported treadmill training 3 to 5 times a week over 8 weeks. Single training sessions lasted up to 45 minutes of total walking time, with gait speed between .42 and .69 m/s and body-weight unloading as low as possible (mean +/- standard deviation, 37%+/-17%).

MAIN OUTCOME MEASURES

Primary outcome measures included the 10-meter walk test, the 6-minute walk test, the Timed Up & Go test, and the Walking Index for Spinal Cord Injury-II tests. Secondary measures included lower-extremity motor scores and spastic motor behaviors to assess their potential contribution to changes in locomotor function. All subjects were tested before, during, and after training.

RESULTS

Locomotor training using the DGO resulted in significant improvements in the subjects' gait velocity, endurance, and performance of functional tasks. There were no significant changes in the requirement of walking aids, orthoses, or external physical assistance. There was no correlation between improvements in walking speed or changes in muscle strength or spastic motor behaviors.

CONCLUSIONS

Intensive locomotor training on a treadmill with the assistance of a DGO results in improved overground walking.

Evidence for force-feedback inhibition in chronic stroke

The presence of force-feedback inhibition was explored during reflex responses in five subjects with known incidence of stroke. Using constant velocity stretches, it was previously found that after movement onset, active reflex force progressively increases with increasing joint angle, at a rate proportional to a fractional exponent of the speed of stretch. However, after the reflex force magnitude exceeds a particular level, it begins rolling off until maintaining a steady-state value. The magnitudes of these force plateaus are correlated with the speed of stretch, such that higher movement speeds result in higher steady-state forces. Based upon these previous studies, we hypothesized that force plateau behavior could be explained by a force-feedback inhibitory pathway. To help facilitate an understanding of this stretch reflex force roll off, a simple model representing the elbow reflex pathways was developed. This model contained two separate feedback pathways, one representing the monosynaptic stretch reflex originating from muscle spindle excitation, and another representing force-feedback inhibition arising from force sensitive receptors. It was found that force-feedback inhibition altered the stretch reflex response, resulting in a force response that followed a sigmoidal shape similar to that observed experimentally. Furthermore, simulated reflex responses were highly dependent on force-feedback gain, where predicted reflex force began plateauing at decreasing levels with increases in this force-feedback gain. The parameters from the model fits indicate that the force threshold for force-sensitive receptors is relatively high, suggesting that the inhibition may arise from muscle free nerve endings rather than Golgi tendon organs. The experimental results coupled with the simulations of elbow reflex responses suggest the possibility that after stroke, the effectiveness of force-feedback inhibition may increase to a level that has functional significance. Practical implications of these findings are discussed in relation to muscle weakness commonly associated with stroke.

Bifurcation and stability analysis in musculoskeletal systems: a study in human stance

Reflexes are important in the control of such daily activities as standing and walking. The goal of this study is to establish how reflexive feedback of muscle length, velocity, and force can lead to stable equilibria (i.e., posture) and limit cycles (e.g., ankle clonus and gait). The influence of stretch reflexes on the behavior and stability of musculoskeletal systems was examined using a model of human stance. We computed branches of fold and Hopf bifurcations by numerical bifurcation analysis of the model. These fold and Hopf branches divide the parameter space, constructed by the reflexive feedback gains, into regions of different behavior: unstable posture, stable posture, and stable limit cycles. These limit cycles correspond to a neural deficiency, termed ankle clonus. We also linked bifurcation analysis to known biomechanical concepts by linearizing the model: the fold branch corresponds to zero ankle stiffness and defines the minimal muscle length feedback necessary for stable posture; the Hopf branch is related to unstable reflex loops. Crossing the Hopf branch can lead to the above-mentioned stable limit cycles. The Hopf branch reduces with increasing time delays, making the subject's posture more susceptible to unstable reflex loops. This might be one of the reasons why elderly people, or those with injuries to the central nervous system, often have trouble with standing and other posture tasks. The influence of cocontraction and force feedback on the behavior of the posture model was also investigated. An increase in cocontraction leads to an increase in ankle stiffness (i.e., intrinsic muscle stiffness) and a decrease in the effective reflex loop gain. On the one hand, positive force feedback increases the ankle stiffness (i.e., intrinsic and reflexive muscle stiffness); on the other hand it makes the posture more susceptible to unstable reflex loops. For negative force feedback, the opposite is true. Finally, we calculated areas of reflex gains for perturbed stance and quiet stance in healthy subjects by fitting the model to data from the literature. The overlap of these areas of reflex gains could indicate that stretch reflexes are the major control mechanisms in both quiet and perturbed stance. In conclusion, this study has successfully combined bifurcation analysis with the more common biomechanical concepts and tools to determine the influence of reflexes on the stability and quality of stance. In the future, we will develop this line of research to look at rhythmic tasks, such as walking.

A numeric study of the noise-induced tremor in a mathematical model of the stretch reflex

A mathematical model of the stretch reflex for the cat soleus muscle is presented. The time-delay differential equations of the model are solved using the fourth-order Runge-Kutta algorithm, introducing a Gaussian-noise term to simulate the environmental noise. The muscle response dynamics are then studied under various levels of average muscle activation. Finally, the feasibility of explaining the so-called physiological tremor from the properties of the stretch reflex mechanisms is discussed by comparing our results with reported experimental evidence.

Clonus after human spinal cord injury cannot be attributed solely to recurrent muscle-tendon stretch

Clonus, presented behaviorally as rhythmic distal joint oscillation, is a common pathology that occurs secondary to spinal cord injury (SCI) and other neurological disabilities. There are two predominant theories as to the underlying mechanism of clonus. The prevailing one is that clonus results from recurrent activation of stretch reflexes. An alternative hypothesis is that clonus results from the action of a central oscillator. We present evidence that the mechanism underlying clonus in individuals with SCI is not solely related to muscle stretch. We studied electromyography (EMG) of the soleus (SOL), medial gastrocnemius (MG), tibialis anterior (TA), medial and lateral hamstrings, vastus medialis, vastus lateralis, and rectus femoris from subjects with clinically complete and clinically incomplete SCI during stretch-induced ankle clonus, stepping, and non-weight-bearing standing. Clonic EMG of the SOL, MG, and TA occurred synchronously and were not consistently related to muscle-tendon stretch in any of the conditions studied. Further, EMG activity during stretch-induced ankle clonus, stepping, and non-weight-bearing standing had similar burst frequency, burst duration, silent period duration, and coactivation among muscles, indicating that clonic EMG patterns occurred over a wide range of kinematic and kinetic conditions, and thus proprioceptive inputs. These results suggest that the repetitive clonic bursts could not be attributable solely to immediate afferent feedback such as recurrent muscle stretch. However, these results support the theory that the interaction of central mechanisms and peripheral events may be responsible for clonus.

Spinally generated electromyographic oscillations and spasms in a low-thoracic complete paraplegic

We have measured some oscillatory properties of severe lower limb spasms experienced by a low-thoracic complete paraplegic during assisted standing. Electromyograms (EMG) were recorded from the leg muscles while the patient stood passively in a standing frame. The patient also stood using functional electrical stimulation (FES) while ground and handle reaction force vectors were measured together with EMG activity. During passive standing, spasms appeared simultaneously in all leg muscle groups on one side. The interval between spasms varied between 3 and 30 seconds. Within the spasms, there was a tendency of repetitive grouped discharge of motor units as well as a strong 10-Hz component in the EMG that was coherent across ipsilateral muscle groups. Thus, the spasms were inherently oscillatory. During FES-assisted standing, clinically similar spasms were observed. However, the interspasm interval became relatively fixed at around 16 seconds, which may indicate entraining of the spasm cycle by FES. There are similarities between this patient's spasms and the pathological motor activities seen in other movement disorders that may also be of spinal origin.