Phase-dependent and task-dependent modulation of stretch reflexes during rhythmical hand tasks in humans

EMG-Torque Dynamics Change With Contraction Bandwidth

An accurate model for ElectroMyoGram (EMG)-torque dynamics has many uses. One of its applications which has gained high attention among researchers is its use, in estimating the muscle contraction level for the efficient control of prosthesis. In this paper, the dynamic relationship between the surface EMG and torque during isometric contractions at the human ankle was studied using system identification techniques. Subjects voluntarily modulated their ankle torque in dorsiflexion direction, by activating their tibialis anterior muscle, while tracking a pseudo-random binary sequence in a torque matching task. The effects of contraction bandwidth, described by torque spectrum, on EMG-torque dynamics were evaluated by varying the visual command switching time. Nonparametric impulse response functions (IRF) were estimated between the processed surface EMG and torque. It was demonstrated that: 1) at low contraction bandwidths, the identified IRFs had unphysiological anticipatory (i.e., non-causal) components, whose amplitude decreased as the contraction bandwidth increased. We hypothesized that this non-causal behavior arose, because the EMG input contained a component due to feedback from the output torque, i.e., it was recorded from within a closed-loop. Vision was not the feedback source since the non-causal behavior persisted when visual feedback was removed. Repeating the identification using a nonparametric closed-loop identification algorithm yielded causal IRFs at all bandwidths, supporting this hypothesis. 2) EMG-torque dynamics became faster and the bandwidth of system increased as contraction modulation rate increased. Thus, accurate prediction of torque from EMG signals must take into account the contraction bandwidth sensitivity of this system.

The Dynamics of Voluntary Force Production in Afferented Muscle Influence Involuntary Tremor

Voluntary control of force is always marked by some degree of error and unsteadiness. Both neural and mechanical factors contribute to these fluctuations, but how they interact to produce them is poorly understood. In this study, we identify and characterize a previously undescribed neuromechanical interaction where the dynamics of voluntary force production suffice to generate involuntary tremor. Specifically, participants were asked to produce isometric force with the index finger and use visual feedback to track a sinusoidal target spanning 5-9% of each individual's maximal voluntary force level. Force fluctuations and EMG activity over the flexor digitorum superficialis (FDS) muscle were recorded and their frequency content was analyzed as a function of target phase. Force variability in either the 1-5 or 6-15 Hz frequency ranges tended to be largest at the peaks and valleys of the target sinusoid. In those same periods, FDS EMG activity was synchronized with force fluctuations. We then constructed a physiologically-realistic computer simulation in which a muscle-tendon complex was set inside of a feedback-driven control loop. Surprisingly, the model sufficed to produce phase-dependent modulation of tremor similar to that observed in humans. Further, the gain of afferent feedback from muscle spindles was critical for appropriately amplifying and shaping this tremor. We suggest that the experimentally-induced tremor may represent the response of a viscoelastic muscle-tendon system to dynamic drive, and therefore does not fall into known categories of tremor generation, such as tremorogenic descending drive, stretch-reflex loop oscillations, motor unit behavior, or mechanical resonance. Our findings motivate future efforts to understand tremor from a perspective that considers neuromechanical coupling within the context of closed-loop control. The strategy of combining experimental recordings with physiologically-sound simulations will enable thorough exploration of neural and mechanical contributions to force control in health and disease.

From spontaneous motor activity to coordinated behaviour: a developmental model

In mammals, the developmental path that links the primary behaviours observed during foetal stages to the full fledged behaviours observed in adults is still beyond our understanding. Often theories of motor control try to deal with the process of incremental learning in an abstract and modular way without establishing any correspondence with the mammalian developmental stages. In this paper, we propose a computational model that links three distinct behaviours which appear at three different stages of development. In order of appearance, these behaviours are: spontaneous motor activity (SMA), reflexes, and coordinated behaviours, such as locomotion. The goal of our model is to address in silico four hypotheses that are currently hard to verify in vivo: First, the hypothesis that spinal reflex circuits can be self-organized from the sensor and motor activity induced by SMA. Second, the hypothesis that supraspinal systems can modulate reflex circuits to achieve coordinated behaviour. Third, the hypothesis that, since SMA is observed in an organism throughout its entire lifetime, it provides a mechanism suitable to maintain the reflex circuits aligned with the musculoskeletal system, and thus adapt to changes in body morphology. And fourth, the hypothesis that by changing the modulation of the reflex circuits over time, one can switch between different coordinated behaviours. Our model is tested in a simulated musculoskeletal leg actuated by six muscles arranged in a number of different ways. Hopping is used as a case study of coordinated behaviour. Our results show that reflex circuits can be self-organized from SMA, and that, once these circuits are in place, they can be modulated to achieve coordinated behaviour. In addition, our results show that our model can naturally adapt to different morphological changes and perform behavioural transitions.

Phase-dependent modulation of soleus H-reflex amplitude induced by rhythmic arm cycling

Rhythmic arm cycling is known to suppress the Hoffmann (H-) reflex amplitudes in the soleus (Sol) muscles of stationary legs. However, it has remained unclear if this suppression is modulated according to the phase of movement in the cycle path or is rather a general setting of excitability level related to rhythmic movement. In the present study we investigated the phase-dependent modulation of the Sol H-reflex induced by rhythmic arm cycling by examining reflex amplitudes at 12 phases of the arm cycle movement. Arm cycling tasks consisted of bilateral, ipsilateral and contralateral movement. Additionally, data were also sampled at 12 static arm positions mimicking those occurring during movement. H-reflexes were evoked and recorded at constant motor wave amplitudes across all conditions. Suppression of Sol H-reflex amplitude was dependent upon the phase of movement (main effect p<0.0001) during arm cycling, but not during static positioning. Results suggest that locomotor central pattern generators may contribute to the phasic reflex modulation observed in this study. The phasic modulation was more pronounced during bilateral movement, however aspects of the neural control driving this modulation were also present during ipsilateral and contralateral movement.

Temporal development of anticipatory reflex modulation to dynamical interactions during arm movement

It is known that somatosensory reflex during voluntary arm movement is modulated anticipatorily according to given tasks or environments. However, when and how reflex amplitude is set remains controversial. Is the reflex modulation completed preparatorily before movement execution or does it vary with the movement? Is the reflex amplitude coded in a temporal manner or in a spatial (or state-dependent) manner? Here we studied these issues while subjects performed planar reaching movements with upcoming opposite (rightward/leftward) directions of force fields. Somatosensory reflex responses of shoulder muscles induced by a small force perturbation were evaluated at several points before the arm encountered predictable force fields after movement start. We found that the shoulder flexor reflex responses were generally higher for the rightward than for the leftward upcoming force fields, whereas the extensor reflex responses were higher for the leftward force field. This reflex amplitude depending on the upcoming force field direction became prominent as the reflex was evoked closer to the force fields, indicating continuous changes in reflex modulation during movement. An additional experiment further showed that the reflex modulation developed as a function of the temporal distance to the force fields rather than the spatial distance. Taken together, the results suggest that, in the force field interaction task, somatosensory reflex amplitude during the course of movement is set anticipatorily on the basis of an estimate of the time-to-contact rather than the state-to-contact, to upcoming dynamical interaction during voluntary movement.

Poor penmanship in children correlates with abnormal rhythmic tapping: a broad functional temporal impairment

Timing is crucial for proficient motor tasks; temporal impairments may lead to dysfunctional motor activities. Although much research has been dedicated to the study of movement timing, clinical examination often overlooks temporal impairment of motor activity. The authors hypothesize that some children have a global temporal impairment leading to dysfunctional motor skills. This article checks whether temporal abnormalities detected on a simple tapping task correlate with temporal dysfunction during complex motor skills such as handwriting. Twenty-three school-aged children, 8-14 years (11.1 +/- 1.3 years), underwent tests to assess finger tapping and cursive handwriting. Handwriting samples were rated by experienced teachers. Children with abnormal tapping had lower handwriting rating scores. Temporal features were similar in both tasks; variability on the tapping test correlated with handwriting variability. Temporal variability was not significantly higher for children with poor penmanship as a whole but rather specific to the subgroup of children with a tapping abnormality. Poor penmanship could be attributed in certain children to global temporal impairment reflected as variable finger tapping and handwriting. Evaluation of dysfunctional motor performance should include temporal aspects, and further studies are needed to better delineate and address treatment of "dysrhythmia."

Modulation of reflex responses in hand muscles during rhythmical finger tasks in a subject with writer’s cramp

The objective of this study was to examine phase- and task-dependent modulation of stretch reflexes during repetitive finger movements in writer's cramp, and compare them with normal controls from our previous study. A subject with writer's cramp conducted two rhythmic tasks, index finger abduction (RFA) and a pen-squeezing (RPS) task akin to handwriting. Stretch reflexes were evoked by mechanical perturbations at random phases of each task. Surface electromyograms (EMG) were recorded from two hand muscles, first dorsal interosseous (FDI) and flexor digitorum superficialis (FDS). The reflex response and background EMG activity of each muscle were modulated in a phase-dependent manner in both tasks. However, they varied largely in phase during the RFA task, but in approximately inverse phase-relationship during RPS. Reflex sensitivity, as represented by the slope of the linear regression between response and background, was much lower for both muscles in the 'writing' task (RPS) than in the RFA task with its positively correlated responses. These phase- and task-related modulation patterns differed dramatically from those observed in our control subjects, where reflex responses were modulated largely in phase with background activity and reflex sensitivity was much higher, particularly in FDI during RFA and FDS during RPS. The altered reflex modulation patterns in writer's cramp may reflect deficiencies of integration of proprioceptive afferent inputs and reduced inhibition at cortical and spinal levels during writing performance. Results from this case study support clinically identified task-specific feature of focal hand dystonia.