Electromyographic response to pseudo-random torque disturbances of human forearm position

A framework for identification of brain network dynamics using a novel binary noise modulated electrical stimulation pattern

Modeling and identification of brain network dynamics is of great importance both for understanding brain function and for closed-loop control of brain states. In this work, we present a multi-input-multi-output (MIMO) linear state-space model (LSSM) to describe the brain network dynamics in response to electrical stimulation. The LSSM maps the parameters of electrical stimulation, such as frequency, amplitude and pulse-width to recorded brain signals such as electrocorticography (ECoG) and electroencephalography (EEG). Effective identification of the LSSM in open-loop stimulation experiments, however, is strongly dependent on the open-loop input stimulation design. We propose a novel input design to accurately identify the LSSM by integrating the concept of binary noise (BN) with practical constraints on stimulation waveforms. The designed input pattern is a pulse train modulated by stochastic BN parameters. We show that this input pattern both satisfies the necessary spectral condition for accurate system identification and can incorporate any desired pulse shape. Using numerical experiments, we show that the quality of identification depends heavily on the input signal pattern and the proposed binary noise modulated pattern achieves satisfactory identification results, reducing the relative estimation error more than 300 times compared with step sequence modulated, single-sinusoid modulated and multi-sinusoids modulated input patterns.

Linear modeling of human hand-arm dynamics relevant to right-angle torque tool interaction

OBJECTIVE

A new protocol was evaluated for identification of stiffness, mass, and damping parameters employing a linear model for human hand-arm dynamics relevant to right-angle torque tool use.

BACKGROUND

Powered torque tools are widely used to tighten fasteners in manufacturing industries. While these tools increase accuracy and efficiency of tightening processes, operators are repetitively exposed to impulsive forces, posing risk of upper extremity musculoskeletal injury.

METHODS

A novel testing apparatus was developed that closely mimics biomechanical exposure in torque tool operation. Forty experienced torque tool operators were tested with the apparatus to determine model parameters and validate the protocol for physical capacity assessment.

RESULTS

A second-order hand-arm model with parameters extracted in the time domain met model accuracy criterion of 5% for time-to-peak displacement error in 93% of trials (vs. 75% for frequency domain). Average time-to-peak handle displacement and relative peak handle force errors were 0.69 ms and 0.21%, respectively. Model parameters were significantly affected by gender and working posture.

CONCLUSION

Protocol and numerical calculation procedures provide an alternative method for assessing mechanical parameters relevant to right-angle torque tool use. The protocol more closely resembles tool use, and calculation procedures demonstrate better performance of parameter extraction using time domain system identification methods versus frequency domain.

APPLICATION

Potential future applications include parameter identification for in situ torque tool operation and equipment development for human hand-arm dynamics simulation under impulsive forces that could be used for assessing torque tools based on factors relevant to operator health (handle dynamics and hand-arm reaction force).

Simulation studies of descending and reflex control of fast movements

Several neurological control strategies for fast head movements are considered using computer simulations of a stretch reflex model. Each control strategy incorporates a different amount of proprioceptive feedback contributing to braking and/or clamping the movement. The model behavior for each control strategy is qualitatively compared to experimental data that includes the agonist and antagonist EMGs, and the head position, velocity, and acceleration. Significance of the study is discussed with respect to the characteristic tri-phasic EMG pattern for fast voluntary movements and the possible roles that the stretch reflex may have in contributing to this pattern of activation.

Variation of magnitude and timing of wrist flexor stretch reflex across the full range of voluntary activation

This paper reports an investigation of the magnitude and timing of the stretch reflex over the full range of activation of flexor carpi radialis. While it is well established that the magnitude of the reflex increases with the level of muscle activation, there have been few studies of reflex magnitude above 50% of maximum voluntary contraction (MVC) and virtually no study of the timing of the response in relation to activation level. Continuous small amplitude (approximately 2 degrees) perturbations were applied to the wrist of 12 normal subjects while they maintained contraction levels between 2.5-95% MVC, monitored via surface electromyography (EMG). Both narrow band (4-5 Hz) and broad band (0-10 Hz) stretch perturbations were employed. The gain (EMG output/stretch input) and phase advance of the reflex varied with the level of muscle activation in a similar manner for both types of stretch, but there were significant differences in the patterns of change due to stretch bandwidth. Consistent with previous studies, the group average reflex gain initially increased with muscle activation level and then saturated. Inspection of individual data, however, revealed that the gain reached a peak at about 60% MVC and then decreased at higher contraction levels, the pattern across the full range of activation being well described by quadratic functions (mean r2=0.82). This quadratic pattern has not been reported previously for the neural reflex response in any muscle but is consistent with the pattern that has been reliably observed in studies of the mechanical reflex response in lower limb muscles. In contrast to the pattern for reflex gain, the phase advance of the reflex (at a stretch frequency of 4.5 Hz) decreased linearly from approximately 130 degrees at the lowest contraction levels to approximately 50 degrees as maximum voluntary contraction was reached (mean r2=0.69). This decrease corresponds to a delay of 49 ms introduced centrally in reflex pathways. All subjects showed clearly defined quadratic functions relating reflex gain and linear functions relating reflex phase to activation level, but there were considerable individual differences in the slopes of these functions which point to systematic differences in synaptic behaviour of the motoneuron pool. Thus, there was wide inter-subject variation in both the contraction level at which the reflex gain reached a peak (31-69% MVC) and the highest target contraction level that could be sustained during reflex measurement (47-95% MVC). A high correlation between these variables (r2=0.78) suggests a linear relation between afferent support of contraction and muscle fatigability. The decline in reflex gain at high levels of muscle activation signals a failure of muscle afferent input and subjects in whom the gain reached a peak and declined early were unable to sustain higher target contraction levels. The results of the study show that both the timing and magnitude of the stretch reflex vary markedly over the full range of voluntary muscle activation. The pattern of variation may account for why the stretch reflex contributes most effectively to muscle mechanics over the lower half of the range of activation, while progressive reductions in both gain and phase advance at higher levels render the reflex mechanically less effective and make tremor more likely.

Comparison of spinal myotatic reflexes in human adults investigated with cross-correlation and signal averaging methods

A cross-correlation method for recording spinal myotatic reflexes has been developed to meet the need for brief test periods in babies and children and subjects with central neurological pathology. In normal adult subjects the method has been validated by comparing excitatory and inhibitory reflexes obtained with cross-correlation with those obtained with conventional signal averaging. In the cross-correlation method a pseudo-random binary sequence of 64 brief tendon taps was delivered in <1.5 s, and in the averaging method 20-150 taps at one per second. The reflexes were expressed as unit impulse responses to enable direct, quantitative comparisons to be made. With cross-correlation the responses were slightly expanded in time, had lower peak amplitudes, and onset latencies advanced by 10 ms, the clock period of the pseudo-random binary sequence. The amplitude of biceps phasic stretch reflex increased with muscle contraction in a similar manner with both methods. In tests for stationarity the amplitude of biceps phasic stretch reflex varied <10% in the first six repeats of the pseudo-random binary sequence. The tap force required at threshold for cross-correlation was approximately half that for averaging, but with both methods the magnitude of biceps phasic stretch reflex varied linearly with tap force over the range of one to two times threshold. The validity of responses obtained with cross-correlation was assessed by a statistical procedure. In conclusion, the cross-correlation method is robust and gives similar results to those obtained with averaging.

Serial processing in human movement production

Parallel processing is often considered to be synonymous with biological computation, but a great deal of evidence points to serial computation being used by animals to solve specific types of problems. In particular, the observation of movement intermittency (fluctuations in limb kinematic variables that cannot be explained by low-level dynamics of the system) seems to imply a serial temporal segmentation strategy in the planning of arm movements. This paper discusses prior observations of movement intermittency in different task contexts, possible theoretical and physiological origins of the phenomenon, and implications for human movement strategies.

Force regulation is deficient in patients with parietal lesions: a system-analytic approach

By means of a quantitative system-analytic investigation strategy, the postural motor control of the fingers was evaluated, to characterise the possible deficit of force regulation in patients with parietal lesions. In spite of a normal response to short torque pulses, the parietal-lesion patients had difficulties in returning to the preload level after the application of an additional step torque load to fingers II-IV of their left or right hands. The control offset (measured 500 ms after step torque application) was significantly larger in the patient group. This deficit in the investigated patients with parietal lesions to compensate for step torque loads was not due to a paresis, but rather resulted from a disturbance in the generation of a sufficient counterforce against the applied step torque within an adequate time window and motor pattern. This distinct force-regulation deficit was found in patients with left- and right-sided parietal lesions.

The simple frequency response of human stretch reflexes in which either short- or long-latency components predominate

1. The stretch reflexes of the human abductor digiti minimi (ADM) and biceps brachii muscles were compared using small-amplitude sinusoidal stretching at 10-50 Hz and recording the surface EMG. The stimulus was applied either to the relevant proximal phalanx or to the biceps tendon while the muscle studied was contracting; the same amplitude was used for all frequencies (range 0.5-2 mm for ADM, 0.1-1 mm for biceps). 2. As the frequency increased, the response of ADM decreased while that of biceps increased. Neither muscle showed a minimum at 20-25 Hz, as previously found for wrist muscles and attributed to an interaction between short- and long-latency components of the reflex. 3. For both muscles, the phase of the response lagged behind the stimulus by an amount which increased approximately linearly with frequency, without the gross inflexion found for wrist muscles. Such linearity would be found for a system dominated by a fixed time delay; its value sets the slope. The slope for biceps was half that for ADM. The values of reflex delay calculated from the slope of the phase plots agreed reasonably with the absolute latencies of the responses evoked by tap or ramp stimulation. Part of the difference between the muscles was due to differences in peripheral conduction time, since ADM lies more distally. Most of it, however, was due to different reflexes being involved, with biceps being predominantly controlled by short-latency pathways and ADM by long-latency pathways. 4. For both muscles, the phase lag at any given frequency was less than that expected from the reflex latency, determined from the slope of the phase plot. Thus, sensory transduction and central transmission had produced a phase advance in the reflex. The 'neural phase advance' of biceps was appreciably larger than that of ADM, and more than would be expected from the behaviour of its spindle afferents. The excess is suggested to be due to the action of Renshaw inhibition, which ADM may lack. 5. The results were substantiated by recording from single motor units in biceps. Stretching at the present amplitudes had rather little effect on the overall rhythmic behaviour, as shown by interspike interval histograms. However, cycle histograms showed that the discharge was modulated reasonably sinusoidally by the stretching, whatever its frequency (i.e. the probability of the occurrence of a spike varied over the cycle). Cyclic changes were also found in autocorrelograms and amplitude spectra of the spike trains.(ABSTRACT TRUNCATED AT 400 WORDS)

Servo Hypotheses for the Biological Control of Movement

An analysis is made of equilibrium-point models for motor control, describing these models in the context of servo control mechanisms. We considered issues of speed and stiffness scaling that are incompatible with current formulations of the equilibrium-point models. A modification of the equilibrium-point models is proposed in which the central nervous system controls velocity as well as positions during the course of fast 1imb movements. Numerical simulations are presented that verify that such a servo control mechanism could successfully produce fast limb movements, as observed in human subjects

Spastic cerebral palsy: possible spinal interneuronal contributions

The author explores the possibility that abnormal and immature spinal interneuronal circuits play a rôle in spastic cerebral palsy. Interneuronal abnormalities could account for a number of characteristic signs: sensitivity to normally innocuous stimuli, hyperreflexia, abnormal and inappropriate co-ordination patterns, and limitations in acquiring, planning, executing and correcting skillful actions. The precise pattern of interneuronal anomalies present will depend on the original site of the lesion, age at onset, and on how central and segmental development were affected. Although the number of relevant studies is very small, there are indications that long-term training can produce changes in segmental response, both by altering descending inhibition and by producing lasting changes in spinal neuronal organisation and responsiveness. Modern recording techniques have introduced the prospect of tracing immature and abnormal segmental components and of establishing their impacts on movement control.

Transient responses to load perturbations of the forearm in a monkey with a chronic lesion in the internal capsule

Small electrolytic lesions were produced in the internal capsule of a monkey. The changes in muscle tone were quantified by studying the EMG responses of elbow muscles and the mechanical responses of the forearm to pseudo-random torque perturbations applied to the elbow joint. Immediately following the lesion, the EMG responses of both biceps and triceps muscles were depressed. Subsequently, biceps responses recovered and became eventually greater than in the control. Triceps responses, instead, remained low throughout the follow-up period (3 months). The mechanical behavior of the forearm was characterized in terms of the dynamic relationship between the applied torque perturbations and the resulting changes in elbow angle. After the lesion, the damping of the elbow responses decreased relative to the control. Possible mechanisms for the observed changes in the EMG and mechanical behavior are discussed.

Anticipatory and reflex coactivation of antagonist muscles in catching

Reflex and anticipatory coactivation of antagonist muscles is demonstrated to occur when human subjects catch a ball. Amplitude and time course of the electromyographic (EMG) responses are strongly modulated by the presence of visual information. It is argued that these responses are centrally preset to stabilize the limb after ball impact.

Myoelectric response of the human triceps brachii to displacement-controlled oscillations of the forearm

The dynamic relations between the surface myoelectric activity in tonically contracting triceps brachii and the forearm rotation (proportional to triceps stretch) were measured by imposing small, sinusoidal, displacement-controlled perturbations on the forearm position. Three normal, adult, male subjects participated in these experiments. The amplitude of the forearm rotation, the driving frequency, and the tonic contraction level were all carefully regulated. The mean rectified triceps EMG (the output) showed a strong harmonic at the driving frequency, and the frequency-response characteristics were computed directly by comparing the amplitude and phase of this harmonic to that of the forearm flexion angle (the input). The (electrical) reflex gain is defined as the amplitude ratio of output to input. The system response was measured from 2 to 18 Hz, at two tonic contraction levels and two forearm rotation amplitudes, about a mean position of 90 degrees forearm flexion. The results show clearly that the system response is nonlinear: the reflex gain decreases with forearm rotation amplitude. (This gain also increases with tonic contraction level for sufficiently low values of the latter variable.) The measured frequency-response characteristics of the system can be modeled approximately as a second-order linear lead filter with a single time delay, followed by a saturating nonlinearity. Both model-independent estimates and least-squares model fitting, yielded values of the time delay of the order of 25 ms, suggesting that a segmental mechanism mediates reflex activity. Simplified calculations and limited measurements are presented to show that a nonlinear system of the type we have identified with constant displacement driving may appear linear under constant torque driving. Our directly-measured frequency-response characteristics differ from those reported by investigators employing random, rather than periodic, driving; possible reasons for these apparent discrepancies are discussed.

Phasic relations in 90 degree abduction-adduction of the arm: the ARIMA representation

Myoelectric signals (MES's) from the medial deltoid, posterior deltoid and medial trapezius were analyzed during 90 degree abduction-adduction cycles of arm movement. Two healthy males were utilized as subjects. The MES's records were divided into sub-sections of 50 ms of duration, and each segment was described by a suitable model. The models were based on autoregressive integrated moving average (ARIMA) processes. Several distinct phases were discerned within a given movement cycle, each phase being associated with a distinct type of ARIMA process. Phasic relations among the three muscles were then revealed. A simple test was suggested to detect the onset of muscle activity in real-time situations.

Time series modeling of neuromuscular system

The dynamic response of the human ankle joint to a bandlimited random torque perturbation superimposed on a constant bias torque is observed in normal human subjects. The applied torque input, the joint angular rotation output, and the electromyographic activity using surface electrodes from the extensor and the flexor muscles of the ankle joint were recorded. Transfer function models using time series techniques were developed for the torque - angular rotation input-output pair and for the angular rotation - electromyographic activity input-output pair. A parameter constraining technique was applied to develop more reliable models. It is shown that the asymptotic behavior of the system must be taken into account during parameter optimization to develop better predictive models.

Effects of muscle model parameter dispersion and multi-loop segmental interaction on the neuromuscular system performance

The effects of parameter dispersion among motor units on the neuromuscular system performance as well as interaction between muscle segments and spinal cord mechanisms are investigated. Elementary components of the system are modeled to simulate with simple models their input-output characteristics. A leaky SS-IPFM encoder with a time-dependent threshold simulates the motor-neuron encoding characteristics. An amplitude and time dependent nonlinear model represent the motor unit mechanical output to neuronal input relationship. The dispersion of parameters in the components of the whole muscle control model is investigated in the open loop mode. It is shown that the dispersion of parameters in the multi-efferent channels converging on a common tendon provides a spatial filtration generating a smoother muscle force in addition to extending the linear dynamic range compared to a similar system having identical motor units. Muscle segmental interaction is investigated in this distributed model by closing the loop through a coupling matrix, representing afferent-motorneuron interaction on the spinal cord level. A diagonal matrix represents no segmental interaction and a uniform matrix represents a uniform interaction between segments through the muscle spindles and Golgi tendon feedback elements. The close loop simulation studied shows that (a). The type of segmental interaction has little effect on the overall system performance, i.e., range of linerity and stability, which is the result of having a muscle system with a large number of motor units. (b) There are only minor differences in results between the uniform and normal parameter distributions tested. (c) A loop gain of 4 divided by 8 in the distributed model can provide linearity through the full physiological force range. (d) Type of segmental interaction has significant effects on the individual segment. A uniform matrix provides a more stable segment due to the spatial filtration resulting from the segmental interaction, while the diagonal noninteracting matrix shows instabilities on the local segmental level despite global stability. The more realistic exponentially decaying spatial interaction matrix yields both global neuromuscular and local segmental stability with the same linear dynamic range generated with the uniform or diagonal matrices.

The mechanical behavior of the human forearm in response to transient perturbations

Static and dynamic components of mechanical impedance of human forearm were evaluated by applying two kinds of perturbations: 1) large viscoelastic loads, and 2) small pseudo-random perturbations. When the task involved the active resistance of the perturbations, both stiffness and viscosity increased relatively to their values in the passive task, the increment in stiffness being larger than that in viscosity. The time course of such changes was investigated during the transition between the two operating points defined by the instructions "do not resist" and "resist" the applied perturbations. The changes in stiffness and viscosity were relatively slow, those in the latter lagging behind those in the former.

Changes in short and long latency stretch responses during the transition from posture to movement

Experiments were performed in 18 normal subjects to estimate the time course of changes in the gains of pathways mediating short- and long-latency responses to muscle stretch during the transition from a maintained posture against a steady load to a rapid ballistic movement. Subjects were instructed to rapidly flex or extend their forearm in response to a tone from an initial position of 90 degree of elbow flexion. Torque pulses stretching the biceps muscle were applied to the forearm at 8 different times before and after the signal to initiate the movement, and the gains of short- and long-latency pathways were estimated from averages of rectified biceps EMG activity for 20 trials at each time interval between the onsets of the tone and torque pulse. The findings demonstrate that changes in the magnitude of long-latency responses (M2, M3) occur during the period between the onset of the auditory signal and the voluntary motor response. However, the magnitude of the short-latency response (M1) remains unchanged until after the onset of voluntary motor activity. The differences in the timing of short- and long-latency stretch responses suggests that activity in long-latency pathways may play an important preparatory role in facilitating the transition from posture to movement.

Myotatic reflexes and the on-off effect in patients with Parkinson's disease

Reflex activity in the biceps and triceps muscles evoked by applied torque perturbations was studied in patients with Parkinson's disease. The perturbations consisted of single pulses or of pseudo-random sequences of pulses of torque. The patients were treated with levodopa and some exhibited marked fluctuations in their clinical disabilities ("on-off" effect). The study was undertaken to see if reflex activity changed in parallel with the fluctuations of their clinical symptoms. It was found that the reflex activity in these patients could be classified into two types, a Type I response differing little from normal and a Type II response exhibiting marked high-frequency (8-14 Hz) oscillations in EMG activity. Both Type I and Type II responses were virtually the same in the "on" as in the "off" state.

Frequency response characteristics of a multi-loop representation of the segmental muscle stretch reflex

This paper continues the investigation of a three-loop representation of the segmental muscle stretch reflex system introduced in a preceding communication. Frequency response characteristics were computed for open-loop conditions, control and disturbance signal inputs under a variety of conditions: (i) "in parallel" and "in series" peripheral arrangements of muscle compartments, (ii) various patterns of central connectivity, (iii) various recruitment levels of motor units, (iv) various overall reflex gains, (v) absence or presence of muscle spindle acceleration sensitivity. These computations disclose a number of mechanisms by which the nervous system might improve system stability and behaviour. These mechanisms are discussed with regard to physiological data.

Modulation of the myotatic reflex gain in man during intentional movements

Human subjects were asked to perform sinusoidal tracking movements (0.5--3.0 Hz) with their forearms while external torque disturbances were applied at the elbow. The changes in angular position, velocity, and acceleration produced by these disturbances were found to be represented in the reflex changes in EMG activity of both biceps and triceps muscles. The gain of each of these reflex components varied during the tracking task, their maximal being about the same as those measured when the torque disturbances were applied in the absence of movements and the subjects attempted to maintain a constant forearm position. Such changes in gain were found to be centrally regulated since they were shown not to depend on the movement itself, being also present during force tracking, i.e. under nearly isometric conditions. Also their minima and maxima did not coincide with those of the EMG activity. These results suggest that an internal plan (or model) of the learned task is present, whereby reflex gains can be regulated independently from the motion and alpha-motoneuron activity. Such regulation effectively uncouples the reflex motor output from the intentionally controlled motion and maintains spindle sensitivity to external disturbances independent of large changes in muscle length. These conclusions are discussed in the context of the functional role of gamma-motoneurons in the control of movements.

An evaluation of nonlinearities in the motor output response to applied torque perturbations in man

Nonlinearities present in the motor output response of biceps and triceps muscles in normal human subjects to applied torque perturbations were evaluated quantitatively. When the applied perturbations were relatively continuous, consisting of a pseudo-random train of pulses, the identified nonlinearities were small, never exceeding 20% of the amplitude of the linear component of the response and usually being much less.