PubMed Health⌕ Search

Biomedical subjects

G C Agarwal

Publications and source records attributed to G C Agarwal.

At least 19 recordsLinked to original sources
PubMed1992

Organizing principles for single joint movements: V. Agonist-antagonist interactions.

1. Normal human subjects made discrete elbow flexions in the horizontal plane under different task conditions of initial or final position, inertial loading, or instruction about speed. We measured joint angle, acceleration, and electromyographic signals (EMGs) from two agonist and two antagonist muscles. 2. For many of the experimental tasks, the latency of the antagonist EMG burst was strongly correlated with parameters of the first agonist EMG burst defined by a single equation, expressed in terms of the agonist's hypothetical excitation pulse. Latency is proportional to the ratio of pulse duration to pulse intensity, making it proportional to movement distance and inertial load and inversely proportional to planned movement speed. However, these rules are not sufficient to define the timing of every possible single joint movement. 3. For movements described by the speed-insensitive strategy, the quantity of both antagonist and agonist muscle activity can be uniformly associated with selected kinetic measures that incorporate muscle force-velocity relations. 4. For movements collectively described by the speed-sensitive strategy, (i.e., that have direct or indirect constraints on speed), no single rule can describe all the combinations of agonist-antagonist coordination that are used to perform these diverse tasks. 5. Estimates of joint viscosity were made by calculating the amount of velocity-dependent torque used to terminate movements on target. These estimates are similar to those that have previously been made of limb viscosity during postural maintenance. They imply that a significant component of muscle activity must be used to overcome these forces. 6. These and previous results are all consistent with a dual-strategy hypothesis for those single-joint movements that are sufficiently fast to require pulse-like muscle activation patterns. The major features of such patterns (pulse intensities, durations, and latencies) are determined by central commands programmed in advance of movement initiation. The selection between speed-insensitive or speed-sensitive rules of motoneuron pool excitation is implicitly specified by the nature of speed constraints of the movement task.

Electromyography↗
PubMed1990

Organizing principles for single joint movements. III. Speed-insensitive strategy as a default.

1. Human subjects made discrete elbow flexions in a horizontal plane over different distances, from a stationary initial position to a visually defined stationary target 9 degrees wide. We measured joint angle, acceleration, and electromyograms (EMGs) from two agonist and two antagonist muscles. 2. Subjects made movements over four different distances following one of four different instructions. The first instructed the subject simply to choose a comfortable speed. The other three explicitly emphasized either speed, accuracy, or maintenance of the "same" speed over different distances. These instructions produced a wide range of movement velocities. 3. The initial rises of the acceleration (and therefore of the inertial torque), as well as the initial slope of the agonist EMG, were all invariant over changes in the target distance for any single instruction but were all sensitive to the given instruction. 4. Our results demonstrate that the speed-insensitive strategy is a standard or default pattern for performing movements that may be carried out for different instructions over a wide range of speeds. A uniform intensity of excitation pulse is not a byproduct of moving at maximal speed. Submaximal intensities are associated with submaximal speeds and are a selected feature of the pattern of movement control.

Acceleration↗
PubMed1990

Organizing principles for single-joint movements. IV. Implications for isometric contractions.

1. Normal human subjects made isometric pulse and step contractions about the elbow to visually defined target torques of different amplitudes and at different rates. We measured joint torque and electromyograms (EMG) from two agonist and two antagonist muscles. 2. When the task specification requires that the subject explicitly alter the rate at which torque is increased, the rates of rise of the agonist and antagonist EMG bursts covary with the rate of rise of the torque. For pulses of torque the duration of motoneuron excitation varies with the duration of the task-defined contractile event. 3. When a subject is asked to generate torques of different amplitudes without specifying a time interval, torque amplitude is positively correlated with how long, and therefore how high, the EMG rose. Subjects usually proportionately covary the strength of the agonist and antagonist contractions but are not constrained to do so. Some subjects use a strategy of varying the antagonist inversely with the agonist contraction. 4. We extend the organizing principles for the control of movement about a single joint to the control of isometric torque. These rules state that control of torque about a single joint is exercised by one of two strategies: the speed-sensitive strategy modulates the rate at which contraction rises by varying the intensity of motoneuron-pool excitation. The speed-insensitive strategy varies the duration over which contraction rises but does not change the rate. These two respective patterns of torque emerge from pulse-height and pulse-width modulation of motoneuron-pool excitation. 5. The rules defining speed-sensitive and speed-insensitive strategies for movements are broadened for isometric contractions because of the wider range of torque patterns that we observe under these conditions. We propose a step-excitation component for prolonged isometric step contractions and slowly rising ramp patterns of excitation for contractions that develop over several hundreds of milliseconds. 6. The choice of strategies is based on task-specific torque requirements. The same two strategies that control torque to produce movement apply to the control of isometric torque. Unlike movements, however, isometric tasks are more often controlled by a blending of the two patterns. Possible reasons for this are discussed.

Electromyography↗
PubMed1990

Reflex responses to ankle perturbations during electrical stimulation of muscle: 1. Measurement techniques and preliminary examples.

Electrical stimulation of muscle has been proposed as a technique to restore function to paralyzed muscles. But, from a control stand-point, little is known about how such artificial activation interacts with the still intact spinal reflex loops. We have developed instrumentation to measure and compare ankle compliance and muscle EMG activity when the ankle is subjected to perturbations in torque or angular position from bias positions that are achieved volitionally or via electrical muscle stimulation. We deliver precise torque or position perturbations (step, ramp, sinusoidal, random) to the ankle via a pivoting footplate driven by a computer-controlled torque motor. Angular displacement, torque, acceleration, and 2 to 4 channels of electromyographic (EMG) data are collected on analog (VHS) tape and simultaneously digitized and stored. Torque or position biases to normal ankle equilibrium position are applied volitionally (for the neurologically intact) or via electrical stimulation of the Gastroc/Soleus or the Tibialis Anterior muscle (for either the neurologically intact or spinal cord injured). A special stimulator/recording amplifier permits the recording of EMG signals from the muscle being stimulated. An overview of the features and response characteristics of the perturbation system and a comparison from preliminary studies of responses at different biases achieved volitionally versus those achieved by stimulation are presented.

Ankle Joint↗
PubMed1989

Organizing principles for single-joint movements. I. A speed-insensitive strategy.

1. Normal human subjects made discrete elbow flexions and extensions in the horizontal plane from a stationary initial position to visually defined targets at different distances with a constant inertial load or made flexions to a visually defined target with different inertial loads. We measured joint angle, acceleration, and electromyograms (EMGs) from two agonist and two antagonist muscles. 2. Subjects were instructed to move their limbs accurately but quickly to the targets. Movements of greater distances or lesser loads were performed at higher velocities. 3. Peak inertial torque, acceleration and velocity, movement time, and integrated, rectified EMG were all highly correlated with the task variables, distance and inertial load. We show that peak inertial torque can be used as a linking variable that is almost sufficient to explain all correlations between the tasks, the EMG, and movement kinematics. 4. The rate at which subjects initially developed torque to accelerate their movements was invariant over changes in the value of either task variable. The rising phase of the agonist EMG was also independent of the distance or load moved. 5. Two components were distinguished in the antagonist EMG. The first had a relatively constant latency and amplitude. It terminated on the onset of the second and larger component at a latency that was delayed as both distance and load increased. 6. The integrated, rectified antagonist EMG was proportional to inertial load and peak decelerating torque for changes in inertial load. When target distance varied, proportionality between peak decelerating torque and antagonist EMG could be found if correction was made for the effects of muscle length on the torque-EMG relationship. 7. We propose organizing principles for the control of single-joint human movements in which tasks are performed by one of two strategies. These are called speed-insensitive and speed-sensitive strategies. 8. A model is described in which movements made under a speed-insensitive strategy are executed by controlling the duration and the relative timing of amplitude invariant patterns of activation to the spinal motoneuron pools.

Elbow Joint↗
PubMed1989

Organizing principles for single-joint movements. II. A speed-sensitive strategy.

1. Normal human subjects made discrete flexions of the elbow over a fixed distance in the horizontal plane from a stationary initial position to a visually defined target. We measured joint angle, acceleration, and electromyograms (EMGs) from two agonist and two antagonist muscles. 2. Changes in movement speed were elicited either by explicit instruction to the subject or by adjusting the target width. Instructions always required accurately stopping in the target zone. 3. Peak inertial torques and accelerations, movement times, and integrated EMGs were all highly correlated with speed. We show that inertial torque can be used as a linking variable that is almost sufficient to explain all correlations between the task, the EMG, and movement kinematics. 4. When subjects perform tasks that require control of movement speed, they adjust the rate at which torque is developed by the muscles. This rate is modulated by the way in which the muscles are activated. The rate at which joint torque develops is correlated with the rate at which the agonist EMG rises as well as with integrated EMG. 5. The antagonist EMG shows two components. The latency of the first is 30-50 ms and independent of movement dynamics. The latency of the second component is proportional to movement time. The rate of rise and area of both components scale with torque. 6. We propose organizing principles for the control of single-joint movements in which tasks are performed by one of two strategies. These are called speed-insensitive and speed-sensitive strategies. 7. A model is proposed in which movements made under a speed-sensitive strategy are executed by controlling the intensity of an excitation pulse delivered to the motoneuron pool. The effect is to regulate the rate at which joint torque, and consequently acceleration, increases. 8. Movements of variable distance, speed, accuracy, and load are shown to be controlled by one of two consistent sets of rules for muscle activation. These rules apply to the control of both the agonist and antagonist muscles. Rules of activation lead to distinguishable patterns of EMG and torque development. All observable changes in movement kinematics are explained as deterministic consequences of these effects.

Elbow Joint↗
PubMed1988

Practice improves even the simplest movements.

Three subjects practiced accurate, fast elbow flexions of 54 degrees to a 3 degrees wide target. Movements of 36 degrees, 54 degrees and 72 degrees were then tested. Comparison over the three distances showed that the normally monotonic relationship between movement distance and movement time is alterable by specific training. Subjects learn to go faster over the practiced distance by refining their neural commands to the muscles. The benefits of practice only partially transfer to other distances. We conclude that many of the relationships seen among movement variables in simple tasks are plastic in nature and affected by prior experience.

Adult↗
PubMed1988

Compliance of single joints: elastic and plastic characteristics.

1. Step changes in torque were applied to the elbow or ankle joint of normal human subjects who exerted constant levels of effort. They were instructed to not react to the torque but to allow their limbs to move to a new equilibrium position. In this experimental paradigm, the joint may be characterized by a nonlinear compliant element. The aim of this study was to characterize the elastic properties of the compliant element. 2. Joint elasticity is described by an S-shaped relation between torque and angle (a "compliant characteristic curve"). The stiffness of a joint is greatest for small perturbations and decreases as the size of the perturbation is increased whether the limb is loaded or unloaded from its initial equilibrium. 3. The S shape of the compliant characteristic curve is relatively constant when measured at different initial joint angles from the same initial joint torque. 4. Higher levels of initial muscle torque increase the steepness of the compliant characteristic curve. 5. All changes in initial joint torque and angle preserve the S shape. The inflection point of the characteristic curve is always at the initial equilibrium angle and torque. This shifting of the inflection point of the torque-angle relation implies a fundamental plasticity in joint compliance. The elastic component is not invariant but changes with the joint's initial equilibrium state. 6. Changes in muscle tension and length that result from a perturbation are accompanied by changes in muscle activation. The relationship between perturbation torque and mean equilibrium EMG is similar to that found for voluntary isometric contraction. It is not possible to conclude what proportion of the late EMG response to perturbation is mediated by segmental reflex mechanisms. 7. At the levels of torque used here, changes in joint stiffness are highly correlated with changes in tonic contraction of the muscle opposing the load. This change in stiffness is not the result of antagonist coactivation, which was minimal. 8. The compliant characteristic curves of elbow and ankle are qualitatively similar. The principal difference is due to the greater passive stiffness of the ankle. 9. Our findings are inconsistent with aspects of the theory of invariant characteristics or with models of movement and load compensation that postulate a control scheme based only on the setting of muscle and reflex equilibrium points. The data are also incompatible with models that only control the elastic stiffness of the muscle.

Ankle Joint↗
PubMed1986

Movement deficits caused by hyperexcitable stretch reflexes in spastic humans.

Spastic patients were instructed to make accurate, rapid ankle dorsiflexion and plantarflexion movements over different distances to a target. Ankle position and surface electromyograms (EMGs) from tibialis anterior (TA) and soleus (SOL) muscles were recorded. In 3 of 8 spastic patients tested, dorsiflexion evoked velocity-dependent activation of the antagonist (SOL) muscle which impeded the movement to the extent that the limb unintentionally reversed movement direction. We propose that this activation is reflex in origin since it is tightly synchronized, has a large peak amplitude, occurs about 50 ms after the initiation of the movement, and is velocity dependent. One of the 3 patients who had reflex-induced antagonist activation in dorsiflexion also demonstrated sustained clonus during plantarflexion. This usually occurred only if the target had been overshot so that the return of the limb stretched the soleus muscle and triggered clonus. We conclude that in some patients, hyperactive stretch reflexes cause movement deficits.

Adult↗
PubMed1986

Stretch reflexes of the normal infant.

Tendon-jerk reflexes were tested in normal, fullterm infants aged one to four days. EMGs were recorded from gastrocnemius-soleus and tibialis anterior muscles. Reflex-like EMGs were evoked when tapping sites which should not excite the muscles from which that activity is recorded--this included the simultaneous activation of antagonistic muscles by a tendon tap. The possible mechanisms which could produce these results are discussed, as are the possible relationships between the proposed pathways and the reciprocally excitatory paths in cerebral palsy.

Electromyography↗
PubMed1979

Response to sudden torques about ankle in man: myotatic reflex.

1. Sudden dorsiflexions and plantarflexions of the foot were imposed on normal human subjects under various states of voluntary activity. 2. Under conditions of constant muscle contraction, the myotatic reflex in soleus and lateral gastrocnemius muscles is linearly and highly correlated with the rate of muscle stretch. The slope of this curve characterizes part of the reflex arc "gain." 3. The gain is linearly proportional to the level of tonic voluntary activation. 4. The gain is reduced by tonic contraction of antagonists. 5. The above statements can be summarized by the following equation (formula: see text), where d theta/dt is the rate of joint rotation. Ts and Tat are measures of voluntary contraction (tension) of all the extensor and flexor muscles acting at the ankle. The term S represents the level of preexisting spinal excitability that can be altered by prior instruction to the subject. 6. A phasic voluntary contraction of the soleus muscle, which leads to muscle shortening, will alter the reflex gain. The gain initially increases with increasing rates of shortening, but at higher rates the gain is reduced. This is in contradiction to the observation for tonic activation as stated above and may be due to an inability of the coactivated fusimotor system to produce sufficiently rapid cocontraction of the spindle fibers. 7. During lengthening of a muscle caused by voluntary contraction of its antagonists, the myotatic reflex gain is reduced. 8. The above facts are interpreted to imply that a functional role for the myotatic reflex in the leg extensors is limited to conditions of postural maintenance or slow, precise movement. During rapid movement, the myotatic reflex is ineffective and load-compensating reactions are mediated by longer latency loops. 9. The duration of the myotatic reflex EMG is from 10 to 40 ms, too brief to be a simple response to a velocity-sensing receptor organ. Either the response is in large measure due to the initial burst of spindle activity that occurs at the start of a ramp stretch, or motoneuron pool dynamics act as a high-pass filter on afferent inputs. 10. In the anterior tibial muscle, the relationships between stretch velocity and reflex amplitude and tonic voluntary contraction and reflex gain are qualitatively similar to those found in the ankle extensors.

Ankle↗
PubMed1978

Stretch and Hoffmann reflexes during phasic voluntary contractions of the human soleus muscle.

Measurements were made of EMG responses in human soleus muscle, following torque disturbances of the foot during phasic voluntary plantarflexions. These were compared with measurements of the Hoffmann reflex during identical voluntary movements. We observe that stretch adequate to evoke a stretch reflex in the relaxed limb often fails to evoke any response during a phasic contraction until about 120 msec after the onset of the stimulus. During this period the H-reflex is facilitated. These findings are discussed in terms of the servo-theory of stretch reflex function and the concept of trans-cortical stretch reflexes.

Adult↗
PubMed1978

Sinusoidal oscillation of the ankle as a means of evaluating the spastic patient.

Sinusoidally modulated torque was applied to rotate the ankle joint of normal subjects and clinically spastic patients. Measurements were made of the effective joint compliance and of the evoked EMG activity. These procedures provide a well-quantified and reliable measure of muscle tone and hyperreflexia which is well correlated with the patient's clinical status.

Adult↗
PubMed1978

Cerebellar stimulation in man. Quantitative changes in spasticity.

The effects of chronic anterior lobe cerebellar stimulation on patients with cerebral palsy have been investigated using a new method of quanitfying muscle rigidity and stretch reflexes. The ankle is oscillated sinusoidally, compliance is measured at several frequencies, and electromyographic activity of the extensor and flexor muscles is recorded. In some patients stimulation can reduce rigidity and coactivation of muscles immediately or slowly over days or months. In others no change is found.

Adolescent↗
PubMed1977

Oscillation of the human ankle joint in response to applied sinusoidal torque on the foot.

1. Low-frequency (3-30 Hz) oscillatory rotation of the ankle joint in plantarflexion-dorsiflexion was generated with a torque motor. Torque, rotation about the ankle and electromyograms (e.m.g.s) for the gastrocnemius-soleus and the anterior tibial muscles were recorded.2. Fourier coefficients at each drive frequency were used to calculate the effective compliance (ratio of rotation and torque). The compliance has a sharp resonance when tonic, voluntary muscle activity is present.3. The resonant frequency of compliance is between 3 and 8 Hz. The location of the resonant frequency and the magnitude of the compliance at resonance depend upon both the degree of tonic muscle activity and the amplitude of the driving torque. The resonant frequency increases with increasing tonic activity.4. With tonic muscle activity, the compliance in the frequency range below resonance increases with increasing amplitudes of driving torque.5. The e.m.g., when evoked by the rhythmic stretch, lags the start of stretching by between 50 and 70 msec.6. When tonic muscle activity is present, the resonant frequency of the stretch reflex is between 5 and 6.5 Hz.7. Following the start of driven oscillation at frequencies near resonance, slowly increasing amplitudes of angular rotation (to a limit) are observed.8. Distortion (from the sinusoidal wave shape) of angular rotation is frequently observed with drive frequencies between 8 and 12 Hz during which there sometimes occur spontaneous recurrences of oscillation at the drive frequency. For the angular rotation, a significant portion of the power may be in subharmonic frequency components of the drive frequency when that frequency is between 8 and 12 Hz.9. Self-sustaining oscillation (clonus) near the resonant frequency of the compliance is sometimes observed after the modulation signal to the motor is turned off. This is most often seen when the gastrocnemius-soleus muscles are fatigued. Clonus may be evoked by driven oscillation at any frequency.10. The hypothesis that physiological tremor, which occurs between 8 and 12 Hz, is a consequence of stretch reflex servo properties seems to be at odds with the observations of resonance in the compliance and of self-generated clonus both occurring in the 5-8 Hz region.

Adult↗