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Biomedical subjects

J F Yang

Publications and source records attributed to J F Yang.

At least 55 records · Page 3Linked to original sources
PubMed1991

H-reflex modulation during walking in spastic paretic subjects.

Hoffman (H) reflexes were elicited from the soleus muscle during treadmill walking in 21 spastic paretic patients. The soleus and tibialis anterior muscles were reciprocally activated during walking in most patients, much like that observed in healthy individuals. The pattern of H-reflex modulation varied considerably between patients, from being relatively normal in some patients to a complete absence of modulation in others. The most common pattern observed was a lack of H-reflex modulation through the stance phase and slight depression of the reflex in the swing phase, considerably less modulation than that of normal subjects under comparable walking conditions. The high reflex amplitudes during periods of the step cycle such as early stance seems to be related to the stretch-induced large electromyogram bursts in the soleus in some subjects. The abnormally active reflexes appear to contribute to the clonus encountered during walking in these patients. In three patients who were able to walk for extended periods, the effect of stimulus intensity was examined. Two of these patients showed a greater degree of reflex modulation at lower stimulus intensities, suggesting that the lack of modulation observed at higher stimulus intensities is a result of saturation of the reflex loop. In six other patients, however, no reflex modulation could be demonstrated even at very low stimulus intensities.

Adult↗
PubMed1991

Factors that determine the magnitude and time course of human H-reflexes in locomotion.

The soleus H-reflex amplitude is deeply modulated during locomotion in humans (Capaday and Stein, 1986). Moreover, at a constant stimulus intensity, the slope of the relationship between the amplitude of the soleus H-reflex and the background electromyogram (EMG) changes with different locomotor tasks (Capaday and Stein, 1987a). Two further aspects are studied here. First, we recorded the reflex during overlapping speeds of walking (2.0-7.5 km/hr) and running (5-9 km/hr) to determine whether the speed, the motor output, or the form of locomotion was most important in setting the slope of this relationship between H-reflex and background EMG. Second, we determined the time course of change in the H-reflex amplitude and the possible site of action for the reflex depression during the transition from standing to walking. The primary determinant of the slope was found to be the form of locomotion. The differences between running and walking could not be explained entirely by either movement speed or motor output. For walking, the slope varied inversely with the speed and the motor output of locomotion. This compensation in slope as a function of motor output may prevent saturation of the motoneuron pool. The appropriate reflex amplitudes for a particular locomotor pattern are activated rapidly and completely within a reaction time, and simultaneously with the activation of muscle activity for the initiation of walking. Mechanisms for the rapid change seen during the initiation of locomotion most likely act presynaptically on the muscle spindle afferents. The time course and magnitude of this change are correlated with the activity of the tibialis anterior muscle.

Electromyography↗
PubMed1990

Methods for estimating the number of motor units in human muscles.

The number of motor units in the thenar muscle group was calculated by dividing the surface electromyogram and twitch force, in maximal stimulation of the median nerve, by estimates of the average electromyogram and twitch force from single units. The following three techniques were used to estimate the average electromyogram from single units: spike-triggered averaging from units recorded with a needle electrode, intramuscular microstimulation of motor nerve branches, and graded whole nerve stimulation at the wrist. The first two techniques also provided independent estimates of motor unit numbers based on the average force generated by single units. The five estimates (three based on the electromyogram and two on force) ranged from 116 to 170 motor units in the thenar group. Correcting for cancellation when unit responses sum to form the compound action potential or twitch increased the estimated number of units, which ranged from 130 to 179. The estimates were not statistically different from one another but were substantially lower than some previous electrophysiological estimates based on graded whole nerve stimulation. The recruitment pattern of single units during whole nerve stimulation was recorded and simulated mathematically. The most likely reason for the higher estimates in previous studies using graded whole nerve stimulation is shown to be alternation of motor units. Potential errors in all the techniques are discussed and compared.

Electric Stimulation↗
PubMed1990

Motor unit numbers and contractile properties after spinal cord injury.

The number of motor units in the thenar muscle group was estimated in 11 patients with cervical spinal cord injuries. The surface electromyogram and twitch force, in response to maximal stimulation of the median nerve was divided by the average surface electromyogram and twitch of single units. The average single unit size was obtained by intramuscular microstimulation of motor nerve branches and by graded whole nerve stimulation, which provided three independent estimates, two based on the electromyogram and one based on force. The motor unit estimates from the patients covered a wide range. Some had essentially normal motor units both in numbers and contractile properties, while others had varying reductions in numbers of units. Those patients who showed a large reduction in motor unit numbers also had greatly enlarged units, which produced an average of up to sixfold the normal force. These enlarged units summed to produce maximal compound action potentials and twitches that were sometimes indistinguishable from normal. Magnetic resonance imaging scans of the cervical spine obtained from some patients provided independent evidence that patients with low motor unit counts had sustained direct injury to the anterior aspect of the spinal cord at the relevant segmental levels. Some patients showed a normal number of motor units long after the injury. No evidence of transneuronal degeneration could be demonstrated in the thenar group in these patients with the current techniques.

Adult↗
PubMed1990

Postural dynamics in the standing human.

The purpose of this study was to develop a mathematical model of the linkage dynamics in upright standing, and to use this model to study output principles for postural control. The standing human was modelled in the sagittal plane as a three-segment linkage. Mechanical disturbances were simulated as forces which could be applied at various points in this linkage. An iterative approach was used to find joint torque combinations which would restore balance within 80 ms of these mechanical disturbances. The model predicted that a specific proportional relationship was necessary between the hip, knee and ankle torques in order for balance to be restored. This proportional relationship was shown to be a function of the model structure, but independent of the location, direction and amplitude of the disturbance. These predictions were tested experimentally. A disturbance apparatus was designed to apply an impulsive force to the subjects. The joint torque responses of the subjects were in quantitative agreement with the predictions of the model. The results suggest that a fixed relationship between joint torques may be required to restore balance, and this fixed relationship may make the task of postural control simpler for the nervous system.

Computer Simulation↗
PubMed1990

Postural dynamics of walking in humans.

The dynamics of postural control in human biped locomotion were studied using (1) a model, and (2) experimentally applied impulsive force disturbances. The model was planar, and contained five rigid segments, articulating at frictionless pin joints. The model was used to identify joint torque combinations which would successfully correct for an impulsive force disturbance applied at different points in the walking cycle. The simulation results suggested that (1) early responses (within 80 ms) can be effective in compensating for impulsive disturbances, (2) the same strategies which successfully counteract similar disturbances during quiet standing are also effective in certain phases of the walking cycle, (3) modifications in the response strategies are needed to accommodate differences in the dynamics over the stride cycle, and (4) the swing leg is ineffective in compensating for disturbances in the short term. These model predictions were tested experimentally. Subject responses to an impulsive force disturbance applied during walking were studied. The electromyographic results generally support the model predictions.

Computer Simulation↗
PubMed1990

Human H-reflexes are smaller in difficult beam walking than in normal treadmill walking.

Hoffman (H) reflexes were elicited from the soleus (SOL) muscle while subjects walked on a treadmill and on a narrow beam (3.5 cm wide, raised 34 cm from the floor). The speed of walking on the treadmill was selected for each subject to match the background activation level of their SOL muscle during beam walking. The normal reciprocal activation pattern of the tibialis anterior and SOL muscles in treadmill walking was replaced by a pattern dominated by co-contraction on the beam. In addition, the step cycle duration was more variable and the time spent in the swing phase was reduced on the beam. The H-reflexes were highly modulated in both tasks, the amplitude being high in the stance phase and low in the swing phase. The H-reflex amplitude was on average 40% lower during beam walking than treadmill walking. The relationship between the H-reflex amplitude and the SOL EMG level was quantified by a regression line relating the two variables. The slope of this line was on average 41% lower in beam walking than treadmill walking. The lower H-reflex gain observed in this study and the high level of fusimotor drive observed in cats performing similar tasks suggest that the two mechanisms which control the excitability of this reflex pathway (i.e. fusimotor action and control of transmission at the muscle spindle to moto-neuron synapse) may be controlled independently.

Electromyography↗
PubMed1990

Phase-dependent reflex reversal in human leg muscles during walking.

1. Reflex responses during walking were elicited in humans by stimulation of the tibial nerve at the ankle. The stimulus intensity was controlled by monitoring the M-wave from an intrinsic foot muscle. Responses were observed in the ipsilateral tibialis anterior (TA), soleus (SO), and rectus femoris (RF) muscles. The most reproducible responses were observed at a middle latency between 50 and 90 ms. The responses were most likely of cutaneous origin, because they closely resembled the responses to stimulation of a purely cutaneous nerve, the sural nerve. 2. A reversal in the direction of the middle latency response from excitation to inhibition was observed for the first time within single muscles during walking. Evidence for a reversal was seen in all three muscles examined and in all seven subjects. 3. The reflex reversal could not be elicited in standing. An inhibition whose amplitude varied in a linear fashion with stimulus intensity and background activation level was always observed at middle latency. The responses elicited during standing resembled those during the stance phase of walking. The two tasks shared some common movement goals and appeared to make use of similar reflex pathways.

Electric Stimulation↗
PubMed1989

Backward walking: a simple reversal of forward walking?

The purpose of this study was to determine whether backward walking represented a simple temporal reversal of forward walking and, hence, could be controlled by a reversed cycling of the same group of neurons. Electromyographic (EMG), joint angle, joint moment, and joint muscle power patterns were compared for forward and backward walking, in 6 subjects. The joint angle patterns with the time-base of the backward walking reversed were similar, with the exception of the ankle. The moment patterns were similar except for the knee, whereas the joint muscle powers were almost reversed-polarity images of each other. This suggests that somewhat similar muscle activation patterns could be used to produce both modes of locomotion, but the temporal cycling of muscle contraction would be reversed: Concentric muscle activity in forward walking would become eccentric activity in backward walking, and visa versa. The EMG results generally supported these findings.

Journal Article↗
PubMed1985

Surface EMG profiles during different walking cadences in humans.

The ensemble electromyogram (EMG) patterns associated with different walking cadences were examined in 11 normal subjects. Five muscle groups were studied: the rectus femoris, vastus lateralis, lateral hamstring, tibialis anterior and soleus muscles of the right lower extremity. The myoelectric signals were telemetered, full-wave rectified and smoothed. Subjects walked at cadences of 115, 95 and 75 steps/min. Footswitches indicated the different phases of the stride. Six or more strides per subject were averaged for each cadence. Cadence-related changes in (1) mean EMG amplitude during stance, and during swing, and (2) the shape of the EMG patterns, were analyzed. One-way repeated-measures analyses of variance on the mean EMG amplitude in stance and in swing revealed significant changes with cadence (P less than 0.05) in all muscles examined. The magnitude of these changes could be related to the mechanical function of the muscles involved. The shape of the EMG patterns generally remained similar at the different cadences. The timing of EMG activity was closely related to the normalized stride time and remained invariant at different cadences.

Adolescent↗