Biomedical subjects

C Capaday

Publications and source records attributed to C Capaday.

26 records · Page 2Linked to original sources

PubMed1988

The trajectory of human wrist movements.

1. To determine the form of human movement trajectories and the factors that determine this form, normal subjects performed wrist flexion movements against various elastic, viscous, and inertial loads. The subjects were instructed with visual and auditory feedback to make a movement of prescribed amplitude in a present period of time, but were free to choose any trajectory that fulfilled these constraints. 2. The trajectories were examined critically to determine if they corresponded to those which would minimize the root mean square (RMS) value of some kinematic variable or of energy consumption. The data agreed better with the trajectory that minimized the RMS value of jerk (the third derivative of length) than that of acceleration. However, systematic deviations from the minimum jerk predictions were consistently observed whenever movements were made against elastic and viscous loads. 3. Improved agreement could generally be obtained by assuming that the velocity profile varied according to a normal (Gaussian) curve. We conclude that minimization of jerk is not a general principle used by the nervous system in organizing voluntary movements, although movements may approach the predicted form, particularly under inertial loading conditions. 4. The EMG of the agonist muscles consisted of relatively simple waveforms containing ramplike increases and approximately exponential decays. The form of the movements could often be predicted quite well by using the EMG as an input to a linear second-order model of the muscle plus load. Rather than rigorously minimizing a kinematic variable or energy consumption, the nervous system may generate simple waveforms and adjust the parameters of these waveforms by trial and error until a trajectory is achieved that meets the requirements for a given load.

Adult↗

PubMed1987

A method for simulating the reflex output of a motoneuron pool.

An analysis of the reflex output of a motoneuron pool in response to a Ia-afferent input is presented. The analysis is based upon a model of the motoneuron pool which includes the subthreshold behavior of motoneurons (integration of synaptic inputs) and the statistical distribution of the motoneurons according to their resting conductance. The latter feature allows for the orderly recruitment of the motoneurons in the order of low resting conductance to high resting conductance. The number of active motoneurons (i.e. the excitation level) is determined by the balance of the excitatory and inhibitory conductances acting on the pool. The reflex output in response to a Ia-EPSP is computed at various excitation levels and with different amounts of presynaptic inhibition. The reflex output is the same for a given excitation level, regardless of the mixture of excitatory and inhibitory postsynaptic conductances used to produce that excitation level. In contrast, presynaptic inhibition markedly affects the relation between reflex output and excitation level.

Action Potentials↗

PubMed1987

Difference in the amplitude of the human soleus H reflex during walking and running.

1. The Hoffman reflex, or H reflex, was strongly modulated in the human soleus muscle during both walking (4 km/h) and running (8 km/h). It was relatively low at the time of heel contact, increased progressively during the stance phase, and reached its maximum amplitude late in the stance phase. During ankle dorsiflexion the H reflex was absent. 2. During running the peak e.m.g. level of the soleus was on average 2.4 times higher than during walking but the maximum amplitude of the H reflex was never larger than during walking. In fact, the H reflex was on average significantly (P less than 0.05 for one-tailed t test) smaller during running than during walking. Furthermore, the slope of the least-squares line fitted to the relation between the H reflex amplitude and the background e.m.g. was always steeper for the walking data than for the running data. 3. The difference in the H reflex in the two tasks is evidence that the size of the H reflex is not simply a passive consequence of the alpha-motoneurone excitation level, as indicated by the e.m.g., but is also influenced by other central neural mechanisms. We suggest that presynaptic inhibition is the most likely mechanism accounting for the change in the slope. 4. The modulation of the reflexes during walking and running can be interpreted in terms of the idea of automatic gain compensation. The decreased gain during running may be appropriate to reduce saturation of motor output and potential instability of the stretch reflex feed-back loop.

H-Reflex↗

PubMed1986

Amplitude modulation of the soleus H-reflex in the human during walking and standing.

Experiments were done to determine the amplitude of the monosynaptically mediated H-reflex of the soleus muscle at various phases of the step cycle, using a computer-based analysis procedure. In all subjects tested the amplitude of the H-reflex was strongly modulated in amplitude during the walking cycle and was highest during the stance phase. In many subjects the peak reflex amplitude occurred at about the same time as the peak soleus electromyographic (EMG) activity, but in others it occurred earlier. The form of the reflex variation (i.e., envelope of H-reflex amplitude versus phase in cycle) during the step cycle could also be quite different from that of the EMG produced during stepping. At an equal stimulus strength and EMG level, the H-reflex was always much larger, up to 3.5 X, during steadily maintained contractions while standing than during walking. The large reflexes when subjects were standing are consistent with the control of position required to maintain a stable posture in this task. Similarly, the reflexes during walking are greatest during the stance phase, when they will assist in maintaining the upright position of the body against gravity. The reflexes are smallest during the swing phase when they would oppose ankle flexion. However, since the reflex amplitude is task-dependent (i.e., greater during standing than during walking at the same EMG and stimulus levels) and is not always closely related to the EMG produced during a given task such as walking, the strong modulation of H-reflex during walking is not simply a passive consequence of the alpha-motoneuron excitation level.(ABSTRACT TRUNCATED AT 250 WORDS)

Ankle↗

PubMed1983

Vibration-induced changes in movement-related EMG activity in humans.

The effect of muscle tendon vibration during voluntary arm movement was studied in normal humans. Subjects made alternating step flexion and extension movements about the elbow. A small vibrator was mounted over either the biceps or the triceps muscle and vibration was applied during flexion or extension movements. The vibrator was turned off between movements. After a period of practice, subjects learned the required movements and were able to make them with their eyes closed. Application of vibration to the muscle antagonist to the movement being performed produced an undershoot of the required end-movement position. The undershoot was 20-30% of the total movement amplitude. In contrast, vibration of the muscle agonist to the movement resulted in no change in movement end position. The vibration-induced undershoot was associated with an increase in the EMG activity of the vibrated (antagonist) muscle and a resultant increase in the ratio of the antagonist to agonist EMG activity. The increase in antagonist EMG produced by the vibration occurred with a latency of approximately 60 ms from vibration onset. The observed results are consistent with vibration-induced activation of muscle spindle receptors in the lengthening muscle during movement. It is suggested that, during movement, the sensitivity of the spindle receptors in the shortening muscle is decreased and the information concerning limb position during movement comes primarily from the lengthening muscle.

Electromyography↗

PubMed1981

The effects of muscle vibration on the attainment of intended final position during voluntary human arm movements.

Muscle tendon vibration was applied during voluntary step-tracking arm target-movements performed by normal human subjects. Vibration (freq. = 120 Hz) was applied over either the biceps or triceps tendons. During non-visually guided (eyes closed) trials, vibration of the muscle antagonistic to the movement being performed resulted in an undershoot of the required target. Thus, biceps vibration produced an undershoot of the extension target and triceps vibration an undershoot of the flexion target. The same effect occurred if the vibration was applied continuously over several movements or only during the course of individual movements. In contrast, vibration of the muscle acting as the prime mover had no effect on the correct attainment of the required target. It is suggested that the central nervous system may monitor muscle afferent activity of the lengthening (antagonist) muscle during simple, step movements.

Arm↗

PubMed1980

Nitrous oxide modifies visual responses in the cat retina, striate cortex and superior colliculus.

Extracellular records from 54 single cells in cat optic tract (14), visual cortex (18) and superior colliculus (22), have shown that ventilation of acute animals with a 70%: 30% mixture of N2O/O2 can modify unit responses to visual stimuli. Results indicate that, under nitrous oxide, (a) responses to flashed or moving stimuli may be severely reduced, and frequently abolished. This may be accompanied by either a sharp decrease; or, conversely, by a dramatic increase, in the resting discharge rate; (b) the degree of directional preference of a given unit, in response to a moving visual stimulus, may be substantially modified; (c) the temporal distribution of unit firing may be modified. While about half (57%) of the units in the optic tract were affected by N2O, only 28% of cortical cells showed any N2O-related response modification. The largest effect was observed in the superior colliculus, where 86% of cells were influenced by the anaesthetic. It is suggested that these results might be explained by a selective interference of N2O with serotonergic transmitter mechanisms.

Animals↗

PubMedDate not supplied

[Improvement of prostheses and orthotic aids for the handicapped using electric stimulation and the registration of bioelectric signals].

Electro-mechanical devices can help a variety of patients with motor disabilities. Surface EMG from remaining muscles in an amputated arm can be used to control powered electronic hands, wrists and elbows. Sensory signals such as knee angle and ankle torque can be used to control the visco-elastic properties of a knee joint for above-knee amputees. Finally, percutaneous electrodes can be used to stimulate paralyzed muscles to replace hand function in quadriplegics and leg function in paraplegics. This article summarizes recent progress in each of these areas.

Artificial Limbs↗