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Biomedical subjects

C B Walter

Publications and source records attributed to C B Walter.

27 records · Page 2Linked to original sources
PubMed1988

Rapid movements with reversals in direction. I. The control of movement time.

Modifications to the underlying motor control of rapid reversal movements (flexion-extension of the elbow) to accommodate experimentally induced changes in the movement time (MT) with constant movement amplitude were examined in man. MT was altered between conditions via instructions and feedback, resulting in seven distinct MT levels (from 100 to 250 ms to the reversal point) with essentially constant movement amplitude. As MT was decreased, the large increases in acceleration were met by two changes in motor control: (a) two- to three-fold increases in the peak accelerations and peak amplitudes of the agonist and antagonist EMGs, and (b) a systematic "compression" of the temporal structure of the entire acceleration-time and EMG-time patterns. This temporal "compression" with increased velocity caused by shifts in MT (distance constant) are considerably different from the constant-duration EMG bursts found when velocity is altered by changing movement distance (where MT is nearly constant). Our findings indicate that MT is a determiner of the temporal structure of rapid actions, and suggest that MT should be regarded as an important controlled variable, and not simply as an emergent property of variations in velocity.

Acceleration↗
PubMed1988

Rapid movements with reversals in direction. II. Control of movement amplitude and inertial load.

Transformations of the underlying movement control of rapid sequential (reversal) responses were examined as the movement amplitude (Experiment 1) and moment of inertia (Experiment 2) were altered, with constant movement time. Increases in amplitude and inertia were both met by sharply increased joint torques with a constant temporal structure, suggesting that the alterations may have been governed by a single gain parameter. The durations of various EMG bursts were essentially constant across changes in inertia, supporting a model in which the output of a fixed temporal representation is amplified to alter joint torques. The EMG amplitudes increased greatly with both amplitude and load. However, the fact that the EMG durations increased systematically with increases in distance provided difficulties for this model of amplitude control. The data suggest an economy in motor control in simple agravitational movements, whereby relatively simple transformations of an underlying representation can accommodate large changes in movement amplitude and moment of inertia.

Acceleration↗
PubMed1988

The influence of agonist premotor silence and the stretch-shortening cycle on contractile rate in active skeletal muscle.

Agonist premotor silence (PMS), a brief period of relative quiescence in active skeletal muscle prior to phasic activation, was investigated in subjects performing maximal contractions. The frequency of occurrence and potential function of the silent period were examined for elbow flexions and extensions. PMS was evident for movements in both directions, indicating that the mechanism is not primarily limited to extensors as previously hypothesized. Flexions demonstrating PMS exhibited increased velocity and acceleration; however, kinematic facilitation was only evident on trials exhibiting the muscular stretch-shortening cycle (SSC). The SSC was present on trials lacking PMS, demonstrating that biceps and triceps silence are not the sole determinants of preparatory agonist lengthening for elbow flexions and extensions, respectively. Taken together, the data indicate that agonist PMS is a mechanism under apparent central control that acts concomitantly with mechanical factors to potentiate elbow flexor contractions.

Adult↗
PubMed1988

The coordination of limb movements with different kinematic patterns.

The principles underlying the coordination of limb movements with different spatiotemporal features were explored. After an initial training session in which the same unidirectional movement had to be performed with both upper limbs, subjects attempted to coordinate two different movements in a second session, i.e., the learned unidirectional movement in the left limb and a new double reversal movement in the right limb. The findings uncovered a wide variety in patterns of interlimb dependence among and within subjects, going from a high degree of dependence to relative independence. The relationship between limbs was studied by means of a detailed analysis of the displacement and acceleration patterns and the electromyographic activity of the major muscles involved. The general underlying principle that appeared to account for the diversity in movement organization was this: higher independence between limb movements is achieved when subjects initiate the movements to be coordinated successively. This asynchrony in movement onset can possibly be viewed as an attempt to safeguard against interference.

Acceleration↗
PubMed1988

The force/force-variability relationship under controlled temporal conditions.

Previously, an inverted U relationship between force and force variability was demonstrated in both static and dynamic responses. Recent research suggests that the inverted U function may be due to a lack of control of the temporal aspects of the response. To investigate this hypothesis, we examined the relationship between force and force variability in rapid movements under controlled temporal conditions. Subjects (N = 4) made rapid reversal responses with a horizontal lever (using elbow flexion and extension) such that the time to reversal (160 ms) and the distance to reversal (45 degrees ) were held constant in each of six load conditions (either 0,.260,.780, 1.040, or 1.560 kg added to the lever). When time to reversal and time to peak acceleration were held constant, a curvilinear relationship between force and force variability resulted, suggesting that the inverted U function is related to control of the temporal aspects of the response.

Journal Article↗
PubMed1986

Electrophoresis in the study of diets and digestive rates of seabirds.

Attempts were made to identify unknown gut contents of seabirds by protein analysis using electrophoresis. Standards of undigested fish and squid muscle tissue were compared with muscle tissue at various stages of digestion. Digested mixtures of squid (Loligo reynaudi), Pelagic Goby (Sufflogobius bibarbatus) and Cape Anchovy (Engraulis capensis) did not resemble the undigested standards of each species respectively. Electrophoresis could prove useful in the study of differential digestion rates of seabird prey species.

Animals↗
PubMed1986

An examination of rapid positioning movements with spatiotemporal constraints.

Unidirectional positioning movements with spatiotemporal constraints were examined as a test of impulse-timing theory (Schmidt, 1976; 1980; Wallace, 1981). Movements were examined at the kinematic, kinetic, and neuromuscular levels in three experiments. In the first experiment, displacement was held constant while five different movement times were examined. Both amplitudes and durations of the EMG and the kinetic variables were related to movement time. The results generally support the impulse-timing model. In the second experiment, movements were performed to a target at each of four distances in a constant movement time. EMG and force amplitudes and, unexpectedly, accelerative-force duration were modulated to achieve changes in displacement when movement time was constant. In the third experiment, movement time and displacement were simultaneously varied resulting in four conditions with equal average velocities. The results of this experiment were not as clear and exhibited individual differences. EMG duration did not always vary with changes in movement time. The results of all three experiments could not be adequately accounted for by the impulse-timing model.

Journal Article↗
PubMed1985

Independent control of initial kinematics and terminal oscillations of rapid positioning movements.

Human subjects performed rapid elbow flexions to visual targets. Subjects were instructed to modulate characteristics of the endpoint oscillations while attempting to hold constant the amplitude and duration of the movement itself. Independent control of the initial kinematics and the frequency of terminal oscillations was observed. The view that positioning movements may be subserved by either a two-stage or time-series control system is supported.

Biomechanical Phenomena↗