Biomedical subjects

Chet T Moritz

Publications and source records attributed to Chet T Moritz.

10 recordsLinked to original sources

PubMed2006

Forelimb movements and muscle responses evoked by microstimulation of cervical spinal cord in sedated monkeys.

Documenting the forelimb responses evoked by stimulating sites in primate cervical spinal cord is significant for understanding spinal circuitry and for potential neuroprosthetic applications involving hand and arm. We examined the forelimb movements and electromyographic (EMG) muscle responses evoked by intraspinal microstimulation in three M. nemestrina monkeys sedated with ketamine. Trains of three stimulus pulses (10-80 muA) at 300 Hz were delivered at sites in regularly spaced tracks from C6 to T1. Hand and/or arm movements were evoked at 76% of the 745 sites stimulated. Specifically, movements were evoked in digits (76% of effective sites), wrist (15% of sites), elbow (26%), and shoulder (17%). To document the muscle activity evoked by a stimulus current just capable of eliciting consistent joint rotation, stimulus-triggered averages of rectified EMG were calculated at each site where a movement was observed. Typically, many muscles were coactivated at threshold currents needed to evoke movements. Out of the 13-15 muscles recorded per animal, only one muscle was active at 14% of the effective sites and two to six muscles were coactivated at 47% of sites. Thus intraspinal stimulation at threshold currents adequate for evoking movement typically coactivated multiple muscles, including antagonists. Histologic reconstruction of stimulation sites indicated that responses were elicited from the dorsal and ventral horn and from fiber tracts in the white matter, with little somatotopic organization for movement or muscle activation. The absence of a clear somatotopic map of output sites is probably a result of the stimulation of complex mixtures of fibers and cells.

Action Potentials↗

PubMed2006

Human hoppers compensate for simultaneous changes in surface compression and damping.

On a range of elastic and damped surfaces, human hoppers and runners adjust leg mechanics to maintain similar spring-like mechanics of the leg and surface combination. In a previous study of adaptations to damped surfaces, we changed surface damping and stiffness simultaneously to maintain constant surface compression. The current study investigated whether hoppers maintain spring-like mechanics of the leg-surface combination when surface damping alone changes (elastic and 1000-4800 N s m(-1)). We found that hoppers adjusted leg mechanics to maintain similar spring-like mechanics of the leg-surface combination and center of mass dynamics on all surfaces. Over the range of surface damping, vertical stiffness of the leg-surface combination increased by only 12% and center of mass displacement decreased by only 6% despite up to 55% less compression of more heavily damped surfaces. In contrast, a simulation predicted a 44% decrease in vertical displacement with no adjustment to leg mechanics. To compensate for the smaller and slower compression of more heavily damped surfaces, the stance legs compressed by up to 4.1 +/- 0.2 cm further and reached peak compression sooner. To replace energy lost by damped surfaces, hoppers performed additional leg work by extending the legs during takeoff by up to 3.1 +/- 0.2 cm further than they compressed during landing. We conclude that humans simultaneously adjust leg compression magnitude and timing, as well as mechanical work output, to conserve center of mass dynamics on damped surfaces. Runners may use similar strategies on natural energy-dissipating surfaces such as sand, mud and snow.

Adult↗

PubMed2006

The Neurochip BCI: towards a neural prosthesis for upper limb function.

The Neurochip BCI is an autonomously operating interface between an implanted computer chip and recording and stimulating electrodes in the nervous system. By converting neural activity recorded in one brain area into electrical stimuli delivered to another site, the Neurochip BCI could form the basis for a simple, direct neural prosthetic. In tests with normal, unrestrained monkeys, the Neurochip continuously recorded activity of single neurons in primary motor cortex for several weeks at a time. Cortical activity was correlated with simultaneously-recorded electromyogram (EMG) activity from arm muscles during free behavior. In separate experiments with anesthetized monkeys, we found that microstimulation of the cervical spinal cord evoked movements of the arm and hand, often involving multiple muscles synergies. These observations suggest that spinal microstimulation controlled by cortical neurons could help compensate for damaged corticospinal projections.

Animals↗

PubMed2005

Prolonged muscle vibration increases stretch reflex amplitude, motor unit discharge rate, and force fluctuations in a hand muscle.

The purpose of this study was to compare the influence of prolonged vibration of a hand muscle on the amplitude of the stretch reflex, motor unit discharge rate, and force fluctuations during steady, submaximal contractions. Thirty-two young adults performed 10 isometric contractions at a constant force (5.0 +/- 2.3% of maximal force) with the first dorsal interosseus muscle. Each contraction was held steady for 10 s, and then stretch reflexes were evoked. Subsequently, 20 subjects had vibration applied to the relaxed muscle for 30 min, and 12 subjects received no vibration. The muscle vibration induced a tonic vibration reflex. The intervention (vibration or no vibration) was followed by 2 sets of 10 constant-force contractions with applied stretches (After and Recovery trials). The mean electromyogram amplitude of the short-latency component of the stretch reflex increased by 33% during the After trials (P < 0.01) and by 38% during the Recovery trials (P < 0.01). The standard deviation of force during the steady contractions increased by 21% during the After trials (P < 0.05) and by 28% during the Recovery trials (P < 0.01). The discharge rate of motor units increased from 10.3 +/- 2.7 pulses/s (pps) before vibration to 12.2 +/- 3.1 pps (P < 0.01) during the After trials and to 11.9 +/- 2.6 pps during the Recovery trials (P < 0.01). There was no change in force fluctuations or stretch reflex magnitude for the subjects in the Control group. The results indicate that prolonged vibration increased the short-latency component of the stretch reflex, the discharge rate of motor units, and the fluctuations in force during contractions by a hand muscle. These adjustments were necessary to achieve the target force due to the vibration-induced decrease in the force capacity of the muscle.

Adolescent↗

PubMed2005

Coherence at 16-32 Hz can be caused by short-term synchrony of motor units.

Time- and frequency-domain measures of discharge times for pairs of motor units are used to infer the proportion of common synaptic input received by motor neurons. The physiological mechanisms that can produce the experimentally observed peaks in the cross-correlation histogram and the coherence spectrum are uncertain. The present study used a computational model to impose synchronization on the discharge times of motor units. Randomly selected discharge times of a unit that was being synchronized to a reference unit were aligned with some of the discharge times of the reference unit, provided the original discharge time was within 30 ms of the discharge by the reference unit. All time-domain measures (indexes CIS, E, and k') were sensitive to changes in the level of imposed motor-unit synchronization (P < 0.01). In addition, synchronization caused a peak between 16 and 32 Hz in the coherence spectrum. The shape of the cross-correlogram determined the frequency at which the peak occurred in the coherence spectrum. Further, the magnitude of the coherence peak was highly correlated with the time-domain measures of motor-unit synchronization (r2 > 0.80), with the highest correlation occurring for index E (r2 = 0.98). Thus the peak in the 16- to 32-Hz band of the coherence spectrum can be caused by the time that individual discharges are advanced or delayed to produce synchrony. Although the in vivo processes that adjust the timing of motor-unit discharges are not fully understood, these results suggest that they may not depend entirely on an oscillatory drive by the CNS.

Humans↗

PubMed2005

Human hopping on very soft elastic surfaces: implications for muscle pre-stretch and elastic energy storage in locomotion.

During hopping in place and running, humans maintain similar center of mass dynamics by precisely adjusting leg mechanics to compensate for moderate changes in surface stiffness. We investigated the limits of this precise control by asking humans to hop in place on extremely soft elastic surfaces. We found that hoppers drastically altered leg mechanics and maintained similar center of mass dynamics despite a sevenfold change in surface stiffness (11-81 kN m(-1)). On the stiffest surfaces, the legs compressed in early stance and then extended in late stance in the pattern that is typical for normal bouncing gaits. On the softest surfaces, however, subjects reversed this pattern so that the legs extended up to 8 cm in early stance and then compressed by a similar distance in late stance. Consequently, the center of mass moved downward during stance by 5-7 cm less than the surface compressed and by a similar distance as on the stiffest surfaces. This unique leg action probably reduced extensor muscle pre-stretch because the joints first extended and then flexed during stance. This interpretation is supported by the observation that hoppers increased muscle activation by 50% on the softest surface despite similar joint moments and mechanical leg work as on the stiffest surface. Thus, the extreme adjustment to leg mechanics for very soft surfaces helps maintain normal center of mass dynamics but requires high muscle activation levels due to the loss of the normal extensor muscle stretch-shorten cycle.

Biomechanical Phenomena↗

PubMed2004

Discharge rate variability influences the variation in force fluctuations across the working range of a hand muscle.

The goal of this study was to improve the ability of a motor unit model to predict experimentally measured force variability across a wide range of forces. Motor unit discharge characteristics were obtained from 38 motor units of the first dorsal interosseus muscle. Motor unit discharges were recorded in separate isometric contractions that ranged from 4 to 85% of the maximal voluntary contraction (MVC) force above recruitment threshold. High-threshold motor units exhibited both greater minimal and peak discharge rates compared with low-threshold units (P < 0.01). Minimal discharge rate increased from 7 to 23 pps, and peak discharge rate increased from 14 to 38 pps with an increase in recruitment threshold. Relative discharge rate variability (CV) decreased exponentially for each motor unit from an average of 30 to 13% as index finger force increased above recruitment threshold. In separate experiments, force variability was assessed at eight force levels from 2 to 95% MVC. The CV for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC (P < 0.01) and remained constant at higher forces (1.2-1.9%; P = 0.14). When the motor unit model was revised using these experimental findings, discharge rate variability was the critical factor that resulted in no significant difference between simulated and experimental force variability (P = 0.22) at all force levels. These results support the hypothesis that discharge rate variability is a major determinant of the trends in isometric force variability across the working range of a muscle.

Action Potentials↗

PubMed2004

Passive dynamics change leg mechanics for an unexpected surface during human hopping.

Humans running and hopping maintain similar center-of-mass motions, despite large changes in surface stiffness and damping. The goal of this study was to determine the contributions of anticipation and reaction when human hoppers encounter surprise, expected, and random changes from a soft elastic surface (27 kN/m) to a hard surface (411 kN/m). Subjects encountered the expected hard surface on every fourth hop and the random hard surface on an average of 25% of the hops in a trial. When hoppers on a soft surface were surprised by a hard surface, the ankle and knee joints were forced into greater flexion by passive interaction with the hard surface. Within 52 ms after subjects landed on the surprise hard surface, joint flexion increased, and the legs became less stiff than on the soft surface. These mechanical changes occurred before electromyography (EMG) first changed 68-188 ms after landing. Due to the fast mechanical reaction to the surprise hard surface, center-of-mass displacement and average leg stiffness were the same as on expected and random hard surfaces. This similarity is striking because subjects anticipated the expected and random hard surfaces by landing with their knees more flexed. Subjects also anticipated the expected hard surface by increasing the level of EMG by 24-76% during the 50 ms before landing. These results show that passive mechanisms alter leg stiffness for unexpected surface changes before muscle EMG changes and may be critical for adjustments to variable terrain encountered during locomotion in the natural world.

Adaptation, Physiological↗

PubMed2003

Neuromuscular changes for hopping on a range of damped surfaces.

Humans hopping and running on elastic and damped surfaces maintain similar center-of-mass dynamics by adjusting stance leg mechanics. We tested the hypothesis that the leg transitions from acting like an energy-conserving spring on elastic surfaces to a power-producing actuator on damped surfaces during hopping due to changes in ankle mechanics. To test this hypothesis, we collected surface electromyography, video kinematics, and ground reaction force while eight male subjects (body mass: 76.2 +/- 1.7 kg) hopped in place on a range of damped surfaces. On the most damped surface, most of the mechanical work done by the leg appeared at the ankle (52%), whereas 23 and 25% appeared at the knee and hip, respectively. Hoppers extended all three joints during takeoff further than they flexed during landing and thereby did more net positive work on more heavily damped surfaces. Also, all three joints reached peak flexion sooner after touchdown on more heavily damped surfaces. Consequently, peak moment occurred during joint extension rather than at peak flexion as on elastic surfaces. These strategies caused the positive work during extension to exceed the negative work during flexion to a greater extent on more heavily damped surfaces. At the muscle level, surface EMG increased by 50-440% in ankle and knee extensors as surface damping increased to compensate for greater surface energy dissipation. Our findings, and those of previous studies of hopping on elastic surfaces, show that the ankle joint is the key determinant of both springlike and actuator-like leg mechanics during hopping in place.

Aged↗

PubMed2003

Human hopping on damped surfaces: strategies for adjusting leg mechanics.

Fast-moving legged animals bounce along the ground with spring-like legs and agilely traverse variable terrain. Previous research has shown that hopping and running humans maintain the same bouncing movement of the body's centre of mass on a range of elastic surfaces by adjusting their spring-like legs to exactly offset changes in surface stiffness. This study investigated human hopping on damped surfaces that dissipated up to 72% of the hopper's mechanical energy. On these surfaces, the legs did not act like pure springs. Leg muscles performed up to 24-fold more net work to replace the energy lost by the damped surface. However, considering the leg and surface together, the combination appeared to behave like a constant stiffness spring on all damped surfaces. By conserving the mechanics of the leg-surface combination regardless of surface damping, hoppers also conserved centre-of-mass motions. Thus, the normal bouncing movements of the centre of mass in hopping are not always a direct result of spring-like leg behaviour. Conserving the trajectory of the centre of mass by maintaining spring-like mechanics of the leg-surface combination may be an important control strategy for fast-legged locomotion on variable terrain.

Biomechanical Phenomena↗