Biomedical subjects

Jane E Butler

Publications and source records attributed to Jane E Butler.

At least 19 recordsLinked to original sources

PubMed2007

Use of motor cortex stimulation to measure simultaneously the changes in dynamic muscle properties and voluntary activation in human muscles.

Force responses to transcranial magnetic stimulation of motor cortex (TMS) during exercise provide information about voluntary activation and contractile properties of the muscle. Here, TMS-generated twitches and muscle relaxation during the TMS-evoked silent period were measured in fresh, heated, and fatigued muscle. Subjects performed isometric contractions of elbow flexors in two studies. Torque and EMG were recorded from elbow flexor and extensor muscles. One study (n = 6) measured muscle contraction times and relaxation rates during brief maximal and submaximal contractions in fresh and fatigued muscle. Another study (n = 7) aimed to 1) assess the reproducibility of muscle contractile properties during brief voluntary contractions in fresh muscle, 2) validate the technique for contractile properties in passively heated muscle, and 3) apply the technique to study contractile properties during sustained maximal voluntary contractions. In both studies, muscle contractile properties during voluntary contractions were compared with the resting twitch evoked by motor nerve stimulation. Measurement of muscle contractile properties during voluntary contractions is reproducible in fresh muscle and reveals faster and slower muscle relaxation rates in heated and fatigued muscle, respectively. The technique is more sensitive to altered muscle state than the traditional motor nerve resting twitch. Use of TMS during sustained maximal contractions reveals slowing of muscle contraction and relaxation with different time courses and a decline in voluntary activation. Voluntary output from the motor cortex becomes insufficient to maintain complete activation of muscle, although slowing of muscle contraction and relaxation indicates that lower motor unit firing rates are required for fusion of force.

Adult↗

PubMed2006

Optimal electrode placement for noninvasive electrical stimulation of human abdominal muscles.

Abdominal muscles are the most important expiratory muscles for coughing. Spinal cord-injured patients have respiratory complications because of abdominal muscle weakness and paralysis and impaired ability to cough. We aimed to determine the optimal positioning of stimulating electrodes on the trunk for the noninvasive electrical activation of the abdominal muscles. In six healthy subjects, we compared twitch pressures produced by a single electrical pulse through surface electrodes placed either posterolaterally or anteriorly on the trunk with twitch pressures produced by magnetic stimulation of nerve roots at the T(10) level. A gastroesophageal catheter measured gastric pressure (Pga) and esophageal pressure (Pes). Twitches were recorded at increasing stimulus intensities at functional residual capacity (FRC) in the seated posture. The maximal intensity used was also delivered at total lung capacity (TLC). At FRC, twitch pressures were greatest with electrical stimulation posterolaterally and magnetic stimulation at T(10) and smallest at the anterior site (Pga, 30 +/- 3 and 33 +/- 6 cm H(2)O vs. 12 +/- 3 cm H(2)O; Pes 8 +/- 2 and 11 +/- 3 cm H(2)O vs. 5 +/- 1 cm H(2)O; means +/- SE). At TLC, twitch pressures were larger. The values for posterolateral electrical stimulation were comparable to those evoked by thoracic magnetic stimulation. The posterolateral stimulation site is the optimal site for generating gastric and esophageal twitch pressures with electrical stimulation.

Abdominal Muscles↗

PubMed2006

Differential activation among five human inspiratory motoneuron pools during tidal breathing.

Neural drive to inspiratory pump muscles is increased under many pathological conditions. This study determined for the first time how neural drive is distributed to five different human inspiratory pump muscles during tidal breathing. The discharge of single motor units (n = 280) from five healthy subjects in the diaphragm, scalene, second parasternal intercostal, third dorsal external intercostal, and fifth dorsal external intercostal was recorded with needle electrodes. All units increased their discharge during inspiration, but 41 (15%) discharged tonically throughout expiration. Motor unit populations from each muscle differed in the timing of their activation and in the discharge rates of their motor units. Relative to the onset of inspiratory flow, the earliest recruited muscles were the diaphragm and third dorsal external intercostal (mean onset for the population after 26 and 29% of inspiratory time). The fifth dorsal external intercostal muscle was recruited later (43% of inspiratory time; P < 0.05). Compared with the other inspiratory muscles, units in the diaphragm and third dorsal external intercostal had the highest onset (7.7 and 7.1 Hz, respectively) and peak firing frequencies (12.6 and 11.9 Hz, respectively; both P < 0.05). There was a unimodal distribution of recruitment times of motor units in all muscles. Neural drive to human inspiratory pump muscles differs in timing, strength, and distribution, presumably to achieve efficient ventilation.

Action Potentials↗

PubMed2006

Decreased input to the motor cortex increases motor cortical excitability.

OBJECTIVE: To investigate whether a short-duration reduction of input to the motor cortex affects excitability in the hand region of the motor cortex. METHODS: Subjects (n=10) received sets of transcranial magnetic stimulation of the motor cortex (TMS) and peripheral ulnar nerve stimulation. Stimuli were delivered before and after 20 min of inactivity of the test hand. The evoked compound muscle action potentials were recorded in two relaxed intrinsic hand muscles using surface EMG. RESULTS: Motor evoked potential size (MEP; expressed relative to the maximal M-wave) increased by approximately 30-40 in both hand muscles (P=0.012) following inactivity. The enlarged MEP was not associated with changes in F-wave size, a marker of motoneurone excitability, or changes in intracortical inhibition and facilitation measured with paired-pulse TMS. CONCLUSIONS: MEP growth most likely reflects an increase in motor cortical excitability. The increased excitability appears to be more associated with reduced voluntary drive to and from the motor cortex rather than reduced afferent input from the periphery. SIGNIFICANCE: These results have important implications for any investigation of motor cortical excitability in relaxed subjects. The outcome of an experimental intervention is the net result of the intervention itself and alterations in cortical excitability produced by the subjects' inactivity.

Adult↗

PubMed2006

Ia-afferent input to motoneurons during shortening and lengthening muscle contractions in humans.

The central nervous system employs different strategies to execute specific motor tasks. Because afferent feedback during shortening and lengthening muscle contractions differs, the neural strategy underlying these tasks may be quite distinct. Cortical drive may be adjusted or afferent input regulated. The exact mechanisms are not clear. Here, we examine the control of synaptic transmission across the Ia synapse during shortening and lengthening muscle contractions. Subjects were instructed to maintain isolated activity in a single tibialis anterior (TA) motor unit while muscle length was varied from flexion to extension and back. At a fixed interval after a firing of the active motor unit, a single electrical stimulus was applied to the common peroneal nerve to activate Ia afferents from the TA muscle. We investigated the stimulus-induced change in firing probability of 19 individual low-threshold TA motor units during shortening and lengthening contractions. Any change in firing probability depends on both pre- and postsynaptic mechanisms. In this experiment, motoneuron firing rate was similar during both contraction types. There was no difference in the firing probability between shortening and lengthening contractions (0.23 +/- 0.03 and 0.20 +/- 0.02, respectively). We suggest that there is no contraction type-specific control of Ia input to the motoneurons during shortening and lengthening muscle contractions. Cortical adjustments may have occurred.

Adult↗

PubMed2006

The origin of activity in the biceps brachii muscle during voluntary contractions of the contralateral elbow flexor muscles.

During strong voluntary contractions, activity is not restricted to the target muscles. Other muscles, including contralateral muscles, often contract. We used transcranial magnetic stimulation (TMS) to analyse the origin of these unintended contralateral contractions (termed "associated" contractions). Subjects (n = 9) performed maximal voluntary contractions (MVCs) with their right elbow-flexor muscles followed by submaximal contractions with their left elbow flexors. Electromyographic activity (EMG) during the submaximal contractions was matched to the associated EMG in the left biceps brachii during the right MVC. During contractions, TMS was delivered to the motor cortex of the right or left hemisphere and excitatory motor evoked potentials (MEPs) and inhibitory (silent period) responses recorded from left biceps. Changes at a spinal level were investigated using cervicomedullary stimulation to activate corticospinal paths (n = 5). Stimulation of the right hemisphere produced silent periods of comparable duration in associated and voluntary contractions (218 vs 217 ms, respectively), whereas left hemisphere stimulation caused a depression of EMG but no EMG silence in either contraction. Despite matched EMG, MEPs elicited by right hemisphere stimulation were approximately 1.5-2.5 times larger during associated compared to voluntary contractions (P < 0.005). Similar inhibition of the associated and matched voluntary activity during the silent period suggests that associated activity comes from the contralateral hemisphere and that motor areas in this (right) hemisphere are activated concomitantly with the motor areas in the left hemisphere. Comparison of the MEPs and subcortically evoked potentials implies that cortical excitability was greater in associated contractions than in the matched voluntary efforts.

Adult↗

PubMed2006

Resolving head rotation for human bipedalism.

Alignment of the body to the gravitational vertical is considered to be the key to human bipedalism. However, changes to the semicircular canals during human evolution suggest that the sense of head rotation that they provide is important for modern human bipedal locomotion. When walking, the canals signal a mix of head rotations associated with path turns, balance perturbations, and other body movements. It is uncertain how the brain uses this information. Here, we show dual roles for the semicircular canals in balance control and navigation control. We electrically evoke a head-fixed virtual rotation signal from semicircular canal nerves as subjects walk in the dark with their head held in different orientations. Depending on head orientation, we can either steer walking by "remote control" or produce balance disturbances. This shows that the brain resolves the canal signal according to head posture into Earth-referenced orthogonal components and uses rotations in vertical planes to control balance and rotations in the horizontal plane to navigate. Because the semicircular canals are concerned with movement rather than detecting vertical alignment, this result shows the importance of movement control and agility rather than precise vertical alignment of the body for human bipedalism.

Adult↗

PubMed2006

Supraspinal fatigue does not explain the sex difference in muscle fatigue of maximal contractions.

Young women are less fatigable than young men for maximal and submaximal contractions, but the contribution of supraspinal fatigue to the sex difference is not known. This study used cortical stimulation to compare the magnitude of supraspinal fatigue during sustained isometric maximal voluntary contractions (MVCs) performed with the elbow flexor muscles of young men and women. Eight women (25.6 +/- 3.6 yr, mean +/- SD) and 9 men (25.4 +/- 3.8 yr) performed six sustained MVCs (22-s duration each, separated by 10 s). Before the fatiguing contractions, the men were stronger than the women (75.9 +/- 9.2 vs. 42.7 +/- 8.0 N.m; P < 0.05) in control MVCs. Voluntary activation measured with cortical stimulation before fatigue was similar for the men and women during the final control MVC (95.7 +/- 3.0 vs. 93.3 +/- 3.6%; P > 0.05) and at the start of the fatiguing task (P > 0.05). By the end of the six sustained fatiguing MVCs, the men exhibited greater absolute and relative reductions in torque (65 +/- 3% of initial MVC) than the women (52 +/- 9%; P < 0.05). The increments in torque (superimposed twitch) generated by motor cortex stimulation during each 22-s maximal effort increased with fatigue (P < 0.05). Superimposed twitches were similar for men and women throughout the fatiguing task (5.5 +/- 4.1 vs. 7.3 +/- 4.7%; P > 0.05), as well as in the last sustained contraction (7.8 +/- 5.9 vs. 10.5 +/- 5.5%) and in brief recovery MVCs. Voluntary activation determined using an estimated control twitch was similar for the men and women at the start of the sustained maximal contractions (91.4 +/- 7.4 vs. 90.4 +/- 6.8%, n = 13) and end of the sixth contraction (77.2 +/- 13.3% vs. 73.1 +/- 19.6%, n = 10). The increase in the area of the motor-evoked potential and duration of the silent period did not differ for men and women during the fatiguing task. However, estimated resting twitch amplitude and the peak rates of muscle relaxation showed greater relative reductions at the end of the fatiguing task for the men than the women. These results indicate that the sex difference in fatigue of the elbow flexor muscles is not explained by a difference in supraspinal fatigue in men and women but is largely due to a sex difference of mechanisms located within the elbow flexor muscles.

Adult↗

PubMed2006

Fatigue-sensitive afferents inhibit extensor but not flexor motoneurons in humans.

The role of group III and IV muscle afferents in controlling the output from human muscles is poorly understood. We investigated the effects of these afferents from homonymous or antagonist muscles on motoneuron pools innervating extensor and flexor muscles of the elbow. In study 1, subjects (n = 8) performed brief maximal voluntary contractions (MVCs) of elbow extensors before and after a 2 min MVC of the extensors. During MVCs, electromyographic responses from triceps were evoked by stimulation of the corticospinal tracts [cervicomedullary motor evoked potentials (CMEPs)]. The same subjects repeated the protocol, but input from fatigue-sensitive afferents was prolonged after the fatiguing contraction by maintained muscle ischemia. In study 2, CMEPs were evoked in triceps during brief extensor MVCs before and after a 2 min sustained flexor MVC (n = 7) or in biceps during brief flexor MVCs before and after a sustained extensor MVC (n = 7). Again, ischemia was maintained after the sustained contractions. During sustained MVCs of the extensors, CMEPs in triceps decreased by approximately 35%. Without muscle ischemia, CMEPs recovered within 15 s, but with maintained ischemia, they remained depressed (by approximately 28%; p < 0.001). CMEPs in triceps were also depressed (by approximately 20%; p < 0.001) after fatiguing flexor contractions, whereas CMEPs in biceps were facilitated (by approximately 25%; p < 0.001) after fatiguing extensor contractions. During fatigue, inputs from group III and IV muscle afferents from homonymous or antagonist muscles depress extensor motoneurons but facilitate flexor motoneurons. The more pronounced inhibitory influence of these afferents on extensors suggests that these muscles may require greater cortical drive to generate force during fatigue.

Adult↗

PubMed2006

Spatial distribution of inspiratory drive to the parasternal intercostal muscles in humans.

The human parasternal intercostal muscles are obligatory inspiratory muscles with a diminishing mechanical advantage from cranial to caudal interspaces. This study determined whether inspiratory neural drive to these muscles is graded, and whether this distribution matches regional differences in inspiratory mechanical advantage. To determine the neural drive, intramuscular EMG was recorded from the first to the fifth parasternal intercostals during resting breathing in six subjects. All interspaces showed phasic inspiratory activity but the onset of activity relative to inspiratory flow in the fourth and fifth spaces was delayed compared with that in cranial interspaces. Activity in the first, second and third interspaces commenced, on average, within the first 10% of inspiratory time, and sometimes preceded inspiratory airflow. In contrast, activity in the fourth and fifth interspaces began after an average 33% of inspiratory time. The peak inspiratory discharge frequency of motor units in the first interspace averaged 13.4 +/- 1.0 Hz (mean +/- s.e.m.) and was significantly greater than in all other interspaces, in particular in the fifth space (8.0 +/- 1.0 Hz). Phasic inspiratory activity was sometimes superimposed on tonic activity. In the first interspace, only 3% of units had tonic firing, but this proportion increased to 34% in the fifth space. In five subjects, recordings were also made from the medial and lateral extent of the second parasternal intercostal. Both portions showed phasic inspiratory activity which began within the first 6% of inspiratory time. Motor units from the lateral and medial portions fired at the same peak discharge rate (10.4 +/- 0.7 versus 10.7 +/- 0.6 Hz). These observations indicate that the distribution of neural drive to the parasternal intercostals in humans has a rostrocaudal gradient, but that the drive is uniform along the mediolateral extent of the second interspace. The distribution of inspiratory neural drive to the parasternal intercostals parallels the spatial distribution of inspiratory mechanical advantage, while tonic activity was higher where mechanical advantage was lower.

Adult↗

PubMed2006

Depression of involuntary activity in muscles paralyzed by spinal cord injury.

Involuntary muscle contractions are common after spinal cord injury (SCI). Increased sensitivity to Ia muscle afferent input may contribute to the development of these spasms. Since tendon vibration results in a period of postactivation depression of the Ia synapse, we sought to determine whether Achilles tendon vibration (80 HZ for 2 s) altered involuntary contractions evoked by superficial peroneal nerve (SPN) stimulation (5 pulses at 300 HZ) in paralyzed leg muscles of subjects with chronic (>1 year) SCI. Responses to SPN stimulation that were conditioned by vibration were reduced in 66% of trials (by 33+/-12% in tibialis anterior and 40+/-16% in soleus). These reductions in electromyographic activity are unlikely to be mediated by changes at the Ia synapse or motoneuron because vibration did not alter the magnitude of the soleus H reflex. The electromyographic reductions may involve long-lasting neuromodulatory effects on spinal inhibitory interneurons or synapses involved in the flexor reflex pathway. Vibration-evoked depression of electromyographic activity may be clinically useful in controlling involuntary muscle contractions after SCI.

Adult↗

PubMed2006

Brief airway occlusion produces prolonged reflex inhibition of inspiratory muscles in obstructive sleep apnea.

STUDY OBJECTIVES: The human inspiratory muscles respond to a brief occlusion of the upper airway during inspiration with a profound short-latency reflex inhibition. This inhibition contrasts with the excitatory stretch reflex of limb muscles and may protect the airway from aspiration. It was postulated that this reflex would be altered in subjects with obstructive sleep apnea (OSA) who have repetitive upper airway occlusion. DESIGN: Subjects underwent overnight polysomnography, as well as muscle reflex studies. For the reflex studies (performed during wakefulness), occlusions lasting 250 milliseconds were delivered during inspiration. Surface electromyogram was recorded over the scalenes, parasternal intercostals, and chest wall (overlying diaphragm). SETTING: Research and sleep laboratories. PARTICIPANTS: Nineteen subjects with untreated OSA (9 moderate and 10 severe) and 9 healthy control subjects. MEASUREMENTS AND RESULTS: In the subjects with severe OSA, the duration of the inhibition was prolonged by at least 25% compared with control subjects. The peak of the inhibitory response for scalenes occurred significantly later for subjects with severe OSA than for control subjects (by 76 +/- 5 ms vs 60 +/- 3 ms [mean +/- SEM], respectively). Onset latencies of the later excitatory response were delayed for scalenes, parasternal intercostals, and chest wall recordings (eg, scalenes: 105 +/- 9 ms for subjects with severe OSA vs 83 +/- 5 ms for control subjects). CONCLUSIONS: The latency of peak inhibition and duration of inhibition were positively correlated with the respiratory disturbance index for all muscle groups. These changes may reflect adaptation in central respiratory paths due to repetitive loading during sleep.

Adult↗

PubMed2005

Tonic and phasic respiratory drives to human genioglossus motoneurons during breathing.

A tongue muscle, the genioglossus (GG), is important in maintaining pharyngeal airway patency. Previous recordings of multiunit electromyogram (EMG) suggest it is activated during inspiration in humans with some tonic activity in expiration. We recorded from populations of single motor units in GG in seven subjects during quiet breathing when awake. Ultrasonography assisted electrode placement. The activity of single units was separated into six classes based on a step-wise analysis of the discharge pattern. Phasic and tonic activities were analyzed statistically with the coefficient of determination (r2) between discharge frequency and lung volume. Of the 110 motor units, 29% discharged tonically without phasic respiratory modulation (firing rate approximately 19 Hz). Further, 16% of units increased their discharge during expiration (expiratory phasic and expiratory tonic units). Only half the units increased their discharge during inspiration (inspiratory phasic and inspiratory tonic units). Units firing tonically with an inspiratory increase had significantly higher discharge rates than those units that only fired phasically (peak rates 25 vs. 16 Hz, respectively). Simultaneous recordings of two or three motor units showed neighboring units with differing respiratory and tonic drives. Our results provide a classification and the first quantitative measures of human GG motor-unit behavior and suggest this activity results from a complex interaction of inspiratory, expiratory, and tonic drives at the hypoglossal motor nucleus. The presence of different drives to GG implies that complex premotor networks can differentially engage human hypoglossal motoneurons during respiration. This is unlike the ordered recruitment of motor units in limb and axial muscles.

Action Potentials↗

PubMed2005

Muscle fiber and motor unit behavior in the longest human skeletal muscle.

The sartorius muscle is the longest muscle in the human body. It is strap-like, up to 600 mm in length, and contains five to seven neurovascular compartments, each with a neuromuscular endplate zone. Some of its fibers terminate intrafascicularly, whereas others may run the full length of the muscle. To assess the location and timing of activation within motor units of this long muscle, we recorded electromyographic potentials from multiple intramuscular electrodes along sartorius muscle during steady voluntary contraction and analyzed their activity with spike-triggered averaging from a needle electrode inserted near the proximal end of the muscle. Approximately 30% of sartorius motor units included muscle fibers that ran the full length of the muscle, conducting action potentials at 3.9 +/- 0.1 m/s. Most motor units were innervated within a single muscle endplate zone that was not necessarily near the midpoint of the fiber. As a consequence, action potentials reached the distal end of a unit as late as 100 ms after initiation at an endplate zone. Thus, contractile activity is not synchronized along the length of single sartorius fibers. We postulate that lateral transmission of force from fiber to endomysium and a wide distribution of motor unit endplates along the muscle are critical for the efficient transmission of force from sarcomere to tendon and for the prevention of muscle injury caused by overextension of inactive regions of muscle fibers.

Adult↗

PubMed2005

Diaphragm length and neural drive after lung volume reduction surgery.

RATIONALE: Patients with chronic obstructive pulmonary disease have shorter inspiratory muscles and higher motor unit firing rates during quiet breathing than do age-matched healthy subjects. Lung volume reduction surgery (LVRS) in patients with chronic obstructive pulmonary disease improves lung function, exercise capacity, and quality of life. OBJECTIVES: We studied the effect of LVRS on length and motor unit firing rates of diaphragm and scalene muscles. METHODS: Diaphragm length was estimated by ultrasound and magnetometers, and firing rates were recorded with needle electrodes in patients (five females and seven males) with severe chronic obstructive pulmonary disease, before and after surgery. MEASUREMENTS AND MAIN RESULTS: Pre-LVRS total lung capacity was 135 +/- 10% predicted (mean +/- SD), and FEV1 was 30 +/- 12% predicted. After surgery, median firing frequency of diaphragmatic motor units fell from 17.3 +/- 4.2 to 14.5 +/- 3.4 Hz (p < 0.001), and scalene motor unit firing rates were reduced from 15.3 +/- 6.9 to 13.4 +/- 3.8 Hz (p < 0.001). Tidal volume and diaphragm length change during quiet breathing did not change, but at end expiration, the zone of apposition length of diaphragm against the rib cage (L(Zapp)) increased (30 +/- 28%, p = 0.004). Improvements in quality-of-life measures and exercise performance after surgery were related to increased forced vital capacity and L(Zapp). CONCLUSIONS: Increased diaphragm length resulted in lower motor unit firing rates and reduced breathing effort, and this is likely to contribute to improved quality of life and exercise performance after LVRS.

Aged↗

PubMed2004

Hyperthermia: a failure of the motor cortex and the muscle.

Fatigue is increased during hyperthermia, and torque declines more rapidly in sustained maximal voluntary contractions (MVCs). This can be caused by a greater decline in voluntary activation of muscle (i.e. 'central fatigue'). The present study aimed to localize the site of failure of voluntary drive during hyperthermia. Seven subjects made brief (2-3 s) and sustained (2 min) MVCs of elbow flexor muscles in two experiments. Core temperature was normal (approximately 37 degrees C) in the first experiment, and elevated (approximately 38.5 degrees C) by passive heating in the second. During some MVCs, transcranial magnetic stimulation of the motor cortex (TMS) was delivered, and the evoked torque (superimposed twitch) and EMG responses were measured. During hyperthermia, voluntary torque was reduced by approximately 2.4% during brief MVCs (P = 0.03), and decreased further (approximately 12%) during sustained MVCs (P = 0.01). The superimposed twitch amplitude in the sustained MVC was approximately 50% larger (P = 0.01). Thus, the ability to drive the muscle maximally in a sustained fashion was decreased, and some motor cortical output, which could have increased torque, remained untapped by voluntary drive. The additional central fatigue was not associated with altered motor cortical 'excitability', as EMG responses produced by TMS were similar at the two temperatures. However, the peak relaxation rate of muscle increased by approximately 20% (P = 0.005) during hyperthermia. Hence, faster motor unit firing rates would be required to produce fusion of force. The increased central fatigue during hyperthermia may represent a failure of descending voluntary drive to compensate for changed muscle properties, despite the availability of additional cortical output.

Adult↗

PubMed2004

Increased blood pressure can reduce fatigue of thenar muscles paralyzed after spinal cord injury.

The aim of this study was to evaluate whether increases in blood pressure, and presumably muscle perfusion pressure, improve the endurance of thenar muscles paralyzed chronically by cervical spinal cord injury (SCI). Resting mean arterial pressure (MAP) was low in all eight subjects (64 +/- 2 mmHg). Muscle fatigue (force decline) was produced on 2 days by intermittent supramaximal electrical stimulation of the median nerve at 20 Hz for 2 min. During one of the fatigue tests, a concurrent sustained voluntary contraction of the contralateral elbow flexors was used to increase resting MAP (by 22%, on average). Although this change in blood pressure resulted in no significant change in mean fatigue for the group, changes in MAP with exercise (median nerve stimulation with and without voluntary contraction) correlated with changes in thenar muscle fatigue in seven subjects. For every 10% increase in MAP, fatigue was reduced by approximately 3%. The data suggest that low blood pressure after chronic cervical SCI and poor blood pressure control during exercise exacerbate the fatigability of paralyzed muscles.

Adult↗

PubMed2003

Responses of human motoneurons to corticospinal stimulation during maximal voluntary contractions and ischemia.

The discharge frequency of human motoneurons declines during a sustained isometric maximal voluntary contraction (MVC) of elbow flexor muscles, but the cause is unresolved. We aimed to determine whether motoneurons were inhibited during a sustained fatiguing contraction of the elbow flexor muscles and whether this inhibition was caused by the discharge of group III and IV muscle afferents. Subjects performed brief MVCs before and after a fatiguing 2 min MVC. During maximal efforts, electromyographic responses recorded from the elbow flexor muscles were evoked by stimulation of the corticospinal tracts at the cervicomedullary level [cervicomedullary motor evoked potentials (CMEPs)] and by supramaximal stimulation over the brachial plexus (Mmax). This revealed a novel decrease in the size of the muscle response to corticospinal tract stimulation during fatigue. During the sustained MVCs, the size of CMEPs decreased to 81 +/- 15 and 78 +/- 15% of the control value for brachioradialis and biceps brachii, respectively (mean +/- SEM; n = 8). This recovered within 15 sec after the fatiguing contraction. In a second set of studies, input from group III and IV muscle afferents was prolonged after the end of the fatiguing contraction by holding the muscle ischemic with a cuff inflated above arterial pressure. Despite the maintained discharge of group III and IV afferents, the CMEPs again recovered within 15 sec of the end of the sustained contraction. These results show a diminished output of spinal motoneurons to stimulation of corticospinal tracts during a fatiguing MVC; however, the mechanisms responsible for this decline are not attributable to activity in group III and IV muscle afferents.

Adult↗