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Biomedical subjects

G M Allen

Publications and source records attributed to G M Allen.

At least 19 recordsLinked to original sources
PubMed1998

Twitch interpolation of the elbow flexor muscles at high forces.

We investigated factors affecting maximal voluntary torque and the assessment of the level of voluntary drive in the elbow flexor muscles. First, the effective compliance of the system was tested by using single, paired, and trains of four stimuli to measure voluntary activation. At high voluntary torques the responses to all these stimuli were identical, suggesting that single stimuli are adequate for estimating voluntary drive. Second, the contribution of torque from synergist elbow flexor muscles was assessed. In attempted maximal voluntary contractions (MVCs), the voluntary activation of brachioradialis (median 91.5%, range 68.9-100%) was lower than for biceps brachii (median 99.1%, range 78.5-100%; P < 0.01). This suggests extra torque may be generated by brachioradialis during elbow flexion, beyond the torque where biceps brachii is maximally activated. Finally, lengthening of the elbow flexors occurred during MVCs, due to slight shoulder movements. This would allow force to increase independently of an increase in voluntary drive.

Adult↗
PubMed1997

Effect of contraction strength on responses in biceps brachii and adductor pollicis to transcranial magnetic stimulation.

The sizes of the motor-evoked potentials (MEPs) and the durations of the silent periods after transcranial magnetic stimulation were examined in biceps brachii, brachioradialis and adductor pollicis in human subjects. Stimuli of a wide range of intensities were given during voluntary contractions producing 0-75% of maximal force (maximal voluntary contraction, MVC). In adductor pollicis, MEPs increased in size with stimulus intensity and with weak voluntary contractions (5% MVC), but did not grow larger with stronger contractions. In the elbow flexors, MEPs grew little with stimulus intensity, but increased in size with contractions of up to 50% of maximal. In contrast, the duration of the silent period showed similar changes in the three muscles. In each muscle it increased with stimulus intensity but was unaffected by changes in contraction strength. Comparison of the responses evoked in biceps brachii by focal stimulation over the contralateral motor cortex with those evoked by stimulation with a round magnetic coil over the vertex suggests an excitatory contribution from the ipsilateral cortex during strong voluntary contractions.

Arm↗
PubMed1997

Pulmonary nodule resection during lung volume reduction surgery.

Lung volume reduction surgery (LVRS) and concomitant pulmonary nodule resection can improve the respiratory function of and remove malignant lesions in patients with chronic obstructive pulmonary disease and lung cancer. Previously, using standard selection criteria, some patients with emphysema who also had lung nodules were denied surgery because of the severity of their pulmonary dysfunction. In this article, the authors report improved pulmonary function in 11 patients with severe emphysema who underwent combined LVRS and nodule resection.

Aged↗
PubMed1997

Quantitative assessments of elbow flexor muscle performance using twitch interpolation in post-polio patients: no evidence for deterioration.

A large number of patients previously affected by polio have symptoms, including increased weakness and fatigue, which are collectively known as a post-polio syndrome. Prospective measurements of strength and endurance using twitch interpolation in post-polio patients are lacking and hence the exact rate of decline in muscle function in these patients is not well defined. We therefore measured performance of the elbow flexor muscles twice, at a mean of 2.5 years apart in a group of selected post-polio subjects (Group A, n = 13) and matched control subjects (n = 11), and in a second group of unselected polio patients from a post-polio clinic (Group B, n = 40) at a mean of 1.7 years apart. All subjects performed 10 attempted maximal voluntary isometric contractions of the elbow flexor muscles, during which voluntary activation of the elbow flexor muscles was measured using a sensitive form of twitch interpolation. The first group of selected polio subjects (Group A) and matched control subjects also performed 45 min of submaximal exercise. During this time, at 5-min intervals, maximal voluntary force, voluntary activation and the amplitude of twitch responses to single and paired stimuli were measured in order to investigate central and peripheral components of muscle fatigue. There was no change in the polio patients' strength, voluntary activation or peripheral muscle endurance between testing sessions, despite an 80% probability of detecting a 2.5% change per year in these variables. The unselected group of patients from the post-polio clinic (Group B) showed no change in maximal voluntary strength or voluntary activation between the first and second test. There was an absence of decline in muscle performance in these polio patients over the test interval, despite a subjective deterioration in muscle function consistent with the 'post-polio syndrome'. This supports the view that symptoms of the post-polio syndrome are not due to a progressive neuronal dysfunction.

Adult↗
PubMed1997

Task failure with lack of diaphragm fatigue during inspiratory resistive loading in human subjects.

Task failure during inspiratory resistive loading is thought to be accompanied by substantial peripheral fatigue of the inspiratory muscles. Six healthy subjects performed eight resistive breathing trials with loads of 35, 50, 75 and 90% of maximal inspiratory pressure (MIP) with and without supplemental oxygen. MIP measured before, after, and at every minute during the trial increased slightly during the trials, even when corrected for lung volume (e.g., for 24 trials breathing air, 12.5% increase, P < 0.05). In some trials, task failure occurred before 20 min (end point of trial), and in these trials there was an increase in end-tidal PCO2 (P < 0.01), despite the absence of peripheral muscle fatigue. In four subjects (6 trials with task failure), there was no decline in twitch amplitude with bilateral phrenic stimulation or in voluntary activation of the diaphragm, even though end-tidal PCO2 rose by 1.6 +/- 0.9%. These results suggest that hypoventilation, CO2 retention, and ultimate task failure during resistive breathing are not simply dependent on impaired force-generating capacity of the diaphragm or impaired voluntary activation of the diaphragm.

Adult↗
PubMed1996

Changes in motor cortical excitability during human muscle fatigue.

1. The excitability of the motor cortex was investigated during fatiguing con of the elbow flexors in human subjects. During sustained contractions at 30 and 1 voluntary force (MVC), the short-latency electromyographic responses (EMG) evoke brachii and brachioradialis by transcranial magnetic stimulation increased in si EMG in the elbow flexors following the evoked muscle potential (silent period), duration during a sustained MVC but not during 30% MVCs nor during a sustained M muscle (adductor pollicis). 2. When the blood supply to brachioradialis was blocked with sphygmomanometer cuff sustained MVC, the changes in EMG responses to transcranial stimulation rapidly control values, This suggests that changes in these responses during fatigue wer small-diameter muscle afferents. 3. Tendon vibration during sustained MVCs indicated that the changes in the resp cortial stimulation were not mediated by reduced muscle spindle inputs. 4. Muscle action potentials evoked in brachioradialis by electrical stimulation cervicomedullary junction did not increase in size during sustained MVCs. Thus, cortically evoked responses during sustained MVCs reflects a change in cortical Although the silent period following cervicomedullary stimulation lengthened, it substantially shorter than the cortically evoked silent period. 5. The altered EMG responses to transcranial stimulation during fatigue suggest exitation and increased inhibition in the motor cortex. As these changes were un manipulation of afferent input they presumably result from intrinsic cortical pr altered voluntary drive to the motor cortex.

Action Potentials↗
PubMed1996

Supraspinal factors in human muscle fatigue: evidence for suboptimal output from the motor cortex.

1. Voluntary activation of elbow flexor muscles can be optimal during brief maximal voluntary contractions (MVCs), although central fatigue, a progressive decline in the ability to drive the muscle maximally, develops during sustained or repeated efforts. We stimulated the motor cortex and motor point in human subjects to investigate motor output during fatigue. 2. The increment in force (relative to the voluntary force) produced by stimulation of the motor point of biceps brachii increased during sustained isometric MVCs of the elbow flexors. Motoneuronal output became suboptimal during the contraction, i.e. central fatigue developed and accounted for a small but significant loss of maximal voluntary force. During 3 min MVCs, voluntary activation of biceps fell to an average of 90.7% from an average of > 99%. 3. The increment in force (relative to the voluntary force) produced by magnetic cortical stimulation was initially small (1.0%) but also increased during sustained MVCs to 9.8% (with a 2 min MVC). Thus, cortical output was not optimal at the time of stimulation nor were sites distal to the motor cortex already acting maximally. 4. A sphygmomanometer cuff around the upper arm blocked blood supply to brachioradialis near the end of a sustained MVC and throughout subsequent brief MVCs. Neither maximal voluntary force nor voluntary activation recovered during ischaemia after the sustained MVC. However, fatigue-induced changes in EMG responses to magnetic cortical stimulation recovered rapidly despite maintained ischaemia. 5. In conclusion, during sustained MVCs, voluntary activation becomes less than optimal so that force can be increased by stimulation of the motor cortex or the motor nerve. Complex changes in excitability of the motor cortex also occur with fatigue, but can be dissociated from the impairment of voluntary activation. We argue that inadequate neural drive effectively 'upstream' of the motor cortex must be one site involved in the genesis of central fatigue.

Adult↗
PubMed1996

Reduced voluntary drive to the human diaphragm at low lung volumes.

Maximal inspiratory and transdiaphragmatic pressures vary with lung volume but the possibility that some of this variability reflects variable voluntary drive to the diaphragm has not been investigated systematically. We assessed the influence of lung volume on the ability to activate the diaphragm with voluntary effort during maximal Mueller manoeuvres. Voluntary activation of the diaphragm was assessed using twitch interpolation with bilateral phrenic nerve stimulation in 6 subjects. Each performed 10 maximal efforts at lung volumes around functional residual capacity (FRC) and additional efforts at volumes above and below FRC. Voluntary activation of the diaphragm was higher at lung volumes above FRC (> 60% TLC; activation 98.3 +/- 2.6%) than at lung volumes around FRC (45-60% TLC; activation 95.5 +/- 3.5%) and below FRC (< 45% TLC; activation 83.3 +/- 15.8%; p < 0.05). Submaximal diaphragm activation at low lung volumes may reflect differences in the length-tension relationships of the various inspiratory muscles and/or reflex inhibition of phrenic motoneurones at low lung volumes.

Adult↗
PubMed1996

Surgical treatment of emphysema using bovine pericardium strips.

Chronic pulmonary emphysema (CPE) damages lung tissue, causing it to lose elasticity and no longer exchange oxygen and carbon dioxide efficiently. In the past, lung transplantation was the only surgical treatment for CPE. Lung volume reduction surgery (LVRS) using bovine pericardium strips is a surgical approach that removes emphysematous lung tissue, allows healthy lung tissue to reexpand, and improves the functioning of the intercostal muscles and diaphragm. Surgical stapling devices buttressed with strips of bovine pericardium are used in LVRS procedures to reduce air leaks along staple lines. Patients with CPE who have undergone LVRS procedures using bovine pericardium strips have experienced a dramatic increase in their ability to exchange air, allowing them to have more normal lifestyles.

Female↗
PubMed1996

Muscle force, perceived effort, and voluntary activation of the elbow flexors assessed with sensitive twitch interpolation in fibromyalgia.

OBJECTIVE: To measure maximal voluntary strength and central activation without fatigue, and to assess both peripheral and central components of muscle fatigue of the elbow flexor muscles, during exercise, in a group of patients with fibromyalgia (FM) (n = 11). Results are compared with data from control subjects (n = 36). METHODS: Maximal voluntary activation and strength of elbow flexors were quantified using twitch interpolation during attempted maximal isometric contractions both in unfatigued muscles and during fatigue produced by 45 min of submaximal exercise. RESULTS: Maximal voluntary strength of the elbow flexors before and during exercise was within the normal range. Central fatigue did not develop to a greater extent in the patient group. No patient had a decline in twitch amplitude during exercise below the 95% confidence limit for the decline in control subjects. However, the increment in perceived effort (Borg Scale) was abnormally large in 5 patients during the fatiguing exercise. CONCLUSION: Neither poor motivation, reflex pain inhibition, nor muscle contractile failure are important in the pathogenesis of fatigue in patients with FM. However, the subjective response to exercise is commonly excessive.

Adult↗
PubMed1995

Reliability of measurements of muscle strength and voluntary activation using twitch interpolation.

We investigated the reproducibility of measurements of maximal voluntary torque and maximal voluntary activation using twitch interpolation. On 5 days, each of 5 subjects performed 10 maximal voluntary isometric contractions of their elbow flexors. Single supramaximal stimuli were delivered over biceps brachii at the measured peak torque during each effort, and in the relaxed muscle 5 s later. A voluntary activation score was calculated from the size of twitches evoked by the stimuli (resolution < 0.15 Nm). Although all subjects were able to drive the stimulated elbow flexor muscles maximally in some trials, they did not do so in 75% of all contractions. Maximal voluntary torques did not vary significantly within a subject between sessions. There were consistent differences in the level of maximal voluntary activation between subjects (P < 0.01), but no differences in voluntary activation within an individual across days in 4 of 5 subjects. Failure to drive the stimulated elbow flexor muscles maximally was not associated with inadvertent co-contraction of the antagonist muscles.

Adult↗
PubMed1995

Central fatigue. Critical issues, quantification and practical implications.

Central fatigue during exercise is the decrease in muscle force attributable to a decline in motoneuronal output. Several methods have been used to assess central fatigue; however, some are limited or not sensitive enough to detect failure in central drive. Central fatigue develops during many forms of exercise. A number of mechanisms may contribute to its development including an increased inhibition mediated by group III and IV muscle afferents along with a decrease in muscle spindle facilitation. In some situations, motor cortical output is shown to be suboptimal. A specific terminology for central fatigue is included.

Exercise↗
PubMed1994

Muscle performance, voluntary activation and perceived effort in normal subjects and patients with prior poliomyelitis.

Many people previously affected by polio complain of increased fatigue, weakness and pain many years after the initial illness. Although electromyographic abnormalities have been found in these patients, the cause of their increased weakness is not well understood. Previous studies have shown decreased strength and impaired exercise performance in those with prior polio, but the level of voluntary drive to the muscle has not been investigated. The present study investigated maximal voluntary activation without fatigue and both peripheral and central components of muscle fatigue during exercise in 21 subjects with poliomyelitis 20-40 years previously, and 20 healthy, age-matched control subjects. Voluntary activation and strength of the elbow flexors were quantified using twitch interpolation during maximal isometric voluntary contractions both at rest, and during fatigue induced by 45 min of repeated isometric contractions. Compared with the control subjects, patients with prior polio had impaired voluntary activation both when the elbow flexors were not fatigued and during fatiguing submaximal exercise. During exercise, polio subjects also had lower twitch amplitudes and increased subjective fatigue. Central and peripheral fatigue were more marked in those with the post-polio syndrome. The impaired voluntary activation with unfatigued muscles in polio subjects indicates that defective central or reflex drive may contribute to their new weakness.

Adult↗
PubMed1994

Impaired voluntary drive to breathe: a possible link between depression and unexplained ventilatory failure in asthmatic patients.

BACKGROUND: Although psychological distress predicts mortality in asthma, an underlying physiological link has not been shown. This study examined relations between impaired voluntary drive to breathe and measures of mood states. METHODS: The level of maximal voluntary activation of the diaphragm and elbow flexors was measured in a previous study using a sensitive modification of the twitch interpolation technique in 11 asthmatic and 10 control subjects. In this study psychological distress was assessed using the Profile of Mood States questionnaire and measures of distress were compared with the muscle voluntary activation results. RESULTS: For the asthmatic subjects, depressed mood increased the risk of impaired maximal voluntary activation of the diaphragm by 3.5 times (95% CI 1.09 to 11.3). No such association was observed in control subjects. CONCLUSIONS: These results suggest that depressed mood may predispose an asthmatic patient to impaired voluntary activation of the diaphragm. Such individuals would be at increased risk of rapidly developing ventilatory failure if faced with severe airway narrowing.

Asthma↗
PubMed1993

Reduced voluntary drive to breathe in asthmatic subjects.

Maximal voluntary drive to the diaphragm and a non-respiratory muscle group (elbow flexors) was compared in 10 control subjects and 11 asthmatics who were studied when well. The degree of voluntary activation during repeated attempted maximal quasi-static efforts was determined using the twitch interpolation technique in the absence of contractile fatigue under both control conditions and following bronchial challenge with histamine. Diaphragm activation was assessed using bilateral phrenic stimulation at the normal resting end-expiratory lung volume after exhalation from TLC. Asthmatic subjects showed lower and more variable voluntary activation than control subjects for both diaphragm (82.0% +/- 18.4 [SD], vs 87.8% +/- 12.0, P < 0.01) and elbow flexors (91.3% +/- 7.6 vs 95.8% +/- 4.1, P < 0.01). Histamine challenge decreased FEV1 in asthmatic subjects to 50% of the initial value, but had no significant effect on voluntary activation in either subject group. The decreased voluntary drive to the diaphragm observed in some asthmatic subjects may predispose to rapid development of ventilatory failure during severe airway narrowing.

Adult↗