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Biomedical subjects

A R Lind

Publications and source records attributed to A R Lind.

At least 37 records · Page 2Linked to original sources
PubMed1980

The influence of temperature on the amplitude and frequency components of the EMG during brief and sustained isometric contractions.

The influence of temperature on the amplitude and frequency components of the EMG power spectra of the surface EMG recorded over the forearm muscles was examined in five male and five female subjects during brief and fatiguing isometric contractions of their handgrip muscles. Brief (3 s) isometric contractions were exerted at tensions ranging between 10 and 100% of each subject's maximum strength while fatiguing contractions were exerted at tensions of 25, 40, and 70% of their maximum strength. The temperature of the muscles during those contractions was varied by placing the forearms of the subjects in a controlled temperature water bath at temperatures of 10, 20, 30, and 40 degrees C. The results of these experiments showed that the center frequency of the power spectra of the surface EMG was directly related to the temperature of the exercising muscles during brief isometric contractions. During fatiguing isometric contractions, the amplitude of the EMG increased while the center frequency of the EMG power spectra decreased for all tensions examined.

Adult↗
PubMed1980

The blood pressure response during isometric exercise in fast and slow twitch skeletal muscle in the cat.

The blood pressure response during fatiguing isometric contractions was examined in a slow twitch muscle (the soleus) and a mixed muscle (the medial gastrocnemius) of the cat. The results of these experiments showed that electrical stimulation of the ventral roots of the spinal cord which carried the efferent innervation to the soleus muscle failed to result in a blood pressure response during isometric exercise. Further, although stimulation of the fast twitch motor units in the medial gastrocnemius muscle was associated with a potent pressor response to isometric exercise, stimulation of the slow twitch motor units was associated with a markedly reduced response throughout the duration of the exercise. These findings infer that the pressor response to isometric exercise may be a function of the fast twitch motor units in the muscle.

Animals↗
PubMed1980

Frequency analysis of the surface electromyogram during sustained isometric contractions.

Four male and four female volunteers served as subjects in these experiments to assess the frequency components of the surface EMG during and following brief (3 s) and sustained isometric contractions of the handgrip muscles. Two types of fatiguing contractions were performed. Contractions were either maintained to fatigue at a constant tension of up to 100% of their strength or were maintained as a sustained maximal effort in the unfatigues or previously fatigued muscle. The frequency components of the surface EMG were assessed by calculating the power spectra of 1.5 s samples of the EMG from a fundamental frequency of 4 Hz through the first 128 harmonics by Fourier analysis; the centre frequencies of the resultant power spectra were then used as an index of the mean frequency of the EMG. The results of these experiments showed that the centre frequency was independent of the tension exerted by the muscle during brief isometric contractions but decreased linearly with time throughout the duration of fatiguing isometric contractions at tensions between 25 and 100% MVC. During sustained maximal effort, the frequency initially decreased linearly with time. However, once the target tension could no longer be maintained, the centre frequency remained constant throughout the remainder of the contraction. The frequency was found to recover within 1 min following exercise at all tensions examined.

Adult↗
PubMed1979

Measurement of forearm blood flow by venous occlusion plethysmography: influence of hand blood flow during sustained and intermittent isometric exercise.

The requirement for using an arterial occlusion cuff at the wrist when measuring forearm blood flows by plethysmography was tested on a total of 8 subjects at rest and during and after sustained and intermittent isometric exercise. The contribution of the venous effluent from the hand to the forearm flow during exercise was challenged by immersing the arm in water at 20, 34, and 40 degrees C. Occlusion of the circulation to the hand reduced the blood flow through the resting forearm at all water temperatures. There was an inverse relationship between the temperature of the water and the proportion in the reduction of forearm blood flow upon inflation of the wrist-cuff, ranging from 45 to 19% at 20 degrees to 40 degrees C, respectively. However, during sustained isometric exercise at 10% of the subjects maximum voluntary contraction (MVC) there was no reduction in the measured forearm flow when an arterial occlusion cuff was inflated aroung the wrist. Similarly, there was no alteration in the blood flow measured 2 s after each of a series of intermittent isometric contractions exerted at 20% or 60% MVC for 2 s whether or not circulation to the hand was occluded nor of the post-exercise hyperemia following 1 min of sustained contraction at 40% MVC. These results indicate that a wrist-cuff is not required for accurate measurement of forearm blood flows during or after isometric exercise.

Forearm↗
PubMed1979

Isometric endurance in fast and slow muscles in the cat.

By use of a method of sequential stimulation described previously, the ability to sustain tensions of 3--100% of the initial strength (maximum isometric tension that could be developed in the fresh muscle) to fatigue (endurance) was assessed in the soleus (slow-twitch), medial gastrocnemius (mixed, fast-twitch), and plantaris (mainly fast-twitch) muscles. For all fractions of the initial strength the endurance was longest in soleus and shortest in plantaris. However, although plantaris fatigued at any tension examined, soleus could maintain a tension of up to 30% of its initial strength indefinitely with no sign of fatigue. Part of the difference in endurance between these two muscles could be accounted for in terms of blood flow because arterial occlusion sharply reduced the endurance of soleus but had only a small effect on endurance in plantaris. However, even with an occluded circulation, soleus still had substantially longer endurance than either medial gastrocnemius or plantaris. The origin of fatigue in any of the three muscles examined appeared to be in the muscle; there was no evidence of failure of transmission at the neuromuscular junction.

Animals↗
PubMed1979

The control of blood flow through human forearm muscles following brief isometric contractions.

1. The blood flow through the forearm was measured 2 sec after single, brief isometric hand-grip contractions. The tension and duration of those contractions varied from 10 to 100% of the maximal voluntary contraction (m.v.c.) and from 2 to 12 sec, respectively. 2. The blood flow increased linearly with tension up to about 60% m.v.c. but further increases in tension, up to 100% m.v.c., did not elicit higher blood flows than were found at 60% m.v.c. The same relationship between tension and the resultant blood flow held for all durations of contractions, from 2 to 12 sec. The blood flow immediately after (2 sec) contractions at a given tension increased linearly with the duration of the contraction, from 2 to 12 sec. Maximal exercise blood flow was approached only in response to the longest contractions (12 sec) at tensions of 60% m.v.c. or higher. 3. Brief alterations (2--5 sec) of transmural pressure across blood vessels did not result in a significant change of blood flow, either in the resting forearm or when the vessels were dilated by brief, isometric contractions. When the tension was applied or released either rapidly or gradually ('ramp' contractions) there was no correlation between the rate of change of stretch on arterial vessels and the resultant blood flow. However, there was a direct relationship between a force--time integral (duration of contraction x peak tension) and blood flow. All these results make it clear that changes in blood flow in the forearm elicited by brief isometric contractions are not the result of a myogenic reflex but are metabolically induced. 4. Successive contractions exerted at 60% m.v.c. for 4 sec induced a blood flow of 21.2 +/- 1.6 ml.min-1.100 ml.-1 when a rest interval of 8 sec was allowed between the contractions. Blood flows remained constant at this submaximal level, even when muscular fatigue was induced, and when there was an accompanying large increase in blood pressure. 5. Isometric muscular activity by the contralateral arm which resulted in fatigue, associated with a large increase in mean blood pressure, did not alter the level of vasodilation that was induced by brief, isometric contractions in the 'test' arm. 6. It is suggested that the vasodilatation in response to intermittent isometric contractions is the result of metabolic vasodilatation of distal segments and continued sympathetic vasoconstriction of the proximal segments of the forearm vascular bed.

Blood Pressure↗
PubMed1978

The lack of influence of reactive hyperemia on exhausting rhythmic or static exercise.

Muller (1958) and Nukada (1955) claimed that the performance of exhausting exercise can be greatly prolonged if the exercise is begun at the start of reactive hyperemia. They reported that arrest of the circulation for 10 min immediately before beginning rhythmic exercise had increased the endurance time nearly 7-fold. We have repeated Muller's and Nukada's experiments. In one series we used rhythmic exercises on a bicycle ergometer, and in another sustained contractions on a hand-grip dynamometer. Our results failed to support those of the above authors. The endurance times for rhythmic and static exercise were unaltered by previous arrest of the circulation.

Adult↗
PubMed1978

Quantitation of thermoregulatory impairment in patients with healed burns.

The basis of our study lies in the clear demonstration that rectal temperature in cool and comfortable environments is a function of the metabolic load but that once a critical level of environmental warmth has been exceeded, the rectal temperature increases above the metabolically controlled level. This effect forms the basis of the presently suggested limits for healthy individuals during heat exposure in industry. We show that impairment of thermoregulation due to extensive burns makes such environmental limits unacceptable. Our experiment was aimed at demonstrating certain tolerance limits above which the extensively burned patient (40% deep second- or third-degree burns) cannot realistically be expected to function under the workload employed. Nine subjects with varying degrees of anhidrosis were subjected to standard workloads under carefully controlled environmental conditions. We found a dramatic diminution in heat tolerance in patients with healed burns covering more than 40% of their body surface area.

Adolescent↗
PubMed1978

Comparison of metabolic and ventilatory responses of men to various lifting tasks and bicycle ergometry.

Four male volunteers served as subjects to examine the metabolic and ventilatory cost of both positive and negative lifting tasks as compared to bicycle ergometry. In different experiments, four boxes weighing 0.91, 6.82, 22.73, and 36.36 kg were lifted up to or down from a height of 60 cm at rates as high as 70 lifts/min for periods of 4 min. The data were then compared to those obtained from bicycling at a rate of 50 rpm at work loads up to 1,500 kmp/min. Work at any given box weight had a substantially higher oxygen and ventilatory cost than similar levels of work on the bicycle ergometer. The reason for these differences appeared to lie in the energy cost of moving parts of the body. When the weight of the boxes was low, there was little difference between the oxygen cost of positive and negative work, but as the weight of the boxes increased, the expected physiological differences in positive and negative work was established.

Adult↗
PubMed1978

Metabolic, cardiovascular, and respiratory factors in the development of fatigue in lifting tasks.

Three well-trained male subjects served as volunteers in these experiments to examine the physiological capacity for extended work during lifting tasks. The maximal oxygen uptake (VO2max) during lifting was always lower than work on the bicycle ergometer. However, the work load during lifting which could be maintained for 1--4 h was 50% of the VO2max for lifting each specific weight of box; the limit for lifting light boxes without fatigue was at an oxygen uptake of about 25% of the VO2max obtained from bicycle ergometry. Significant fatigue in the forearm muscles was found during prolonged lifting as assessed from the endurance of isometric contractions and from the surface electromyogram (EMG), and was more pronounced as the weight of the box increased.

Adult↗
PubMed1978

Influence of posture on isometric fatigue.

The isometric strength of four trained subjects was unaltered by changes in posture. But the endurance of an isometric contraction held to fatigue at 25 and 40% of the maximum voluntary contraction (MVC) was 20% greater in the sitting than in the recumbent posture. This difference was abolished when the exercise was performed with the arm's circulation arrested. At rest, the blood flow through the forearm was greater when the subjects were in the recumbent than in the sitting position but the reverse was true during isometric contractions. In these two postures, there was no difference in the right atrial pressure during the contraction, suggesting that the low-pressure baroreceptors are not responsible for the differences in blood flow during exercise. To date no mechanism is available to explain these observations.

Adult↗
PubMed1976

Isometric strength and endurance during the menstrual cycle.

Seven healthy young women, 3 whom had been taking oral contraceptives, were examined during the course of 2 menstrual cycles to assess their isometric strength, their endurance during a series of 5 fatiguing isometric contractions at a tension of 40% MVC, and their blood pressures and heart rates during those fatiguing contractions. Two sets of experiments were performed, one in which the subject's forearm temperature was allowed to vary as a function of T A, and one with the muscle temperature stabilized by immersion of the forearm in water at 37 degrees C. During exposure to ambient temperatures, isometric strength and both the heart rate and blood pressure responses at rest and at the end of a fatiguing, sustained isometric exercise, were not significantly different during any phase of the menstrual cycle in any subject. In contrast, the isometric endurance in the women not taking oral contraceptives varied sinusoidally in all 5 contractions with a peak endurance midway through the ovulatory phase and the lowest endurance mid-way through the luteal phase of the menstrual cycle. The isometric endurance of the women taking oral contraceptives did not vary during their menstrual cycle. After stabilization of the temperature of the muscles of the forearm in water at 37 degrees C, the isometric endurance of the normal subjects showed a hyperbolic response with the maximal endurance at the beginning and end of their cycles, and the shortest endurance at mid-cycle. Here again, however, the isometric endurance of the women taking oral contraceptives did not vary after immersion of their forearms in the 37 degree C water.

Adult↗
PubMed1975

Isometric strength, endurance, and the blood pressure and heart rate responses during isometric exercise in healthy men and women, with special reference to age and body fat content.

The relationship of sex, age, and body fat content were assessed on the maximal voluntary strength (MVC), the endurance of a sustained contraction held at 40% of the subject's maximal strength, and the associated changes in blood pressure and heart rate. Isometric hand-grip strength was, as expected, greater in men than in women; however, in both sexes, age was inversely related to isometric strength, particularly in women. Body fat content, however, was directly related to strength. In contrast, in both male and female subjects aging was directly related to isometric endurance while body fat content was now inversely related to isometric endurance. The increase in heart rate throughout a fatiguing contraction at 40% MVC was strikingly similar in men and women. Heart rate, which always increased during exercise, attained its highest magnitude during exercise in the subjects who had the highest resting heart rates. However, older subjects displayed a smaller increase in exercising heart rates than younger subjects. The blood pressure at the end of the 40% MVC was directly related to the resting blood pressure. However, aging and body fat content both increased the resting systolic blood pressure in men and women; this aging effect was further exaggerated during the exercise.

Adipose Tissue↗
PubMed1975

The assessment of the static component in rhythmic exercise.

A new approach has been devised to assess the "static component" of dynamic exercise. This technique involves the measurement of the isometric endurance of muscles which have just taken part in rhythmic exercise and depends on the repeatability of trained subjects in isometric effort. The premise is that isometric endurance will be inversely related to the static component of the preceeding dynamic exercise. The subjects worked on a bicycle ergometer at known fractions of their maximal aerobic capacity (max Vo2). The rate of pedalling was varied from 30 to 90 rpm, so that for a given % max Vo2, the belt tension varied inversely with the speed of cycling. At any one speed of cycling, isometric endurance decreased as the belt tension increased. Following exercise at 30 rpm, the isometric endurance was 25 to 50% lower than that found at the most advantageous speed of cycling for our subuects; at these faster rates of cycling two subjects showed least static component following exercise at 90 rpm while the remaining subject performed best after cycling at 50 rpm.

Adult↗
PubMed1975

The relationship of body fat content to deep muscle temperature and isometric endurance in man.

1. The relationship between body fat content, isometric endurance and deep muscle temperature was assessed in eight male and female volunteers trained to static effort. 2. Muscle temperature, measured in the belly of the brachioradialis muscle, was found to be directly related to the subject's body fat content. 3. Associated with a reduction in body fat content, there was a marked decrease in deep muscle temperature and an increase in isometric endurance of the hand-grip muscles; no change in strength was noted. 4. Conversely, an increase in body fat content resulted in an increase in deep muscle temperature and a decrease in isometric endurance. 5. When muscle temperature was stabilized before and after weight loss by immersing the forearm in water at 37 degrees C, a reduction in body fat no longer influenced endurance. 6. It was concluded that the change in isometric endurance associated with either loss or gain of weight can be accounted for entirely by the changes in muscle temperature.

Adipose Tissue↗
PubMed1975

Respiratory responses to simultaneous static and rhythmic exercises in humans.

1. Six male subjects performed simulataneous static (hand-grip) and rhythmic (bicycle ergometer) exercises while their respiratory responses were measured. 2. Oxygen consumption increased with the intensity of rhythmic work load, with a modest additional oxygen consumption accompanying the addition of static effort during the rhythmic exercise. 3. Minute ventilation (VE) increased directly with rhythmic exercise, but increased disproportionately to the metabolic need when static effort was added. The mean increment of VE elicited by the static exercise influence was nearly constant at 20 l/min, regardless of the rhythmic load present. 4. Possible reflex mechanisms which result in disruption of the normally well-matched ventilation and metabolic oxygen demands whenever static effort is present are discussed.

Adult↗