Alexander R. Lind

Frequency analysis of the surface electromyogram during sustained isometric contractions

SummaryFour 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 unfatigued 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.

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

SummaryThe 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 subjects 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‡ 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.

Evaluation of the amplitude and frequency components of the surface EMG as an index of muscle fatigue

The frequency components and amplude of the surface electromyogram (EMG) were measured during both 3-s (tensions of 5-100% of the maximum voluntary contraction (MVC)) and fatiguing contractions at 25, 40 and 70% MVC in the handgrip, biceps, adductor pollicis and quadriceps muscles in six male subjects. For the handgrip and biceps muscles, the experiments were repeated at three different muscle lengths: the length at which the muscle was able to exert its maximum isometric strength, and the muscle length above and below that length which corresponded to a length at which the muscle could exert 80% of it maximum. The frequency components of the EMG were the same during brief fatiguing isometric contractions in any of the muscles examined here as long as the muscles contracted near their optimal length. Shortening the muscle length prior to contraction caused an increase in the power in the low frequencies of the EMG power spectra while stretching a muscle had the opposite effect during isometric contraction...

Isometric Strength and Endurance during the Menstrual Cycle

Seven healthy young women, 3 of 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 subjects forearm temperature was allowed to vary as a function of TA, and one with the muscle temperature stabilized by immersion of the forearm in water at 37‡ 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‡ 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‡ C water.

The effect of handgrip span on isometric exercise performance.

Fourteen male and eight female volunteers served as subjects in these experiments lo determine the effect of hand tool dimensions on isometric strength, endurance, the surface EMG above the active muscle, and the cardiovascular responses to isometric exercise. As reported by others, we found that for each individual, there existed one handgrip size at which he or she could exert the greatest isometric strength. Endurance was the same at any work load relative to the maximum strength for a given grip dimension. The EMG and blood pressure responses to isometric exercise were the same at any given grip span: however, the heart rate response was lowest when subjects worked with their muscles at the optimal grip span.

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

SummaryThe 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 subjects 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.

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

SummaryThe 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.

The influence of temperature on the isometric characteristics of fast and slow muscle in the cat

The maximum tetanic tension (initial strength) and the length of time, 20, 40 or 60%, of that initial strength could be maintained (endurance) was assessed in 2 fast twitch muscles (the plantaris and the medial gastrocnemius) and a slow twitch muscle (the soleus) of the cat at muscle temperatures ranging between 22 and 38°C. The results of these experiments showed that the strength of the soleus was constant between muscle temperatures of 38 to 28°C, and then gradually decreased as the muscle was cooled further. Plantaris, on the other hand, showed a reduction in strength at all muscle temperatures less than 38°C; the response of the medial gastrocnemius was between these two extremes. The longest endurance for the soleus muscle was found to be at a muscle temperature of 28°C, the endurance being as much as 3 times higher at this temperature than at muscle temperatures of 22 or 38°C. The medial gastrocnemius showed a similar response but the endurance only doubled when comparing the endurance at a muscle temperature of 28 to 38°C. In contrast, the plantaris maintained a constant endurance over the temperature range of 38 ot 28°C; below this muscle temperature the endurance decreased markedly. The mechanism for this response appeared to be related to the effect of temperature on the contractile characteristics of the muscles.

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

SummaryThe 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‡ 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‡ to 40‡ 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 around 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.

The assessment of the static component in rhythmic exercise

AbstractA 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 preceding dynamic exercise. The subjects worked on a bicycle ergometer at known fractions of their maximal aerobic capacity (max