JOURNAL
OF
APPLIED
PHYSIOLOGY
Vol. 38, No. 5, M*ay 1975.
Printed
in U.S.A.
Comparison women
of physiological
responses
and men to isometric
JERROLD S. PETROFSKY, Department of Physiology, Saint
RICHARD Louis
University
cardiovascular
responses;
exercise
L. BURSE, AXD ALEXANDER School, St. Louis, Missouri
Medical
PETKOFSKY, JERROLD S., RICHARD L. BURSE, AND ALEXANDER R. LIND. Comparison of physiological responses of women and men to isometric exercise. J. Appl. Physiol. 38(5) : 863868. 1975.-The volunteers for ,this study were 83 women, aged 19-65 yr, drawn from several different occupations. Three minutes after exerting maximal handgrip strength (MVC) each subject held a tension of 40yc MVC to fatigue. Blood pressures and heart rates were measured before, during, and after the endurance contraction. Age was associated with a reduction of strength of the women, whereas their endurance at 40y0 MVC increased. There was evidence that menopause enhanced those age effects for strength and endurance. At rest, age was associated with a decreased heart rate. As expected, the heart rates of all the women increased during the endurance contraction. But that increase was greater for the younger than for the older women, thereby exaggerating the difference due to age already seen at rest. Systolic blood pressure at rest was higher with age and, in a similar manner, that relationship was also exaggerated throughout the isometric contraction. Diastolic blood pressure, however, was not changed with age at rest, and although the diastolic pressure increased during the isometric exercise, the rate of increase was unaffected by age. The results obtained are compared with those from a similarly large number of men examined in identical circumstances. fatigue; muscular contraction; sex differences; blood pressure
of
aging;
WHEN THE ISOMETRIC TENSION exerted is greater than 15% of the subject’s own maximal strength, isometric exercise results in the rapid onset of fatigue (18, 2 1). During the resulting brief fatiguing effort, the cardiovascular system responds with a modest rise in heart rate and a dramatic rise in both systolic and diastolic blood pressures, which return rapidly to control values after exercise (15, 16) ; these studies were performed on relatively few subjects. While the results indicate that these cardiovascular responses represent a characteristic pattern, it is clear that there is quite a large variation in the absolute magnitude of the responses from person to person. The extent of that variation has recently been explored in a large number of men (19); some of the individual variability can be attributed to aging. For example, there was no statistically significant difference in strength or endurance between younger and older men, but the increments in heart rates of the younger men were higher in response to a fatiguing isometric contraction of 40 % of maximal voluntary contraction (MVC) than they were for the older men. In contrast, during such fatiguing contractions, while the
older rate,
R. LIND tZ104
men did not show as great an elevation in heart they did show a larger increase in systolic blood
pressure
.
Although these responses are now clearly established in males, practically nothing is known of the responses of females to isometric exercise. Tenuous evidence on small groups of females (22) suggests that there is no difference in the isometric endurance of the female when compared to her male counterpart if both are working at the same percentage of their maximal strength. While reports of the isometric strength and endurance of females are scant, there is no evidence at all concerning the cardiovascular responses of women during isometric exercise. It is the purpose of the present study to rectify these omissions by examining a large number of female subjects, over a wide their isometric strength and range of ages, to establish endurance values and to measure their cardiovascular responses to this form of exertion. In the course of this report, the data we obtained from a large number of women are compared with those previously obtained from men; the experimental procedures in both studies are identical. METHODS
Subjects. Eighty-three female volunteers from a variety of occupations took part, including nurses, physical therapists, utility workers, teachers, students, sales clerks, and secretaries. Subjects were examined from Fall 1971 to Spring 1973; their physical characteristics are given in Table 1. Similar characteristics for the men studied previously are provided elsewhere (19). Statistical analysis. While some of the statistical analysis of the data was performed against a continuous distribution of age, we found it worthwhile (as in our previous study on men) to examine the data with regard to the responses of age groups: a) 19-29 yr, b) 30--39 yr, c) 40-49 yr, and d) 50-65 yr; for the sake of clarity in the text, these groups The numbers of subjects in are referred to as decades. these respective groups were 33, 18, 10, and 22. Analysis of the data involved calculations of means, variances, standard deviations, and regression lines by the method of least squares. Comparisons were made by F tests and unrelated The level indicating statistical sigt-tests as appropriate. nificance was chosen to be P < 0.05. Experimental procedures. Each subject was examined on a single occasion. The subject’s age, height, weight, resting blood pressure, and brief medical history were obtained in a preliminary examination and interview. Since we wished
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864 TABLE
-.
PETROFSKY,
1.
_ --. -
General charcrcteristks of female subjects __.~_
i%F,
I>tXade
6th Decade
34.7 30-38
43.2 40-48
55.6 50-65
164.5 1.50 . o173.2
164.9 156.9176.5
157 A 150.3167.9
3rd Decade
4th Decade
19-6.5
22.8 19-29
162.7 1.50 .o176.5
164.4 157.5171.5
Total
Group
5th
+
Y’
Mean Range
Height,
36.5
cm
Mean Kange Weight, Mean Range
kg 60 .2 38 . :383 . ii
58.5 45 . o81.8
61.4 50.083.6
63.4 51 .374.8
58.6 38.368.1
10
22
BURSE,
AND
LIND
The sphygmomanometers and the handgrip used in both these studies were calibrated at the start of each experirnental day. Since the duration of the isometric contraction varied from person to person, the heart rates and blood pressures obtained during exercise were assessed directly or by interpolation at 20, 40, 60, and 80 % of the endurance time. The values of heart rate and blood pressure at the end of the contraction were obtained directly or by extrapolation from the values obtained toward the end of the contraction. The physiological responses were thereby normalized for all the subjects examined. RESULTS
No.
83
33
18
to examine. only healthy individuals, volunteers were not accepted as subjects if they had a history of any form of cardiovascular disease, their resting blood pressure exceeded 155/95 mmHg, or there was evidence of an abnormal ECG before or during the exercise stress. In consequence, we rejected more of the older women from our total volunteer group. Applying the above criteria, we eliminated two subjects from the 19- to 29-yr group, four from the 30- to 39-yr group, six from the 40- to 49-yr group, and 16 from the 50- to 65-yr group- Hence, we interviewed a total of 111 women to arrive at our pool of 83 subjects. Once a volunteer was found to be acceptable by these criteria, the experimental protocol was carefully explained to her and she was asked to take part in the experiment and, on agreeing, was required to sign a form of informed consent. Only one individual declined to participate at that point. After a demonstration of the experimental procedure, maximal voluntary strength (MVC) was measured by two brief ( < 3 s) contractions on a portable strain gauge handgrip dynamometer (19) while seated with the upper arm dependent and the forearm held horizontally; 1 min was allowed between the two measurements. The maximum strength was taken to be the stronger of the two contractions. Three minutes following the second determination of MVC, the subject exerted a tension of 40% MVC and held it to fatigue. As a result each subject worked at the same relative work load. The endurance time of the fatiguing contraction was measured to the nearest second. Before and during the endurance contraction, the subject was instructed on the importance of maintaining a steady tension and was continuously exhorted to maintain the tension to the point of fatigue. In practice, the variation in maintained tension was kept to X2 % of the target value. Blood pressure and heart rate measurements were obtained before, during, and after the exercise. Heart rates were measured from a continuously recorded ECG. Blood pressure was measured by auscultation on the left arm; measurements were made twice at rest, as often as possible during the endurance contraction, and at 30, 60, and 90 s after the isometric contraction. One observer was responsible for measuring the blood pressure in all but 17 of our female subjects; he had also measured the blood pressure in all the 100 male subjects in the earlier, cornparable study.
Isometric strength. The maximal handgrip strength values of the 83 women are depicted by the open circles in Fig. 1. The mean values for each of the four age groups are represented by the appropriate hatched bars to the right of the figure. With a correlation coefficient of -0.50, the data displayed a significant linear decrease in strength with increased age (1’ < 0.01). The prediction equation is strength (kg) = 37.22 - 0.226 age (yr). This was reflected in the means for the four age groups by increasing decades, which were, respectively, 31.5 kg, 30.0 kg, 29.5 kg, and 23.9 kg. In our subjects, the lowest strengths were found in women in the 50- to 65-yr age group. The mean strength of all the women was 28.9 & 6.1 kg, which was only 58 % of the average strength of the men (49.2 s 7.7 kg), a highly significant difference (P < 0.01). The handgrip strength of each of the 100 male subjects of our earlier study (19) is shown by the solid triangles in the upper part of Fig. 1; the solid bars to the right of the figure represent the mean strength of each of the four age groups. The tendency toward a slight decrease in strength was not statistically significant. i;or with age (r = -0.12) each of the four age groups, the mean strengths were, by increasing decade, 48.9, 52.4, 48.6, and 47.2 kg, respectively. For these male subjects, analysis of variance showed no significant difference between any of the age groups. Clearly, from the description given above, there was a c
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50
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Age
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FIG. 1. The left panel represents the individual handgrip strengths of each of the 100 men (A) and 83 women (0) of different ages. Regression lines are drawn for each sex. Bar graphs in the righthand panel display the average strength for each decade for men (hollow bars) and women (hatched bars).
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PHYSIOI,OGICAL
RESPONSES
OF
FEMALES
TO
ISOMETRIC
divergence of strength of the men and women with increasing age. In the youngest decade, the women had an ;qverage strength of about 65 % that of the men of the same age, while with each increasing decade, the strength of the women was 57, 60, and 50 %, respectively, when compared to men of the same decades. In summary, aging was associated with a decrease of strength in the women, but not in the men. Isometric endurnnce at 40% MVC. The endurance times of the 40 % MVC isometric handgrip contraction for the women are represented by the open circles in the lower portion of Fig. 2. Their average endurance was 172 & 52 S. The mean endurance times for each age group, by increasing decade, were 156, 178, 166, and 194 s, respectively. regression indicated that endurance Analysis by linear increased significantly with age (I’ < 0.01). The duration of the 40 % MVC handgrip contraction for each man is represented by the solid triangles in the upper part of Fig. 2. The average endurance time of all the men was 139 j= 31 s, some 33 s shorter than the women; thus the women’s mean endurance is significantly greater than the men’s (I’ < 0.01). The men displayed somewhat less variability in endurance and exhibited no difl’erence in endurance time with age. In summary, aging was associated with a significant rise in endurance in the women but not in the men. Hem-t rutes. The average heart rates for all the wornen taken together and for all the men combined before, during, and after the 40 % MVC handgrip contraction to fatigue were very similar. In both cases, the heart rate increases steadily in response to the isometric exercise, with the fastest rate of rise in the early part of the contraction. After the end of the contraction, the heart rate returned rapidly to resting control values. Although the average heart rates of all the women were systematically a few beats per minute
865
EXERCISE
greater than those of all the men, the absolute differences were not statistically significant. But the increment in average heart rate from rest to the end of exercise was significantly different for men and women (I’ < 0.01 ), at 24.3 beats/min for the men and 28.6 beats/min for the women. However, the women held the contraction for a longer time, and therefore their rate of rise of heart rates was not significantly different, averaging 10.5 beats/min for the men and 9.8 beats/min for the women. A similar pattern of response of heart rate during exercise is found to occur in each decade (Fig. 3). There were, however, difl’erences related to age. At rest, the older females had a lower (I’ < 0.05) heart rate than the younger women, a difference that became even rnore evident (1’ < 0.01) at the end of the contraction. For example, the average resting heart rate of the women in the youngest decade was 8 beats/‘min higher than for those in the oldest decade. At the end of the isometric contraction the difference was 17 beats/min. The major contribution to this exaggeration of the age difference with exercise is attributable to the increase in heart rate early in the contraction. By increasing decade, the increments in average heart rate from rest to the value found at 20 % of the duration of the contraction were 19, 10, 7, and 6 beats/‘min, respectively. For the remainder of the contraction, heart rate increased in parallel fashion for each decade. The similarity of the heart rate response of women and men to a 40% MVC isometric contraction is evident in the mean values for the four age decades (Fig. 3). At no time before, during, or after the fatiguing handgrip contraction were the heart rates significantly different between the men and the women in anvJ decade. It is worth noting, 1
1
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1
I
1
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1
30 - 39 yr
1
over 50 yr
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+ 0.28x
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Age (yd 2. Duration of a 40% MVC isometric for each of the 100 males (A) and 83 females Regression lines are illustrated for each sex. FIG.
60
100 306090
0 20
60
100306090
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contraction in seconds (0) of different ages.
3. Each panel shows the average heart rates for men (A) and women (0) in the 4 decades. Values are shown at rest and during and after the fatiguing 40y0 MVC contraction. FIG.
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866
PETROFSKY,
however, that there was a tendency in the three younger age groups for the exercise heart rates to be higher for women, while in the over-50-yr group, that tendency was reversed. Blood pressures. The average systolic and diastolic blood pressure responses were lower for all the women combined than for all the men combined before, during, and after the exercise. These differences in both the systolic and diastolic blood pressures were significant at the 1 % level when point-by-point comparisons were made. Rather striking is the fact that the pattern of response of blood dramatically during the exercise and pressure, increasing thereafter returning rapidly to control values, is clearly parallel for men and women. The average systolic and diastolic blood pressure responses for each age decade of the women are illustrated in Fig. 4. There was no diflerence in the diastolic pressures with age. But the systolic blood pressure was increasingly elevated with age before, during, and after the contraction. As with the heart rate, isometric exercise exaggerated the age differences in systolic pressure. For example, the mean systolic pressure of the youngest decade at rest was 117 mmHg while that of the oldest decade was 140 mmHg, a difference of 23 rnmHg. At the end of the exercise the corresponding values were 158 and 192 mmHg, respectively, a difference of 34 mmHg. These findings showed the same pattern we obtained from the men, as shown in Fig. 4, although the age effect for the men was less striking. In addition, the absolute levels of both diastolic and systolic pressures were lower for the wornen than for the men, as is evident frorn the comparison, decade by decade, shown in Fig. 5. The point-by-point comparison of the under-29and the 30- to 39-yr age groups reveals that, while parallel in response, both during and after exercise the systolic and diastolic blood pressures were lower for women than for men. However, the difference in blood
1 ”
”
“‘I’
?
220 -
I”“““’ I Al!% years A40 - 49 years 0 30 - 39 years
BURSE,
AND
LIND
pressure between men and women decreased with increasing age. For example, the average resting systolic and diastolic blood pressures for the women in the under29-yr group was 117/79 mmHg, while that of the men of the same age was 132/89 mmHg. In the over-50-yr group the value for the women was 140/85 mmHg, while that of the rnen was 143/90 mmHg. Card& work. Cardiac work was calculated as the product of heart rate and mean blood pressure in each of the four age groups by decade, before, during, and 1 min after isometric exercise. The findings indicate no systematic difference in cardiac work at rest, during exercise, or postexercise between men and women or between any two age groups of either sex. In all cases, from a resting value of about 7,600 arbitrary units, cardiac work rose in an approxirnately linear fashion during exercise to a level of about double the resting value (mean value 15,450 arbitrary units) and returned quickly to control values after the exercise. Tension-time index. The tension-time index (TTI) was calculated (heart rate X mean systolic blood pressure X systolic interval) to give an approximation of myocardial oxygen consumption in both groups of subjects (25). As was the case with cardiac work, there was no difference in TTI before, during, or after exercise in any of the four age groups of men or women. In all age groups, the TTI
1 d
0 19 - 29 years
200 I"
;2 ;i
180 -
180
.E 160 Q, 5 $ 140-$ m
120100 80 -
0 20
60
100 30 6090
0 20
60
100306090
80
0 20
60
1003060
90
% Duration--TimeFIG. 4. hlean systolic (4 upper plots) and diastolic (single lower plot) blood pressures before, during, and after the 40% MVC handgrip contraction. The average values for 19- to 29-yr-old (a), 30to 39yr-old (o), 40- to 49-yr-old (A), and >50-yr-old (A) groups for women are shown in the left panel, while the right panel represents the comparable values in men.
t of
40%
MVC
I
(4
I
0 20
60
-%
Duration--Time
I of
40%
MVC
10030 I
6090
(set)
FIG. 5. The average blood pressures of men (A) and women (0) in each of the 4 decades are represented in the separate panels. Systolic and diastolic blood pressures are shown before, during, and after the fatiguing 407& MVC handgrip contraction.
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PHYSIOLOGICAL
RESPONSES
OF
FEMALES
TO
ISOMETRIC
started at approximately 2,000 arbitrary units at rest and increased by some 50 % (to a mean value of 3,100 arbitrary units) by the end of the contraction and, following exercise, returned rapidly to control values. There were no significant correlations between TTI and age before, during, or after exercise in either the men or the women. DISCUSSION
It has been concluded, on somewhat indifferent evidence, that men and women can hold isometric endurance contractions at set proportions of their maximum strengths for the same length of time (18, 21, 22). Our results do not support this view. ‘The average strength of our group of women was, as expected, much lower (58 “r,) than that of the men, a difl’erence that increased with age. It was unexpected, then, to find that the endurance of the women, for a sustained contraction of 40 % MVC, averaged 123 % that of the men. The few earlier studies on the effect of age on the strength of women point to a diminution of strength of the order of 20-25 % as age increases’frorn 20 to 60 yr (2). Our findings the major resupport that view, but in our investigation duction occurred as age exceeded 50 yr, giving rise to the obvious speculation that this change may be related to menopause. Those women who were known to have achieved menopause had an average MVC of 23.9 kg and the duration of their 40 CT0MVC was 201 s. The corresponding values for known prernenopausal women were 29.5 kg and 165 s, respectively. The differences in strength and endurance for these two groups of women were statistically significant (Y < 0.01). There has been a recent accretion of data that may help to explain some of the differences found in strength and endurance as age increases. While cell death in the brain appears to be well substantiated as animals age, there is some argument about the loss of nerve cells in the spinal cord (10). There is agreement that there is no loss of motor neurons (4), but there is a demonstrable loss in the muscle f-iber population with age, which is attributed to a reduction in the maintenance function of motor neurons, probably related to protein synthesis within the neuronal of maintenance function in cell body (13). Th e reduction the neurons results first in the degeneration of the motor end plate and, as a result, to the removal of the trophic influence of the motor nerve on its target fibers. Guttman (13) likens this aging process to the atrophy following section of the nerve, entitling it “functional denervation.” It is well established that the trophic influence of nerves on their target tissues is potent. The characteristics of muscles comprised of predorninan tly red or predominantly white muscle fibers can be reversed by transposition of the nerves serving those muscles (6). These findings, demonstrated in the cat, have also been found to hold for the pig (20) and the rat (9). As well as clearly demonstrated microscopic and functional changes, related changes have been shown in cellular biochemistry-notably an increase in myosin ATPase activity in formerly white fibers (and the decrease in myosin ATPase in formerly red fibers) following transposition of the motor nerves (3, 7, 23). Slow (red) muscle fibers, with a high concentration of
EXERCISE
867
myoglobin, have long been associated with slow response to stimulus but with great capacity for endurance. In contrast, fast (white) muscle fibers confer speed of contraction on a muscle but have little power of endurance. These functional attributes of slow and fast fibers have been clearly demonstrated in animals and have been generally accepted to hold for human muscles, although with little direct evidence. However, a recent study (17) provides telling support. After treating men with curare, which preferentially blocks slow muscle fibers, or decamethonium, which preferentially blocks fast muscle fibers, so that strength was decreased in both cases to 50 % of the maximum, the endurance of isometric exercise was greater following administration of decamethonium. In addition, other recent evidence sheds light on the importance of aging on red and white fiber populations. While both fiber types are present in the adult, only slow iibers are present in the newborn. Differentiation of fast from slow muscle fibers occurs in mammals during the period of growth and maturation (6, 8). Furthermore there is evidence that as aging proceeds an increasing proportion of the fast muscle fibers become undifferentiated and reassume the characteristics of slow muscle fibers (12, 13). The evidence then a) of a reduction of the total number of muscle fibers as described above could account for the reduction of muscle strength with age, and b) of an increasing proportion of red muscle fibers with aging could account for the increase in isometric endurance. But other factors can modify the functional properties of muscle. It has been shown that continued physical activity retards and, up to a point, may temporarily arrest the decrease of muscular strength (2). This may well explain the fact that the men we examined, who were mainly machinists, did not show a loss of strength with age (19), while the women we examined here (whose occupations make it likelv that there was no similar continued daily activity of the forearm muscles) did. Also it seems clear that sex hormones are involved in muscular function. The strength of bovs. and girls is the same up to the point thereafter the boys become much stronger of puberty; than the girls (14). Furthermore it has been shown that the administration of methyltestosterone to eunuchoids and to patients over 50 yr of age who complained of muscle weakness and quickly developing fatigue, resulted in an increase of both their muscle strength and their endurance (26). Young men, with no such symptoms of muscular weakness, showed no change in muscular function after administration of methyltestosterone (24). In addition, our present results strongly suggest that menopause is associated with changes in both strength and endurance. However, definitive assessments of the inherent physiological factors involved in changes of isometric muscular function in aging must await further investigations. While the resting heart rate and the maximum heart rate during running or cycling declines with age for both men and women (1) the heart rates at set, submaximal levels of dynamic exercise appear to be specifically related to the oxygen requirement and do not change significantly with age (1, 28). The underlying mechanisms controlling these changes of resting and maximal heart rate with age are not understood. To explain them, various hypotheses
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868
PETROFSKY,
have been put forward, ranging from the implication of factors in the central and peripheral circulation, to changes in metabolic function of either skeletal or cardiac muscle. These hypotheses are purely speculative and remain unconvincing at present. In our studies, there was a clearly defined decrease of resting heart rate with age for women. This relationship was maintained and indeed exaggerated by the isometric exercise, at the end of which the heart rates were undoubtedly not maximal. In this respect, the influence of dynamic and isometric exercise is different and may be added to a number of other differences of cardiovast ular responses to the two types of exercise (5) The results of this study show that there was no significant diflerence in heart fates for the men and women at rest or during exercise. This point has some reievance to the fact that, despite the large difference in absolute tension exerted by the men and the women, the relative tension was the same, since it has been shown recently that the level of heart rate during isometric exercise depends on the tension exerted (12). The resting systolic blood pressure of our women and our men in&eased with age. Also the resting pressures of our young women were lower than those of our young men,
BURSE,
AND
LIND
but at age 60 there was no difference due to sex. These facts are in agreement with the extensive data compiled by the Society of Actuaries (27). Since the isometric exercise resulted in a parallel rise of blood pressure in both men and women at all ages, the pattern of blood pressure with age found at rest remained the same during the exercise. The only difference was, as with heart rate, an exaggeration of the age effect during the exercise. It is usually considered that at rest the increase in systolic pressure with age is related to a decrease in the distensibility of arteries. The same explanation would iit the fact that the age effect becomes exaggerated during isometric exercise, at a time (when either the cardiac output is increased or some increase of peripheral resistance occurs (16). However, the precise mechanisms that control the increases in heart rate and blood pressure in isometric exercise remain to be uncovered. We are indebted to Dr. P. Miller, Dr. R/I. Ellert, D. Lowery, ,J. S. Rinehart, and S. Ashby for valuable help perimen ts. This research was carried out under Dept. of Health, and Welfare Contract HSM 099-7 l-2 1. Received
for
publication
26 August
LeDonne, in these
S. ex-
Education,
1974.
KEFERENCES ASTRAND, I. Aerobic capacity of men and women with special reference to age. Acta Physiol. Stand. Sufq!~Z. 169, 1960. ASTRAND, P. 0, AND K. RODAHL. Textbook of Work Physiology. New York : McGraw-Hill, 1970. BAKANY, M., AND Ii. CLOSE. The transforrnation of myosin in cross-innervated rat muscles. ,J. Physiol., London 2 13 : 455-474, 1971. BRODY, ,A. Organization of the cerebral cortex. III. A study of aging in the human cerebral cortex. J. Camp. Neural. 102 : 5 1 l556, 1955. BRUCE, 1-L A., A. K. LIND, D. FRANKLIN, A. L. MUIR, H. R. MACDONALD, G. W. MCNICOL, AND K. W. DONALD. The effects of digoxin on fatiguing static and dynamic exercise in man. CZin. sci. 34 : 29-53, 1968. BULLER, A. J ., J. C. ECCI-ES, AND R. M. ECCLES. Interactions betweeli rnotoneurones and muscle in respect to the characteristic speeds of their responses. J. Physiol., London 150 : 417-439, 1960. BULLER, A. J., W. F. H. M. MOMMAERTS, AND K. SIRAYDARIAN. Enzymic properties of myosin in fast and slow twitch muscles of the cat following cross-innervation. J. Physiol., London 205: 58 l-597 1969 . . k. Dynainic properties of fast and slow skeletal muscles CI,OSE, of the rat during development. J. Physiol., London 173: 74-95, 1964. CLOSE, R. Dynamic properties of fast and slow skeletal muscles of the rat after nerve cross-union. J. Physzol., London 204: 331346 1969. CRI;.CHI,EY, IVY. Problems in Ageing, edited by E. V. Cowdrey. Baltimore, Md. : Williams & Wilkins, 1942, p. 5 18. DRAHOI’A, Z., AND E. GUTMAN. Long term influence of the nervous system on some metabolic differences in muscles of different function. Physiol. Rohemoslav. 12 : 339350, 1963. FUNDERBURK, C. F., S. G. HIPSKIND, K. F. WELTON, AND A. R. LIND. The development of and recovery from muscular fatigue induced by static effort at different tensions. J. AppZ. Physiol. 37: 392-396, 1974. GUTMAN, E., AND V. HANZLEKOVA. Age Changes in the Neuromuscular System. Bristol: Scientechnica Ltd., 1972, p. 82. HEUINGER, T. L. Phy.l-iology of Strength. Springfield, Ill. : Thomas, 1961.
15.
16.
17. 18.
19.
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21. 22. 23.
24.
25.
26.
27. 28.
HUMPHREYS, P. W., AND A. R. LIND. The blood flow through active and inactive muscles of the forearm during sustained handgrip contractions. J. Physiol., London 166 : 120-l 35, 1963. LIND, A. R., S. H. TAYI,OR, P. W. HUMPHREYS, B. M. KENELLY, AND K. W. DONALD. Circulatory effects of sustained voluntary muscle contraction. CZin. Sci. 27 : 229-244, 1964. MOLBECH, S., AND S. JOHANSEN. Endurance time in static work during partial curarization. J. A#. Physiol. 27 : 44-48, 1969. MONOII, H., AND J. SCEIERRER. Capacite de travail statiquc d’une groupe musculaire synergique chez l’homme. Compt. &nd. Sot. Biol. 151: 1358-1367, 1957. PETROFSKY, J. S., AND A. I
OF
APPLIED
PHYSIOLOGY
Vol. 38, No. 5, M*ay 1975.
Printed
in U.S.A.
Comparison women
of physiological
responses
and men to isometric
JERROLD S. PETROFSKY, Department of Physiology, Saint
RICHARD Louis
University
cardiovascular
responses;
exercise
L. BURSE, AXD ALEXANDER School, St. Louis, Missouri
Medical
PETKOFSKY, JERROLD S., RICHARD L. BURSE, AND ALEXANDER R. LIND. Comparison of physiological responses of women and men to isometric exercise. J. Appl. Physiol. 38(5) : 863868. 1975.-The volunteers for ,this study were 83 women, aged 19-65 yr, drawn from several different occupations. Three minutes after exerting maximal handgrip strength (MVC) each subject held a tension of 40yc MVC to fatigue. Blood pressures and heart rates were measured before, during, and after the endurance contraction. Age was associated with a reduction of strength of the women, whereas their endurance at 40y0 MVC increased. There was evidence that menopause enhanced those age effects for strength and endurance. At rest, age was associated with a decreased heart rate. As expected, the heart rates of all the women increased during the endurance contraction. But that increase was greater for the younger than for the older women, thereby exaggerating the difference due to age already seen at rest. Systolic blood pressure at rest was higher with age and, in a similar manner, that relationship was also exaggerated throughout the isometric contraction. Diastolic blood pressure, however, was not changed with age at rest, and although the diastolic pressure increased during the isometric exercise, the rate of increase was unaffected by age. The results obtained are compared with those from a similarly large number of men examined in identical circumstances. fatigue; muscular contraction; sex differences; blood pressure
of
aging;
WHEN THE ISOMETRIC TENSION exerted is greater than 15% of the subject’s own maximal strength, isometric exercise results in the rapid onset of fatigue (18, 2 1). During the resulting brief fatiguing effort, the cardiovascular system responds with a modest rise in heart rate and a dramatic rise in both systolic and diastolic blood pressures, which return rapidly to control values after exercise (15, 16) ; these studies were performed on relatively few subjects. While the results indicate that these cardiovascular responses represent a characteristic pattern, it is clear that there is quite a large variation in the absolute magnitude of the responses from person to person. The extent of that variation has recently been explored in a large number of men (19); some of the individual variability can be attributed to aging. For example, there was no statistically significant difference in strength or endurance between younger and older men, but the increments in heart rates of the younger men were higher in response to a fatiguing isometric contraction of 40 % of maximal voluntary contraction (MVC) than they were for the older men. In contrast, during such fatiguing contractions, while the
older rate,
R. LIND tZ104
men did not show as great an elevation in heart they did show a larger increase in systolic blood
pressure
.
Although these responses are now clearly established in males, practically nothing is known of the responses of females to isometric exercise. Tenuous evidence on small groups of females (22) suggests that there is no difference in the isometric endurance of the female when compared to her male counterpart if both are working at the same percentage of their maximal strength. While reports of the isometric strength and endurance of females are scant, there is no evidence at all concerning the cardiovascular responses of women during isometric exercise. It is the purpose of the present study to rectify these omissions by examining a large number of female subjects, over a wide their isometric strength and range of ages, to establish endurance values and to measure their cardiovascular responses to this form of exertion. In the course of this report, the data we obtained from a large number of women are compared with those previously obtained from men; the experimental procedures in both studies are identical. METHODS
Subjects. Eighty-three female volunteers from a variety of occupations took part, including nurses, physical therapists, utility workers, teachers, students, sales clerks, and secretaries. Subjects were examined from Fall 1971 to Spring 1973; their physical characteristics are given in Table 1. Similar characteristics for the men studied previously are provided elsewhere (19). Statistical analysis. While some of the statistical analysis of the data was performed against a continuous distribution of age, we found it worthwhile (as in our previous study on men) to examine the data with regard to the responses of age groups: a) 19-29 yr, b) 30--39 yr, c) 40-49 yr, and d) 50-65 yr; for the sake of clarity in the text, these groups The numbers of subjects in are referred to as decades. these respective groups were 33, 18, 10, and 22. Analysis of the data involved calculations of means, variances, standard deviations, and regression lines by the method of least squares. Comparisons were made by F tests and unrelated The level indicating statistical sigt-tests as appropriate. nificance was chosen to be P < 0.05. Experimental procedures. Each subject was examined on a single occasion. The subject’s age, height, weight, resting blood pressure, and brief medical history were obtained in a preliminary examination and interview. Since we wished
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864 TABLE
-.
PETROFSKY,
1.
_ --. -
General charcrcteristks of female subjects __.~_
i%F,
I>tXade
6th Decade
34.7 30-38
43.2 40-48
55.6 50-65
164.5 1.50 . o173.2
164.9 156.9176.5
157 A 150.3167.9
3rd Decade
4th Decade
19-6.5
22.8 19-29
162.7 1.50 .o176.5
164.4 157.5171.5
Total
Group
5th
+
Y’
Mean Range
Height,
36.5
cm
Mean Kange Weight, Mean Range
kg 60 .2 38 . :383 . ii
58.5 45 . o81.8
61.4 50.083.6
63.4 51 .374.8
58.6 38.368.1
10
22
BURSE,
AND
LIND
The sphygmomanometers and the handgrip used in both these studies were calibrated at the start of each experirnental day. Since the duration of the isometric contraction varied from person to person, the heart rates and blood pressures obtained during exercise were assessed directly or by interpolation at 20, 40, 60, and 80 % of the endurance time. The values of heart rate and blood pressure at the end of the contraction were obtained directly or by extrapolation from the values obtained toward the end of the contraction. The physiological responses were thereby normalized for all the subjects examined. RESULTS
No.
83
33
18
to examine. only healthy individuals, volunteers were not accepted as subjects if they had a history of any form of cardiovascular disease, their resting blood pressure exceeded 155/95 mmHg, or there was evidence of an abnormal ECG before or during the exercise stress. In consequence, we rejected more of the older women from our total volunteer group. Applying the above criteria, we eliminated two subjects from the 19- to 29-yr group, four from the 30- to 39-yr group, six from the 40- to 49-yr group, and 16 from the 50- to 65-yr group- Hence, we interviewed a total of 111 women to arrive at our pool of 83 subjects. Once a volunteer was found to be acceptable by these criteria, the experimental protocol was carefully explained to her and she was asked to take part in the experiment and, on agreeing, was required to sign a form of informed consent. Only one individual declined to participate at that point. After a demonstration of the experimental procedure, maximal voluntary strength (MVC) was measured by two brief ( < 3 s) contractions on a portable strain gauge handgrip dynamometer (19) while seated with the upper arm dependent and the forearm held horizontally; 1 min was allowed between the two measurements. The maximum strength was taken to be the stronger of the two contractions. Three minutes following the second determination of MVC, the subject exerted a tension of 40% MVC and held it to fatigue. As a result each subject worked at the same relative work load. The endurance time of the fatiguing contraction was measured to the nearest second. Before and during the endurance contraction, the subject was instructed on the importance of maintaining a steady tension and was continuously exhorted to maintain the tension to the point of fatigue. In practice, the variation in maintained tension was kept to X2 % of the target value. Blood pressure and heart rate measurements were obtained before, during, and after the exercise. Heart rates were measured from a continuously recorded ECG. Blood pressure was measured by auscultation on the left arm; measurements were made twice at rest, as often as possible during the endurance contraction, and at 30, 60, and 90 s after the isometric contraction. One observer was responsible for measuring the blood pressure in all but 17 of our female subjects; he had also measured the blood pressure in all the 100 male subjects in the earlier, cornparable study.
Isometric strength. The maximal handgrip strength values of the 83 women are depicted by the open circles in Fig. 1. The mean values for each of the four age groups are represented by the appropriate hatched bars to the right of the figure. With a correlation coefficient of -0.50, the data displayed a significant linear decrease in strength with increased age (1’ < 0.01). The prediction equation is strength (kg) = 37.22 - 0.226 age (yr). This was reflected in the means for the four age groups by increasing decades, which were, respectively, 31.5 kg, 30.0 kg, 29.5 kg, and 23.9 kg. In our subjects, the lowest strengths were found in women in the 50- to 65-yr age group. The mean strength of all the women was 28.9 & 6.1 kg, which was only 58 % of the average strength of the men (49.2 s 7.7 kg), a highly significant difference (P < 0.01). The handgrip strength of each of the 100 male subjects of our earlier study (19) is shown by the solid triangles in the upper part of Fig. 1; the solid bars to the right of the figure represent the mean strength of each of the four age groups. The tendency toward a slight decrease in strength was not statistically significant. i;or with age (r = -0.12) each of the four age groups, the mean strengths were, by increasing decade, 48.9, 52.4, 48.6, and 47.2 kg, respectively. For these male subjects, analysis of variance showed no significant difference between any of the age groups. Clearly, from the description given above, there was a c
I
I
I
I
I
I
1
70 60
A
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50
f
40
c Q)
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0
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30
40
50
60
70
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Age
~2930-3940-49
250
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FIG. 1. The left panel represents the individual handgrip strengths of each of the 100 men (A) and 83 women (0) of different ages. Regression lines are drawn for each sex. Bar graphs in the righthand panel display the average strength for each decade for men (hollow bars) and women (hatched bars).
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PHYSIOI,OGICAL
RESPONSES
OF
FEMALES
TO
ISOMETRIC
divergence of strength of the men and women with increasing age. In the youngest decade, the women had an ;qverage strength of about 65 % that of the men of the same age, while with each increasing decade, the strength of the women was 57, 60, and 50 %, respectively, when compared to men of the same decades. In summary, aging was associated with a decrease of strength in the women, but not in the men. Isometric endurnnce at 40% MVC. The endurance times of the 40 % MVC isometric handgrip contraction for the women are represented by the open circles in the lower portion of Fig. 2. Their average endurance was 172 & 52 S. The mean endurance times for each age group, by increasing decade, were 156, 178, 166, and 194 s, respectively. regression indicated that endurance Analysis by linear increased significantly with age (I’ < 0.01). The duration of the 40 % MVC handgrip contraction for each man is represented by the solid triangles in the upper part of Fig. 2. The average endurance time of all the men was 139 j= 31 s, some 33 s shorter than the women; thus the women’s mean endurance is significantly greater than the men’s (I’ < 0.01). The men displayed somewhat less variability in endurance and exhibited no difl’erence in endurance time with age. In summary, aging was associated with a significant rise in endurance in the women but not in the men. Hem-t rutes. The average heart rates for all the wornen taken together and for all the men combined before, during, and after the 40 % MVC handgrip contraction to fatigue were very similar. In both cases, the heart rate increases steadily in response to the isometric exercise, with the fastest rate of rise in the early part of the contraction. After the end of the contraction, the heart rate returned rapidly to resting control values. Although the average heart rates of all the women were systematically a few beats per minute
865
EXERCISE
greater than those of all the men, the absolute differences were not statistically significant. But the increment in average heart rate from rest to the end of exercise was significantly different for men and women (I’ < 0.01 ), at 24.3 beats/min for the men and 28.6 beats/min for the women. However, the women held the contraction for a longer time, and therefore their rate of rise of heart rates was not significantly different, averaging 10.5 beats/min for the men and 9.8 beats/min for the women. A similar pattern of response of heart rate during exercise is found to occur in each decade (Fig. 3). There were, however, difl’erences related to age. At rest, the older females had a lower (I’ < 0.05) heart rate than the younger women, a difference that became even rnore evident (1’ < 0.01) at the end of the contraction. For example, the average resting heart rate of the women in the youngest decade was 8 beats/‘min higher than for those in the oldest decade. At the end of the isometric contraction the difference was 17 beats/min. The major contribution to this exaggeration of the age difference with exercise is attributable to the increase in heart rate early in the contraction. By increasing decade, the increments in average heart rate from rest to the value found at 20 % of the duration of the contraction were 19, 10, 7, and 6 beats/‘min, respectively. For the remainder of the contraction, heart rate increased in parallel fashion for each decade. The similarity of the heart rate response of women and men to a 40% MVC isometric contraction is evident in the mean values for the four age decades (Fig. 3). At no time before, during, or after the fatiguing handgrip contraction were the heart rates significantly different between the men and the women in anvJ decade. It is worth noting, 1
1
I
1
I
1
1
1
1
30 - 39 yr
1
over 50 yr
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I
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120
r 290
-
230
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1
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y = 128
+ 0.28x
; ';
80
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m I 0
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00
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Age (yd 2. Duration of a 40% MVC isometric for each of the 100 males (A) and 83 females Regression lines are illustrated for each sex. FIG.
60
100 306090
0 20
60
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contraction in seconds (0) of different ages.
3. Each panel shows the average heart rates for men (A) and women (0) in the 4 decades. Values are shown at rest and during and after the fatiguing 40y0 MVC contraction. FIG.
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866
PETROFSKY,
however, that there was a tendency in the three younger age groups for the exercise heart rates to be higher for women, while in the over-50-yr group, that tendency was reversed. Blood pressures. The average systolic and diastolic blood pressure responses were lower for all the women combined than for all the men combined before, during, and after the exercise. These differences in both the systolic and diastolic blood pressures were significant at the 1 % level when point-by-point comparisons were made. Rather striking is the fact that the pattern of response of blood dramatically during the exercise and pressure, increasing thereafter returning rapidly to control values, is clearly parallel for men and women. The average systolic and diastolic blood pressure responses for each age decade of the women are illustrated in Fig. 4. There was no diflerence in the diastolic pressures with age. But the systolic blood pressure was increasingly elevated with age before, during, and after the contraction. As with the heart rate, isometric exercise exaggerated the age differences in systolic pressure. For example, the mean systolic pressure of the youngest decade at rest was 117 mmHg while that of the oldest decade was 140 mmHg, a difference of 23 rnmHg. At the end of the exercise the corresponding values were 158 and 192 mmHg, respectively, a difference of 34 mmHg. These findings showed the same pattern we obtained from the men, as shown in Fig. 4, although the age effect for the men was less striking. In addition, the absolute levels of both diastolic and systolic pressures were lower for the wornen than for the men, as is evident frorn the comparison, decade by decade, shown in Fig. 5. The point-by-point comparison of the under-29and the 30- to 39-yr age groups reveals that, while parallel in response, both during and after exercise the systolic and diastolic blood pressures were lower for women than for men. However, the difference in blood
1 ”
”
“‘I’
?
220 -
I”“““’ I Al!% years A40 - 49 years 0 30 - 39 years
BURSE,
AND
LIND
pressure between men and women decreased with increasing age. For example, the average resting systolic and diastolic blood pressures for the women in the under29-yr group was 117/79 mmHg, while that of the men of the same age was 132/89 mmHg. In the over-50-yr group the value for the women was 140/85 mmHg, while that of the rnen was 143/90 mmHg. Card& work. Cardiac work was calculated as the product of heart rate and mean blood pressure in each of the four age groups by decade, before, during, and 1 min after isometric exercise. The findings indicate no systematic difference in cardiac work at rest, during exercise, or postexercise between men and women or between any two age groups of either sex. In all cases, from a resting value of about 7,600 arbitrary units, cardiac work rose in an approxirnately linear fashion during exercise to a level of about double the resting value (mean value 15,450 arbitrary units) and returned quickly to control values after the exercise. Tension-time index. The tension-time index (TTI) was calculated (heart rate X mean systolic blood pressure X systolic interval) to give an approximation of myocardial oxygen consumption in both groups of subjects (25). As was the case with cardiac work, there was no difference in TTI before, during, or after exercise in any of the four age groups of men or women. In all age groups, the TTI
1 d
0 19 - 29 years
200 I"
;2 ;i
180 -
180
.E 160 Q, 5 $ 140-$ m
120100 80 -
0 20
60
100 30 6090
0 20
60
100306090
80
0 20
60
1003060
90
% Duration--TimeFIG. 4. hlean systolic (4 upper plots) and diastolic (single lower plot) blood pressures before, during, and after the 40% MVC handgrip contraction. The average values for 19- to 29-yr-old (a), 30to 39yr-old (o), 40- to 49-yr-old (A), and >50-yr-old (A) groups for women are shown in the left panel, while the right panel represents the comparable values in men.
t of
40%
MVC
I
(4
I
0 20
60
-%
Duration--Time
I of
40%
MVC
10030 I
6090
(set)
FIG. 5. The average blood pressures of men (A) and women (0) in each of the 4 decades are represented in the separate panels. Systolic and diastolic blood pressures are shown before, during, and after the fatiguing 407& MVC handgrip contraction.
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PHYSIOLOGICAL
RESPONSES
OF
FEMALES
TO
ISOMETRIC
started at approximately 2,000 arbitrary units at rest and increased by some 50 % (to a mean value of 3,100 arbitrary units) by the end of the contraction and, following exercise, returned rapidly to control values. There were no significant correlations between TTI and age before, during, or after exercise in either the men or the women. DISCUSSION
It has been concluded, on somewhat indifferent evidence, that men and women can hold isometric endurance contractions at set proportions of their maximum strengths for the same length of time (18, 21, 22). Our results do not support this view. ‘The average strength of our group of women was, as expected, much lower (58 “r,) than that of the men, a difl’erence that increased with age. It was unexpected, then, to find that the endurance of the women, for a sustained contraction of 40 % MVC, averaged 123 % that of the men. The few earlier studies on the effect of age on the strength of women point to a diminution of strength of the order of 20-25 % as age increases’frorn 20 to 60 yr (2). Our findings the major resupport that view, but in our investigation duction occurred as age exceeded 50 yr, giving rise to the obvious speculation that this change may be related to menopause. Those women who were known to have achieved menopause had an average MVC of 23.9 kg and the duration of their 40 CT0MVC was 201 s. The corresponding values for known prernenopausal women were 29.5 kg and 165 s, respectively. The differences in strength and endurance for these two groups of women were statistically significant (Y < 0.01). There has been a recent accretion of data that may help to explain some of the differences found in strength and endurance as age increases. While cell death in the brain appears to be well substantiated as animals age, there is some argument about the loss of nerve cells in the spinal cord (10). There is agreement that there is no loss of motor neurons (4), but there is a demonstrable loss in the muscle f-iber population with age, which is attributed to a reduction in the maintenance function of motor neurons, probably related to protein synthesis within the neuronal of maintenance function in cell body (13). Th e reduction the neurons results first in the degeneration of the motor end plate and, as a result, to the removal of the trophic influence of the motor nerve on its target fibers. Guttman (13) likens this aging process to the atrophy following section of the nerve, entitling it “functional denervation.” It is well established that the trophic influence of nerves on their target tissues is potent. The characteristics of muscles comprised of predorninan tly red or predominantly white muscle fibers can be reversed by transposition of the nerves serving those muscles (6). These findings, demonstrated in the cat, have also been found to hold for the pig (20) and the rat (9). As well as clearly demonstrated microscopic and functional changes, related changes have been shown in cellular biochemistry-notably an increase in myosin ATPase activity in formerly white fibers (and the decrease in myosin ATPase in formerly red fibers) following transposition of the motor nerves (3, 7, 23). Slow (red) muscle fibers, with a high concentration of
EXERCISE
867
myoglobin, have long been associated with slow response to stimulus but with great capacity for endurance. In contrast, fast (white) muscle fibers confer speed of contraction on a muscle but have little power of endurance. These functional attributes of slow and fast fibers have been clearly demonstrated in animals and have been generally accepted to hold for human muscles, although with little direct evidence. However, a recent study (17) provides telling support. After treating men with curare, which preferentially blocks slow muscle fibers, or decamethonium, which preferentially blocks fast muscle fibers, so that strength was decreased in both cases to 50 % of the maximum, the endurance of isometric exercise was greater following administration of decamethonium. In addition, other recent evidence sheds light on the importance of aging on red and white fiber populations. While both fiber types are present in the adult, only slow iibers are present in the newborn. Differentiation of fast from slow muscle fibers occurs in mammals during the period of growth and maturation (6, 8). Furthermore there is evidence that as aging proceeds an increasing proportion of the fast muscle fibers become undifferentiated and reassume the characteristics of slow muscle fibers (12, 13). The evidence then a) of a reduction of the total number of muscle fibers as described above could account for the reduction of muscle strength with age, and b) of an increasing proportion of red muscle fibers with aging could account for the increase in isometric endurance. But other factors can modify the functional properties of muscle. It has been shown that continued physical activity retards and, up to a point, may temporarily arrest the decrease of muscular strength (2). This may well explain the fact that the men we examined, who were mainly machinists, did not show a loss of strength with age (19), while the women we examined here (whose occupations make it likelv that there was no similar continued daily activity of the forearm muscles) did. Also it seems clear that sex hormones are involved in muscular function. The strength of bovs. and girls is the same up to the point thereafter the boys become much stronger of puberty; than the girls (14). Furthermore it has been shown that the administration of methyltestosterone to eunuchoids and to patients over 50 yr of age who complained of muscle weakness and quickly developing fatigue, resulted in an increase of both their muscle strength and their endurance (26). Young men, with no such symptoms of muscular weakness, showed no change in muscular function after administration of methyltestosterone (24). In addition, our present results strongly suggest that menopause is associated with changes in both strength and endurance. However, definitive assessments of the inherent physiological factors involved in changes of isometric muscular function in aging must await further investigations. While the resting heart rate and the maximum heart rate during running or cycling declines with age for both men and women (1) the heart rates at set, submaximal levels of dynamic exercise appear to be specifically related to the oxygen requirement and do not change significantly with age (1, 28). The underlying mechanisms controlling these changes of resting and maximal heart rate with age are not understood. To explain them, various hypotheses
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868
PETROFSKY,
have been put forward, ranging from the implication of factors in the central and peripheral circulation, to changes in metabolic function of either skeletal or cardiac muscle. These hypotheses are purely speculative and remain unconvincing at present. In our studies, there was a clearly defined decrease of resting heart rate with age for women. This relationship was maintained and indeed exaggerated by the isometric exercise, at the end of which the heart rates were undoubtedly not maximal. In this respect, the influence of dynamic and isometric exercise is different and may be added to a number of other differences of cardiovast ular responses to the two types of exercise (5) The results of this study show that there was no significant diflerence in heart fates for the men and women at rest or during exercise. This point has some reievance to the fact that, despite the large difference in absolute tension exerted by the men and the women, the relative tension was the same, since it has been shown recently that the level of heart rate during isometric exercise depends on the tension exerted (12). The resting systolic blood pressure of our women and our men in&eased with age. Also the resting pressures of our young women were lower than those of our young men,
BURSE,
AND
LIND
but at age 60 there was no difference due to sex. These facts are in agreement with the extensive data compiled by the Society of Actuaries (27). Since the isometric exercise resulted in a parallel rise of blood pressure in both men and women at all ages, the pattern of blood pressure with age found at rest remained the same during the exercise. The only difference was, as with heart rate, an exaggeration of the age effect during the exercise. It is usually considered that at rest the increase in systolic pressure with age is related to a decrease in the distensibility of arteries. The same explanation would iit the fact that the age effect becomes exaggerated during isometric exercise, at a time (when either the cardiac output is increased or some increase of peripheral resistance occurs (16). However, the precise mechanisms that control the increases in heart rate and blood pressure in isometric exercise remain to be uncovered. We are indebted to Dr. P. Miller, Dr. R/I. Ellert, D. Lowery, ,J. S. Rinehart, and S. Ashby for valuable help perimen ts. This research was carried out under Dept. of Health, and Welfare Contract HSM 099-7 l-2 1. Received
for
publication
26 August
LeDonne, in these
S. ex-
Education,
1974.
KEFERENCES ASTRAND, I. Aerobic capacity of men and women with special reference to age. Acta Physiol. Stand. Sufq!~Z. 169, 1960. ASTRAND, P. 0, AND K. RODAHL. Textbook of Work Physiology. New York : McGraw-Hill, 1970. BAKANY, M., AND Ii. CLOSE. The transforrnation of myosin in cross-innervated rat muscles. ,J. Physiol., London 2 13 : 455-474, 1971. BRODY, ,A. Organization of the cerebral cortex. III. A study of aging in the human cerebral cortex. J. Camp. Neural. 102 : 5 1 l556, 1955. BRUCE, 1-L A., A. K. LIND, D. FRANKLIN, A. L. MUIR, H. R. MACDONALD, G. W. MCNICOL, AND K. W. DONALD. The effects of digoxin on fatiguing static and dynamic exercise in man. CZin. sci. 34 : 29-53, 1968. BULLER, A. J ., J. C. ECCI-ES, AND R. M. ECCLES. Interactions betweeli rnotoneurones and muscle in respect to the characteristic speeds of their responses. J. Physiol., London 150 : 417-439, 1960. BULLER, A. J., W. F. H. M. MOMMAERTS, AND K. SIRAYDARIAN. Enzymic properties of myosin in fast and slow twitch muscles of the cat following cross-innervation. J. Physiol., London 205: 58 l-597 1969 . . k. Dynainic properties of fast and slow skeletal muscles CI,OSE, of the rat during development. J. Physiol., London 173: 74-95, 1964. CLOSE, R. Dynamic properties of fast and slow skeletal muscles of the rat after nerve cross-union. J. Physzol., London 204: 331346 1969. CRI;.CHI,EY, IVY. Problems in Ageing, edited by E. V. Cowdrey. Baltimore, Md. : Williams & Wilkins, 1942, p. 5 18. DRAHOI’A, Z., AND E. GUTMAN. Long term influence of the nervous system on some metabolic differences in muscles of different function. Physiol. Rohemoslav. 12 : 339350, 1963. FUNDERBURK, C. F., S. G. HIPSKIND, K. F. WELTON, AND A. R. LIND. The development of and recovery from muscular fatigue induced by static effort at different tensions. J. AppZ. Physiol. 37: 392-396, 1974. GUTMAN, E., AND V. HANZLEKOVA. Age Changes in the Neuromuscular System. Bristol: Scientechnica Ltd., 1972, p. 82. HEUINGER, T. L. Phy.l-iology of Strength. Springfield, Ill. : Thomas, 1961.
15.
16.
17. 18.
19.
20.
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