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Muscle strength differences in urban and rural populations in Sweden
1315
Muscle Strength Differences in Urban and Rural Populations in Sweden Karin Ringsberg, RPT ABSTRACT. Ringsberg K. Muscle strength differences in urban and rural populations in Sweden. Arch Phys Med Rehabil1993;74:1315-18. l The isometric extension and flexion strength of the right knee was investigated in 396 men and 537 women. The probands were randomly selected from the National Population Records, either from the city of Malmo, Sweden, or from a typical agricultural region 60km outside the city. Both the extension and the flexion strength decreased with age in both groups. Between 50 years and 80 years of age the decrease was almost 40%. The urban probands had significantly lower muscle strength in both sexes. The difference was most pronounced for the knee extension strength. Also, the flexion strength was less in the urban population, the difference significant only in octogenarians. 0 1993 bv the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and ”
Rehabi&tion
Muscle strength is known to indicate habitual level of physical activity’.* and to be correlated with the function of systems such as the cardiovascular, the autonomic nervous, the neurophysiologic, the endocrine, the skeletal, and the psychosocial.*-5 Sernbo and coworker@ found that the incidence of hip fracture is significantly higher in the city of Malmij than in the county of Sjiibo, a farming district 60km east of the city. The same difference in hip fracture incidence between urban and rural districts has been found by others.‘,’ Gardsell and colleagues’ found that the bone mineral content differs between urban and rural using the same probands as in the present study. Both women and men in Malmij had significantly less bone mineral content than the rural population of SjSbo. Several studies have shown that athletes have a significantly higher bone mass than sedentary people regardless of age.‘@‘* Mechanical skeletal loading contributes to muscle contraction, which, in accordance with Wolf’s laws increases bone mass and muscle strength.4,‘3 The purpose of this study was to determine if there is a difference in muscle strength between an urban and a rural population. If there is a difference, is it larger in persons who have never lived in the country, or in persons who have never lived in a city? An additional purpose was to create normative data on muscle strength in persons more than 40 years of age. Also of interest is whether the age-related decline in muscle strength is the same in the urban and rural groups. From the Department of Orthopaedics, MalmB General Hospital. Lund University, S-214 01 MalmG, Sweden. Submitted for publication October 2 I. 199 I. Accepted in revised form August 3. 1992. This research supported by grants from the Greta and Johan Kock, the Alfred 6sterlund foundations and Lund University Research Grants. No commercial party having a direct or indirect interest in the subject matter ofthis article has or will confer a benefit upon the author or upon any organization with which the author is associated. Reprint requests to Karin Ringsberg. RPT. Department of Orthopaedics, Malmii General Hospital. Lund University, S-214 01 Malm6. Sweden. Q I993 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation 0003-9993/93/7412-0231$3.00/O
MATERIAL
AND
METHODS
Malmo, with 232,000 residents, is the third largest city in Sweden. Sjobo, 60km east of Malmo, is a typical agricultural community with 15,000 residents. Invited to participate in the population-based study were men and women of Scandinavian background, born in the years 1908, 1918, 1928, 1938, and women born in 1948. We tested premenopausal women to see if they differed in muscle strength from postmenopausal women more than the expected age-related decline. The subjects from Malmb (urban) and Sjobo (rural) were randomly selected from the National Population Records and matched for age and sex. The urban and rural cohorts were subdivided. One group consisted of individuals who had always lived in the city and the other of individuals who had never lived in a city. The subgroups are referred to as true urban (TU) and true rural (TR). Altogether, 553 subjects from Malmo and 380 from Sjobo participated. Only 46% of the urban women born in 1908 participated, due to illness and to transportation problems. The age and sex-specific participation and its percentage of all invited to the study are presented in table 1. The characteristics of the urban and the rural group and Sweden in general were obtained from the Central Bureau of Statistics (table 2). The weight and height were registered (table 3) and correlated with muscle strength. The population-based study was done simultaneously in Malmo and in Sjiibo in 1989 by the same examiner. The strength of the quadriceps and the hamstrings muscles in the right leg was tested. The urban population was tested with a Cybex II.* The rural population was tested with a transducer fixed to a movable arm on a quadriceps training table. The transducer was padded and placed proximal to the ankle joint and the pressured value was in kilopond. The length from the lateral joint space of the knee to the transducer was measured in meters. The axis of rotation was at the joint axis for both instruments. The transducer was movable according to the leg length and was placed proximal to the ankle joint and perpendicular to the limb as much alike the test procedure of the Cybex as possible. The transducer was manufactured by the Mechanical and ElecArch Phys Med Rehabil Vol74, December
1993
MUSCLE STRENGTH DIFFERENCES, Ringsberg
1316
Table 1: Participation (N) and Percentage of all Individuals Invited to the Study and Percentage of Participants Who Were True Urban and True Rural Rural
Urban Age
N
%
TU%
N
%
TR%
40 Women 50 Men Women 60 Men Women 70 Men Women 80 Men Women
70 39 60 52 59 91 81 57 44
75 71 72 81 71 81 68 76 46
53 69 48 35 49 47 44 47 45
48 39 45 39 44 50 53 29 33
87 76 88 85 81 82 85 67 62
56 92 82 87 93 90 94 97 100
Total
553
380
Abbreviations: TU, true urban: TR. true rural.
troengineering department at the hospital and was calibrated together with the Cybex apparatus before and during the test period. The calibration was performed with constant weights of 1,2,4, and 1Okg with the test arm in horizontal position. The test procedure consisted of three isometric contractions for the quadriceps and hamstrings muscle groups in sitting position with the right knee in 60” offlexion. At both test situations the participants were fixed with straps around the waist and femurs and both tables had adjustable backs. To find possible differences between the two test methods 40 healthy persons of both sexes and between the ages of 18 and 65 years were examined. Three contractions for the quadriceps and the hamstrings muscle groups with a pause of ten seconds between the contractions were performed with the two test methods and a ten minutes’ pause between the methods. The best result of three trials was recorded. The linear coefficient for correlation between the two methods was for quadriceps strength Y= 0.77 with the regression equation y = 41.18 + 1.033x. The hamstrings
muscle group had the correlation coefficient r = 0.82 and the regression equation y = 13.01 + 0.87x. In this calculation x represents the value of the transducer and y the Cybex value. Because the two test methods were correlated we used the regression equation to transform the results from the test method of the rural population. All the results are expressed in Newton meters. Student’s t test was used for the statistical evaluation.
RESULTS
Men The knee extension strength of the right leg in men was less in the urban population in all ages as compared with the rural population. In the age groups 50,60, and 70 years the differences were significant. The same appeared in the TR and TU subgroups (table 4). The flexion strength for men in the urban and true urban groups was significantly less at the age of 80. There was also a tendency for the TU group to be weaker in all age groups (table 4).
Women The same tendency of difference between the urban and rural populations and the subgroups was found for women. The urban group had significantly less extension strength in age groups 40,50,60, and 80 years. The TU group showed the same difference, except for the age group of 40 (table 5). The flexion strength was only significantly less for the urban and TU groups at the age of 80 (table 5). But at the age of 40 years the rural and true rural groups had less strength than the urban groups of the same age. The age-related decline between the youngest and the oldest was less in rural women. Extension strength had a decline of 32% and flexion strength 19% compared with urban women who had 44% in both tests. In men the extension strength declined about 42% in both populations and flexion strength declined 33% in the rural group and 43% in the urban group.
Table 2: Facts from Statistics Sweden, 1989 Sweden Inhabitants per km2 Age 65 years and over (in W) Men living alone (in W) (1985) Women living alone (in %) (1985) Old age pensioners receiving social domestic help (in %) ( 1987) Persons receiving social benefits (in %) ( 1987) Persons gainfully employed in agriculture and forestry (in %) Employment density, aged 60 to 64 (in %) Men Women Total mean income (XSEK 1,000) Men Women Pensioners on early retirement per 1.OOO All adults aged 16 to 64 ( 1987) Upper age group, 60 to 64 (1987) Number of persons per 100 households (I 985) Households of retired pensioners (in %) (1985) Living in one family homes (in %) (1985)
Arch Phys Med Rehabil Vol74, December 1993
19 18 16 20 18 6 5
104 68 6.3 30.7 7 73 I._ 23 46
MalmS
SjGbo
I.487 21 20 27 I7 13
I
30 20 15 15 10 6 18
59.8 46.5
70.9 45.8
103 69 7.6 32.5 1.88 26 17
87 57 5.5 25.9 2.40 26 84
MUSCLE STRENGTH DIFFERENCES, Ringsberg
1317
Table 3: Mean Weight and Height in Relation to Age in a True Urban, Urban, Rural, and True Rural Population 40 Years
Women
50 Years
60 Years
70 Years
80 Years
(W/H)
(W/H)
(W/H)
(W/H)
(W/H)
True urban Urban Rural
651165 641166 64/163
671165 661164 70/163
681163 691163 71/160
681161 68/161 74/161
601160 641160
True rural
66/163
701 I63
72/161
741161
841179 81/178 811177 81/177
80/176 781175 811172 81/173
751174 761175 81/173 821173
68/157 741174 751174 771169 771168
Men
True urban Urban Rural True rural
Weight (kg)/height
(cm).
The true rural men and women had a slightly higher body weight than the true urban in the oldest age groups and the rurals of 80 were three to six centimeters shorter (table 2). There was no consistent positive correlation between muscle strength and weight. DISCUSSION The urban population and the subgroup TU had less muscle strength when population-based groups were compared-knee extension strength was less in all age groups over age 40, and flexion strength was less only in the oldest group. The measurements were isometric. It is well known that isokinetic strength testing may demonstrate different torque recordings in the same group of subjects when two different isokinetic machines made by the same manufacturer are used.14 This conclusion may also probably hold some merit for isometric strength determination. This influence should, however, be considerably less because the sitting position is easier to adjust, the range of movement in the knee differs in elderly people, the speed of reaction is slower in the elderly, and elderly persons understand the instructions more easily when isometric measurements are performed. In this study the extension strength in men and women of age 80 in the TU population was not different from the corresponding values published in an earlier study in Copenhagen.15 Despite the limited participation of women in the urban group due to illness and transportation difficul-
68/157
ties, we could assume that those who took part in this investigation were more healthy, but had nevertheless lower muscle strength than the women in the rural group. Aniansson and associates’6 measured the isometric quadriceps muscle strength in 70-year-old men and women in Gothenburg. Also their values were equivalent with the actual data from the true urban group but only for women. The men in Malmii had 17% less strength than the men in Gothenburg. The decrease in extension strength between 40 and 80 in rural women was 32%. Young and coworkers” found the same decrease between 20 and 80 in women of urban environment. In the present study the decrease in extension strength for urban women was even higher, in spite of the low value of extension strength in 40-year-olds. In men the decrease of extension strength was the same, about 40%, for the two populations, which is slightly more than the decrease in bone mass in the same samples.’ Larsson and associates” showed the same decrease of muscle strength in men between 50 and 69. The reason no significant difference in flexion strength was found, except in the oldest, could be due to the specific function in daily living of the hamstrings muscle group. Schtildt and colleagues” using electromyography found a very low muscle activity during rising from a chair. The maximum capacity of the hamstrings muscle group may not be fully used in daily living, this muscle group may function mainly as a postural stabilizer. In old age when the function of other muscle groups declines, an increasing
Table 4: Mean Flexion and Extension Strength in Relation to Age in a True Urban, Urban, Rural, and True Rural Population: Men 50 years 12
Extension strength (Newton meters) True urban Urban Rural True rural
60 years
x+SD
I1
27 39 39 36
213 + 48 212 + 45 241 + 50 244+51
18 52 39 34
37 39 31 36
97 103 104 104
18 52 39 34
70 years
x k SD
159 167 214 214
n
t 37 rt 52 rt_ 35 f 35
43 91 50 45
81 + 18 85+21 86? 17 87* 15
43 91 49 44
80 years
x i SD
158 159 175 177
n
x k SD
120 + 41 131 ? 40 140 ZL42 140-+43
2 & ” f
43 36 38 37
26 57 29 28
72 ? 75 i 79 * 80?
23 22 20 19
26 57 29 28
Flexion strength True urban Urban Rural True rural
i & t i
29 29 33 34
56t 58i 70 * 70 f
18 17 22 2’
Arch Phys Med Rehabil Vol74, December 1993
MUSCLE STRENGTH DIFFERENCES, Ringsberg Table 5: Mean Flexion and Extension Strength in Relation to Age in a True Urban, Urban, Rural, and True Rural Population: Women 40 years
Extension strength True urban Urban Rural True rural Rexion strength True urban Urban Rural True rural
50
60 years
years
x k
70
?I
x t SD
n
x ? SD
n
35 70 48 31
141 * 34 135 t 37 150 + 39 156 rf: 30
29 60 45 37
113+29 120 + 32 147 f 36 145? 37
29 59 44 41
108 + 107 + 133 * 136 4
28 33 32 31
36 81 53 50
29 59 44 41
50+ 48* 49* 52*
13 15 II 13
36 81 53 50
35 70 48 31
64 + 63% 602 62%
22 17 I4 I4
29 60 45 37
56 * 55 + 57* 57*
23 19 15 15
load may be taken by the hamstrings. The more sedentary life of the urban population is therefore not revealed until old age.” Although the rural population was slightly heavier, this could not explain the differences in muscle strength because no correlation was found between muscle strength and weight. On the other hand it is possible that their presumed heavier work load can influence their muscle strength.2’ The figures in table 2 showed that the residents in Sjijbo worked more years before retirement, lived in one-family homes, lived together with someone, and received less social domestic help and benefits. Johnell and Sernbo22 showed that fracture patients more often lived alone and needed more home aid before the fracture than did the controls. Furthermore, they were not able to do their own shopping and had fewerer outdoor activities. Some of these factors are more noticeable in an urban population in contrast to a rural population23 (table 2), which to some extent could explain the higher prevalence of hip fracture in the urban population.6 This study found that the muscle strength was significantly higher in a rural population compared with an urban population. This difference could be due to a more strenuous and larger physical activity of life in the rural cohort. The decreased muscle strength and the decreased bone mineral content in the urban population could explain both the increased incidence of hip fracture in urban as compared to rural populations. The differences between the urban and the rural groups also stress the necessity of using correct normative data when comparing background factors from different populations. References Cooper C, Barker DJP, Wickham C. Physical activity. muscle strength, and calcium intake in fracture of the proximal femur in Britain. Br Med J 1988;297:1443-8. Skinner JS, Tipton CM, Vailas AC. Exercise. physical training, and the ageing process. In: Vidik A, editor. Lectures on gerontology. on biology of ageing 1 (part B). Orlando, FL: Academic. 1982. Fitts RH. Aging and skeletal muscle. Exercise and aging, the scientific basis. Presented at the American College of Sports Medicine, Las Vegas, May 28-30. 1980. Sandier R. Muscle strength assessmentsand the prevention ofosteoporosis. A hypothesis. JAGS 1989;37: I 192-7. Arch Phys Med Rehabil Vol74, December 1993
SD
I1
80 years
years
n
x * SD
103 + 28 101 + 29 Ill *30 III t29
20 44 33 33
79 f 29 76 ? 27 104+20 104 + 20
47 * 25 46+-21 50+ I5 51 + I5
20 44 ::
x ‘-
SD
35 * 36 + 492 49*
12 II I7 17
5. Snow-Hatter C. Bouxsein M. Lewis B. Charette S, Weinstein P, Marcus R. Muscle strength asa predictor ofbone mineral density in young women. J Bone Mineral Res 1990;5:589-95. 6 Sernbo I, Johnell 0. Andersson T. Differences in the incidence of hip fracture. Comparison of an urban and rural population in Southern Sweden. Acta Orthop Stand 1988:59:382-5. 7 Falch JA, Ilebekk A, Slungaard U. Epidemiology of hip fractures in Norway. Acta Orthop Stand 1985:56: 12-6. 8. Mannius S. Mellstrom D, Odtn A, Rundgren A. Zetterberg C. Incidence of hip fracture in Western Sweden 1974-1982. Acta Orthop Stand 1987:58:38-42. 9. Gardsell P. Johnell 0, Nilsson BE, Sernbo I: Bone mass in an urban and a rural population. A comparative, population-based study in Southern Sweden. J Bone Mineral Res 1990;6:67-75. IO. Jacobson PC, Beaver W, Grubb SA, Taft TN, Talmage RV. Bone density in women: college athletes and older athletic women. J Orthop Res 1984:2:328-32. 11. Lane NE. Bloch DA. Jones HH, Marshall Jr WH. Wood PD, Fries JF. Long-distance running, bone density and osteoarthritis. JAMA 1986:255: I 147-5 I. 12. Nilsson BE, Westlin NE. Bone density in athletes. Clin Orthop I97 I ;?l: 179-82. 13. Aloia JF. Cohn SH. Babu T. Skeletal mass and body composition in marathon runners. Metabolism 1978:27:1793-6. 14. Cress M. Johnson J. Agre JC. lsokinetic strength testing in older women: a comparison of two systems. J Orthop Sports Phys Therapy I99 1: 13: 199-202. 15. Danneskiold-Samsoe B, Kofod V. Munter J, Grimby G, Schnohr P. Jensen G. Muscle strength and functional capacity in 78-8 l-year-old men and women. Eur J Appl Physiol 1984:52:3 10-4. 16. Aniansson A, Grimby G. Rundgren A. Isometric and isokinetic quadriceps muscle strength in 70-year-old men and women. Stand J Rehab Med 1980:12:161-8. 17. Young A. Stokes M, Crowe M. Size and strength of the quadriceps muscles of old and young women. Eur J Clin Invest 1984;14:282-7. 18. Larsson L. Grimby G, Karlsson. J. Muscle strength and speed ofmovement in relation to age and muscle morphology. J Appl Physiol I979;46:45 l-6. 19. Schiildt K. Ekholm .I, NCmeth G. Arborelius UP. HarmsRindahl K. Knee load and muscle activity during exercises in rising. In: Winter DA. ed. Biomechanics of human movement. Toronto: Wiley &Sons. 1979. 20. Skog R. Regional medborgarrapport Skaraborgs Ian. Institutet fdr social forskning (in Swedish). 21. Grimby G, Saltin B. The aging muscle. Clin Physiol 1983:3:209-18. 22. Johnell 0, Sernbo 1. Health and social status in patients with hip fractures and controles. Age Ageing 1986: I5:285-9 1. 23. Statistics Sweden. Central Bureau of Statistics, Stockholm, Sweden. Supplier a. Cybex II. Division of Lumex Incorporated, 2100 Smithtown Avenue, Ronkonkoma. NY 11779.
Muscle Strength Differences in Urban and Rural Populations in Sweden Karin Ringsberg, RPT ABSTRACT. Ringsberg K. Muscle strength differences in urban and rural populations in Sweden. Arch Phys Med Rehabil1993;74:1315-18. l The isometric extension and flexion strength of the right knee was investigated in 396 men and 537 women. The probands were randomly selected from the National Population Records, either from the city of Malmo, Sweden, or from a typical agricultural region 60km outside the city. Both the extension and the flexion strength decreased with age in both groups. Between 50 years and 80 years of age the decrease was almost 40%. The urban probands had significantly lower muscle strength in both sexes. The difference was most pronounced for the knee extension strength. Also, the flexion strength was less in the urban population, the difference significant only in octogenarians. 0 1993 bv the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and ”
Rehabi&tion
Muscle strength is known to indicate habitual level of physical activity’.* and to be correlated with the function of systems such as the cardiovascular, the autonomic nervous, the neurophysiologic, the endocrine, the skeletal, and the psychosocial.*-5 Sernbo and coworker@ found that the incidence of hip fracture is significantly higher in the city of Malmij than in the county of Sjiibo, a farming district 60km east of the city. The same difference in hip fracture incidence between urban and rural districts has been found by others.‘,’ Gardsell and colleagues’ found that the bone mineral content differs between urban and rural using the same probands as in the present study. Both women and men in Malmij had significantly less bone mineral content than the rural population of SjSbo. Several studies have shown that athletes have a significantly higher bone mass than sedentary people regardless of age.‘@‘* Mechanical skeletal loading contributes to muscle contraction, which, in accordance with Wolf’s laws increases bone mass and muscle strength.4,‘3 The purpose of this study was to determine if there is a difference in muscle strength between an urban and a rural population. If there is a difference, is it larger in persons who have never lived in the country, or in persons who have never lived in a city? An additional purpose was to create normative data on muscle strength in persons more than 40 years of age. Also of interest is whether the age-related decline in muscle strength is the same in the urban and rural groups. From the Department of Orthopaedics, MalmB General Hospital. Lund University, S-214 01 MalmG, Sweden. Submitted for publication October 2 I. 199 I. Accepted in revised form August 3. 1992. This research supported by grants from the Greta and Johan Kock, the Alfred 6sterlund foundations and Lund University Research Grants. No commercial party having a direct or indirect interest in the subject matter ofthis article has or will confer a benefit upon the author or upon any organization with which the author is associated. Reprint requests to Karin Ringsberg. RPT. Department of Orthopaedics, Malmii General Hospital. Lund University, S-214 01 Malm6. Sweden. Q I993 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation 0003-9993/93/7412-0231$3.00/O
MATERIAL
AND
METHODS
Malmo, with 232,000 residents, is the third largest city in Sweden. Sjobo, 60km east of Malmo, is a typical agricultural community with 15,000 residents. Invited to participate in the population-based study were men and women of Scandinavian background, born in the years 1908, 1918, 1928, 1938, and women born in 1948. We tested premenopausal women to see if they differed in muscle strength from postmenopausal women more than the expected age-related decline. The subjects from Malmb (urban) and Sjobo (rural) were randomly selected from the National Population Records and matched for age and sex. The urban and rural cohorts were subdivided. One group consisted of individuals who had always lived in the city and the other of individuals who had never lived in a city. The subgroups are referred to as true urban (TU) and true rural (TR). Altogether, 553 subjects from Malmo and 380 from Sjobo participated. Only 46% of the urban women born in 1908 participated, due to illness and to transportation problems. The age and sex-specific participation and its percentage of all invited to the study are presented in table 1. The characteristics of the urban and the rural group and Sweden in general were obtained from the Central Bureau of Statistics (table 2). The weight and height were registered (table 3) and correlated with muscle strength. The population-based study was done simultaneously in Malmo and in Sjiibo in 1989 by the same examiner. The strength of the quadriceps and the hamstrings muscles in the right leg was tested. The urban population was tested with a Cybex II.* The rural population was tested with a transducer fixed to a movable arm on a quadriceps training table. The transducer was padded and placed proximal to the ankle joint and the pressured value was in kilopond. The length from the lateral joint space of the knee to the transducer was measured in meters. The axis of rotation was at the joint axis for both instruments. The transducer was movable according to the leg length and was placed proximal to the ankle joint and perpendicular to the limb as much alike the test procedure of the Cybex as possible. The transducer was manufactured by the Mechanical and ElecArch Phys Med Rehabil Vol74, December
1993
MUSCLE STRENGTH DIFFERENCES, Ringsberg
1316
Table 1: Participation (N) and Percentage of all Individuals Invited to the Study and Percentage of Participants Who Were True Urban and True Rural Rural
Urban Age
N
%
TU%
N
%
TR%
40 Women 50 Men Women 60 Men Women 70 Men Women 80 Men Women
70 39 60 52 59 91 81 57 44
75 71 72 81 71 81 68 76 46
53 69 48 35 49 47 44 47 45
48 39 45 39 44 50 53 29 33
87 76 88 85 81 82 85 67 62
56 92 82 87 93 90 94 97 100
Total
553
380
Abbreviations: TU, true urban: TR. true rural.
troengineering department at the hospital and was calibrated together with the Cybex apparatus before and during the test period. The calibration was performed with constant weights of 1,2,4, and 1Okg with the test arm in horizontal position. The test procedure consisted of three isometric contractions for the quadriceps and hamstrings muscle groups in sitting position with the right knee in 60” offlexion. At both test situations the participants were fixed with straps around the waist and femurs and both tables had adjustable backs. To find possible differences between the two test methods 40 healthy persons of both sexes and between the ages of 18 and 65 years were examined. Three contractions for the quadriceps and the hamstrings muscle groups with a pause of ten seconds between the contractions were performed with the two test methods and a ten minutes’ pause between the methods. The best result of three trials was recorded. The linear coefficient for correlation between the two methods was for quadriceps strength Y= 0.77 with the regression equation y = 41.18 + 1.033x. The hamstrings
muscle group had the correlation coefficient r = 0.82 and the regression equation y = 13.01 + 0.87x. In this calculation x represents the value of the transducer and y the Cybex value. Because the two test methods were correlated we used the regression equation to transform the results from the test method of the rural population. All the results are expressed in Newton meters. Student’s t test was used for the statistical evaluation.
RESULTS
Men The knee extension strength of the right leg in men was less in the urban population in all ages as compared with the rural population. In the age groups 50,60, and 70 years the differences were significant. The same appeared in the TR and TU subgroups (table 4). The flexion strength for men in the urban and true urban groups was significantly less at the age of 80. There was also a tendency for the TU group to be weaker in all age groups (table 4).
Women The same tendency of difference between the urban and rural populations and the subgroups was found for women. The urban group had significantly less extension strength in age groups 40,50,60, and 80 years. The TU group showed the same difference, except for the age group of 40 (table 5). The flexion strength was only significantly less for the urban and TU groups at the age of 80 (table 5). But at the age of 40 years the rural and true rural groups had less strength than the urban groups of the same age. The age-related decline between the youngest and the oldest was less in rural women. Extension strength had a decline of 32% and flexion strength 19% compared with urban women who had 44% in both tests. In men the extension strength declined about 42% in both populations and flexion strength declined 33% in the rural group and 43% in the urban group.
Table 2: Facts from Statistics Sweden, 1989 Sweden Inhabitants per km2 Age 65 years and over (in W) Men living alone (in W) (1985) Women living alone (in %) (1985) Old age pensioners receiving social domestic help (in %) ( 1987) Persons receiving social benefits (in %) ( 1987) Persons gainfully employed in agriculture and forestry (in %) Employment density, aged 60 to 64 (in %) Men Women Total mean income (XSEK 1,000) Men Women Pensioners on early retirement per 1.OOO All adults aged 16 to 64 ( 1987) Upper age group, 60 to 64 (1987) Number of persons per 100 households (I 985) Households of retired pensioners (in %) (1985) Living in one family homes (in %) (1985)
Arch Phys Med Rehabil Vol74, December 1993
19 18 16 20 18 6 5
104 68 6.3 30.7 7 73 I._ 23 46
MalmS
SjGbo
I.487 21 20 27 I7 13
I
30 20 15 15 10 6 18
59.8 46.5
70.9 45.8
103 69 7.6 32.5 1.88 26 17
87 57 5.5 25.9 2.40 26 84
MUSCLE STRENGTH DIFFERENCES, Ringsberg
1317
Table 3: Mean Weight and Height in Relation to Age in a True Urban, Urban, Rural, and True Rural Population 40 Years
Women
50 Years
60 Years
70 Years
80 Years
(W/H)
(W/H)
(W/H)
(W/H)
(W/H)
True urban Urban Rural
651165 641166 64/163
671165 661164 70/163
681163 691163 71/160
681161 68/161 74/161
601160 641160
True rural
66/163
701 I63
72/161
741161
841179 81/178 811177 81/177
80/176 781175 811172 81/173
751174 761175 81/173 821173
68/157 741174 751174 771169 771168
Men
True urban Urban Rural True rural
Weight (kg)/height
(cm).
The true rural men and women had a slightly higher body weight than the true urban in the oldest age groups and the rurals of 80 were three to six centimeters shorter (table 2). There was no consistent positive correlation between muscle strength and weight. DISCUSSION The urban population and the subgroup TU had less muscle strength when population-based groups were compared-knee extension strength was less in all age groups over age 40, and flexion strength was less only in the oldest group. The measurements were isometric. It is well known that isokinetic strength testing may demonstrate different torque recordings in the same group of subjects when two different isokinetic machines made by the same manufacturer are used.14 This conclusion may also probably hold some merit for isometric strength determination. This influence should, however, be considerably less because the sitting position is easier to adjust, the range of movement in the knee differs in elderly people, the speed of reaction is slower in the elderly, and elderly persons understand the instructions more easily when isometric measurements are performed. In this study the extension strength in men and women of age 80 in the TU population was not different from the corresponding values published in an earlier study in Copenhagen.15 Despite the limited participation of women in the urban group due to illness and transportation difficul-
68/157
ties, we could assume that those who took part in this investigation were more healthy, but had nevertheless lower muscle strength than the women in the rural group. Aniansson and associates’6 measured the isometric quadriceps muscle strength in 70-year-old men and women in Gothenburg. Also their values were equivalent with the actual data from the true urban group but only for women. The men in Malmii had 17% less strength than the men in Gothenburg. The decrease in extension strength between 40 and 80 in rural women was 32%. Young and coworkers” found the same decrease between 20 and 80 in women of urban environment. In the present study the decrease in extension strength for urban women was even higher, in spite of the low value of extension strength in 40-year-olds. In men the decrease of extension strength was the same, about 40%, for the two populations, which is slightly more than the decrease in bone mass in the same samples.’ Larsson and associates” showed the same decrease of muscle strength in men between 50 and 69. The reason no significant difference in flexion strength was found, except in the oldest, could be due to the specific function in daily living of the hamstrings muscle group. Schtildt and colleagues” using electromyography found a very low muscle activity during rising from a chair. The maximum capacity of the hamstrings muscle group may not be fully used in daily living, this muscle group may function mainly as a postural stabilizer. In old age when the function of other muscle groups declines, an increasing
Table 4: Mean Flexion and Extension Strength in Relation to Age in a True Urban, Urban, Rural, and True Rural Population: Men 50 years 12
Extension strength (Newton meters) True urban Urban Rural True rural
60 years
x+SD
I1
27 39 39 36
213 + 48 212 + 45 241 + 50 244+51
18 52 39 34
37 39 31 36
97 103 104 104
18 52 39 34
70 years
x k SD
159 167 214 214
n
t 37 rt 52 rt_ 35 f 35
43 91 50 45
81 + 18 85+21 86? 17 87* 15
43 91 49 44
80 years
x i SD
158 159 175 177
n
x k SD
120 + 41 131 ? 40 140 ZL42 140-+43
2 & ” f
43 36 38 37
26 57 29 28
72 ? 75 i 79 * 80?
23 22 20 19
26 57 29 28
Flexion strength True urban Urban Rural True rural
i & t i
29 29 33 34
56t 58i 70 * 70 f
18 17 22 2’
Arch Phys Med Rehabil Vol74, December 1993
MUSCLE STRENGTH DIFFERENCES, Ringsberg Table 5: Mean Flexion and Extension Strength in Relation to Age in a True Urban, Urban, Rural, and True Rural Population: Women 40 years
Extension strength True urban Urban Rural True rural Rexion strength True urban Urban Rural True rural
50
60 years
years
x k
70
?I
x t SD
n
x ? SD
n
35 70 48 31
141 * 34 135 t 37 150 + 39 156 rf: 30
29 60 45 37
113+29 120 + 32 147 f 36 145? 37
29 59 44 41
108 + 107 + 133 * 136 4
28 33 32 31
36 81 53 50
29 59 44 41
50+ 48* 49* 52*
13 15 II 13
36 81 53 50
35 70 48 31
64 + 63% 602 62%
22 17 I4 I4
29 60 45 37
56 * 55 + 57* 57*
23 19 15 15
load may be taken by the hamstrings. The more sedentary life of the urban population is therefore not revealed until old age.” Although the rural population was slightly heavier, this could not explain the differences in muscle strength because no correlation was found between muscle strength and weight. On the other hand it is possible that their presumed heavier work load can influence their muscle strength.2’ The figures in table 2 showed that the residents in Sjijbo worked more years before retirement, lived in one-family homes, lived together with someone, and received less social domestic help and benefits. Johnell and Sernbo22 showed that fracture patients more often lived alone and needed more home aid before the fracture than did the controls. Furthermore, they were not able to do their own shopping and had fewerer outdoor activities. Some of these factors are more noticeable in an urban population in contrast to a rural population23 (table 2), which to some extent could explain the higher prevalence of hip fracture in the urban population.6 This study found that the muscle strength was significantly higher in a rural population compared with an urban population. This difference could be due to a more strenuous and larger physical activity of life in the rural cohort. The decreased muscle strength and the decreased bone mineral content in the urban population could explain both the increased incidence of hip fracture in urban as compared to rural populations. The differences between the urban and the rural groups also stress the necessity of using correct normative data when comparing background factors from different populations. References Cooper C, Barker DJP, Wickham C. Physical activity. muscle strength, and calcium intake in fracture of the proximal femur in Britain. Br Med J 1988;297:1443-8. Skinner JS, Tipton CM, Vailas AC. Exercise. physical training, and the ageing process. In: Vidik A, editor. Lectures on gerontology. on biology of ageing 1 (part B). Orlando, FL: Academic. 1982. Fitts RH. Aging and skeletal muscle. Exercise and aging, the scientific basis. Presented at the American College of Sports Medicine, Las Vegas, May 28-30. 1980. Sandier R. Muscle strength assessmentsand the prevention ofosteoporosis. A hypothesis. JAGS 1989;37: I 192-7. Arch Phys Med Rehabil Vol74, December 1993
SD
I1
80 years
years
n
x * SD
103 + 28 101 + 29 Ill *30 III t29
20 44 33 33
79 f 29 76 ? 27 104+20 104 + 20
47 * 25 46+-21 50+ I5 51 + I5
20 44 ::
x ‘-
SD
35 * 36 + 492 49*
12 II I7 17
5. Snow-Hatter C. Bouxsein M. Lewis B. Charette S, Weinstein P, Marcus R. Muscle strength asa predictor ofbone mineral density in young women. J Bone Mineral Res 1990;5:589-95. 6 Sernbo I, Johnell 0. Andersson T. Differences in the incidence of hip fracture. Comparison of an urban and rural population in Southern Sweden. Acta Orthop Stand 1988:59:382-5. 7 Falch JA, Ilebekk A, Slungaard U. Epidemiology of hip fractures in Norway. Acta Orthop Stand 1985:56: 12-6. 8. Mannius S. Mellstrom D, Odtn A, Rundgren A. Zetterberg C. Incidence of hip fracture in Western Sweden 1974-1982. Acta Orthop Stand 1987:58:38-42. 9. Gardsell P. Johnell 0, Nilsson BE, Sernbo I: Bone mass in an urban and a rural population. A comparative, population-based study in Southern Sweden. J Bone Mineral Res 1990;6:67-75. IO. Jacobson PC, Beaver W, Grubb SA, Taft TN, Talmage RV. Bone density in women: college athletes and older athletic women. J Orthop Res 1984:2:328-32. 11. Lane NE. Bloch DA. Jones HH, Marshall Jr WH. Wood PD, Fries JF. Long-distance running, bone density and osteoarthritis. JAMA 1986:255: I 147-5 I. 12. Nilsson BE, Westlin NE. Bone density in athletes. Clin Orthop I97 I ;?l: 179-82. 13. Aloia JF. Cohn SH. Babu T. Skeletal mass and body composition in marathon runners. Metabolism 1978:27:1793-6. 14. Cress M. Johnson J. Agre JC. lsokinetic strength testing in older women: a comparison of two systems. J Orthop Sports Phys Therapy I99 1: 13: 199-202. 15. Danneskiold-Samsoe B, Kofod V. Munter J, Grimby G, Schnohr P. Jensen G. Muscle strength and functional capacity in 78-8 l-year-old men and women. Eur J Appl Physiol 1984:52:3 10-4. 16. Aniansson A, Grimby G. Rundgren A. Isometric and isokinetic quadriceps muscle strength in 70-year-old men and women. Stand J Rehab Med 1980:12:161-8. 17. Young A. Stokes M, Crowe M. Size and strength of the quadriceps muscles of old and young women. Eur J Clin Invest 1984;14:282-7. 18. Larsson L. Grimby G, Karlsson. J. Muscle strength and speed ofmovement in relation to age and muscle morphology. J Appl Physiol I979;46:45 l-6. 19. Schiildt K. Ekholm .I, NCmeth G. Arborelius UP. HarmsRindahl K. Knee load and muscle activity during exercises in rising. In: Winter DA. ed. Biomechanics of human movement. Toronto: Wiley &Sons. 1979. 20. Skog R. Regional medborgarrapport Skaraborgs Ian. Institutet fdr social forskning (in Swedish). 21. Grimby G, Saltin B. The aging muscle. Clin Physiol 1983:3:209-18. 22. Johnell 0, Sernbo 1. Health and social status in patients with hip fractures and controles. Age Ageing 1986: I5:285-9 1. 23. Statistics Sweden. Central Bureau of Statistics, Stockholm, Sweden. Supplier a. Cybex II. Division of Lumex Incorporated, 2100 Smithtown Avenue, Ronkonkoma. NY 11779.