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Bone mineral density of visually handicapped women Kirsti Uusi-Rasi, Harri SievaÈnen, Marjo Rinne, Pekka Oja and Ilkka Vuori UKK Institute for Health Promotion Research, Tampere, Finland Received 29 January 2001; accepted 26 February 2001 Correspondence: Kirsti Uusi-Rasi, UKK Institute, PO Box 30, FIN-33501 Tampere, Finland

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Summary

While physical activity is an essential factor for muscle performance and development and also for the maintenance of bone mass in the loaded bones, apparently low intensity of physical activity of blind persons may compromise the muscle performance and bone mineral density (BMD). Therefore, the aim was to study whether there are differences in BMD of the weight-bearing or non-weight-bearing bones between visually handicapped persons and those with normal sight. Nineteen visually handicapped premenopausal women and their matched pairs were recruited to the study. The mean age of the visually handicapped women was 39á9 years (SD 8á1) and that of the women with normal vision 39á7 years (6á5). BMD of the distal radius, femoral neck and trochanter was measured with dual energy X-ray absorptiometry (DXA), and isometric muscle strength of the extremities and trunk with a dynamometer. Between-group differences were compared with paired Student's t-test. The BMD at the femur was 8% higher in favour of the group with normal sight, whereas radial BMD was similar in the two groups. The t-score was )1á0 (95% con®dence interval )1á5 to )0á5) for the femoral neck BMD and )0á7 ()1á1 to )0á2) for the trochanter BMD in the group with impaired vision. The respective t-scores for the group with normal sight were )0á3 ()0á9 to 0á3) for the femoral neck and 0 ()0á7 to 0á7) for the trochanter. Visual handicap seems to be a risk for lower BMD of the weightbearing proximal femur, but not for lower BMD of the non-weight-bearing distal radius.

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Keywords: bone mineral density, physical activity, physical ®tness, visual handicap. Introduction

Physical activity is an essential factor for muscle performance and development and also behind the maintenance of bone mass in the loaded bones, while substantially reduced activity and immobilization are known to result in muscle wasting and bone loss (Drinkwater, 1993; Mosekilde, 1995). Studies on athletes have shown high bone mineral density (BMD) in loaded bones when compared with unloaded contralateral bone sites or bones of less active control subjects (Fehling et al., 1995; Heinonen et al., 1995; Kannus et al., 1995; Etherington et al., 1996). Not only athletic exercise, but also more common, long-term physical activity during leisure time has been shown to be associated with greater muscle strength and bone mass (Kerr et al., 1996; Uusi-Rasi et al., 1998, 1999). The issue of how much and what kind of physical activity is needed to gain and maintain an adequate bone mass remains unclear. The amount and, especially, intensity of physical activity of blind persons are deemed to be relatively low on the average for the apparent reasons of the lack of safety, guidance and adequate facilities. Given that blind persons have not encountered previous fractures, metabolic diseases or used drugs that would affect bone or calcium metabolism, this group would provide an useful opportunity for searching the minimum level of physical activity needed to maintain adequate bone mass.

Clinical Physiology 21, 4, 498±503 · Ó 2001 Blackwell Science Ltd

K. Uusi-Rasi et al. · Bone mineral density of visually handicapped women ............................................................................................................................................................................................................................................................................................................................

The purpose of this matched-pair cross-sectional study was, therefore, to investigate whether there is any difference between the BMD of visually handicapped persons and those with normal sight and, if so, to ®nd possible explanations for the difference. Subjects and methods

Nineteen visually handicapped premenopausal women and 19 healthy women with normal sight were recruited for the study. The visually handicapped women were obtained via the Finnish Register of Visual Impairment from the whole area of Finland. In addition to visual handicap, they had no other mental or physical disability. The control data were obtained from our database without knowledge on BMD. The control subjects were individually matched for age (‹5 years), body mass index (‹2 kg m)2) and smoking. The mean age of the visually handicapped women was 39á9 years (SD 8á1, range from 27 to 50) and that of the normal sighted women 39á7 years (SD 6á5, range from 27 to 50). The mean height was 163á7 (SD 7á6) and 163á4 cm (SD 7á0) and the mean body weight was 69á7 (SD 15á4) and 69á4 kg (SD 15á4) for those groups, respectively. The visually handicapped subjects did not participate in any leisure time physical activity provided for them. None of the subjects had chronic diseases associated with osteoporosis, such as hypogonadism, hyperparathyroidism, hyperthyroidism, hypercorticolism or eating disorders, and none used drugs affecting bone metabolism. They had not sustained any severe bone fractures in adulthood. The study protocol was approved by an independent ethical committee for clinical investigations, and each subject gave their informed consent. Health status

Before the measurements a physician made a clinical screening examination. In addition, information on the history and current status of physical activity, occupational work load, and consumption of alcohol and cigarettes was requested from all the subjects. The reason for the visual defect, its degree and its duration were obtained in an interview. Nine visually handicapped women were classi®ed as having Ó 2001 Blackwell Science Ltd · Clinical Physiology 21, 4, 498±503

impaired vision (visual acuity under 0á3±0á05) and 10 were blind (visual acuity under 0á05). By the international classi®cation of severity of visual impairment, visual acuity is considered impaired, if visual acuity repaired by glasses is under 0á3 and deeply impaired, if it is under 0á05 (WHO, 1992). The mean duration of the handicap was 35 years (range from 3 to 50 years). The handicap had been caused by a disturbance in development in ®ve women, ®ve were born blind, one was prematurely born, two had an atrophy of the optic nerve, two had hereditary retinopathy, one had diabetic retinopathy, two had been blinded accidentally, and one had a brain injury. Anthropometry

Height, weight and body mass index (BMI, calculated as weight/height2 in kg m)2) were measured for each subject during her visit. Calcium intake

Current total calcium intake and the use of calcium supplements were estimated by the 48-h recall method. The calcium intake data were calculated by Micro-Nutrica software (Social Insurance Institution, Helsinki, Finland). Physical ®tness and physical activity

The cardiorespiratory capacity of the visually handicapped women was determined by an incremental bicycle ergometer test up to the symptom-free volitional maximum. Maximal oxygen consumption was measured with an automatic metabolic analyzer (2900Z, Sensor Medics Corp., Anaheim, CA, USA). Two of the blind subjects could not be tested because of glaucoma. The 2-km Walk Test (Oja et al., 1991) was used to estimate the cardiorespiratory ®tness of the reference subjects. The maximal voluntary isometric strength of the trunk ¯exors and extensors, leg extensors and dominant upper-limb ¯exors was measured by strain gauge dynamometers (Heinonen et al., 1994). The most common type of physical activity of the visually handicapped subjects was walking. Sixteen women reported walking at least once a week at low to moderate intensity. Five women swam occasionally. 499

Bone mineral density of visually handicapped women · K. Uusi-Rasi et al. ...........................................................................................................................................................................................................................................................................................................................

Two women had an exercise cycle at home, and one woman did tandem biking in the summertime and skiing in the wintertime. One woman practised bowling once a week and one went horseback riding. The women with impaired vision were able to move without help during the day in familiar surroundings, while the blind women had to use a white cane or other type of aid when moving outside. The most typical leisure-time exercise of the reference group was also walking. In addition, three reported biking, three participated in recreational gymnastics once a week, two were engaged in gardening and two liked to ski. Bone densitometry

The areal BMD (g cm)2) in the weight-bearing proximal femur region (femoral neck and trochanter) and non-weight-bearing dominant distal radius were measured by dual energy X-ray absorptiometry (DXA, Norland XR-26, Norland Corp., WI, USA) (SievaÈnen et al., 1992, 1996). In our laboratory, the in vivo precision (coef®cient of variation) of these BMD measurements is better than 1% (SievaÈnen et al., 1992). Statistical analysis

The BMD and muscle strength data were compared with the use of paired Student's t-test. The t-scores for the BMD values of the subjects were calculated using data gathered in our laboratory from 86 healthy women aged 20±40 years (Haapasalo et al., 1996). Pearson's product±moment correlation coef®cient was used to assess the strength of the association between the BMD and potential explanatory factors.

The present sample size provides 90% statistical power to detect an intergroup difference of about one standard deviation at signi®cance level of P