In this prospective, population-based, 13-year cohort study, low physical activity was a strong risk factor for cervical, but not for trochanteric, hip fractures. Additionally, functional mobility, measured with the TUG test, and low BMI were associated with the risk of both hip fracture types. Other clinical factors appeared to differ between the hip fracture sites: hypertension and coffee consumption of more than three cups per day decreased the risk of cervical hip fractures, while daily smoking increased the risk of trochanteric hip fractures.
Physical activity helps to maintain muscle strength and mobility and thus prevent falls in older people . In contrast, a more active lifestyle has been shown to increase the incidence of nonsyncope falls and related traumas, e.g., wrist fractures [14, 26]. Our results showed that the women with high physical activity at baseline had a lower risk of future hip fractures (especially the risk of cervical fractures) than women with moderate or low activity. A similar dose–response relationship has been reported in both male  and female populations [6, 28]. Based on our results, regardless of earlier lifetime physical activity, an active lifestyle in the postmenopausal years decreases the fracture risk. Both Feskanich et al.  and Michaëlsson et al.  reported that by increasing physical activity during the lifespan, the fracture risk could be diminished, whereas Høidrup et al. reported that increments in physical activity during follow-up did not influence the risk of hip fractures. In our study, no longitudinal data on physical activity during the follow-up period were available.
Baseline physical activity was associated with cervical, but not with trochanteric, fractures in our study population. This finding might indicate an association between physical activity and femoral geometry. Previously, it has been shown that bone geometry is associated with cervical fracture risk, while bone density is more strongly related to the risk of trochanteric fractures . Furthermore, it has been shown that mechanical loading is a strong external determinant of the structure and concomitant strength of the femoral neck . Therefore, this finding may arise from the structural weakening of the femoral neck caused by a low physical activity level. In particular, the thinning with age of the superolateral femoral neck cortex leads to the loss of elastic stability due to under loading of this site, exposing it to local buckling of the cortex [31, 32]. Based on the current results, it can be assumed that this structural weakening may accelerate in individuals with low levels of physical activity, thus increasing the risk of femoral neck failure.
In this study, we evaluated functional mobility using the “Timed Up & Go” (TUG) test. The TUG test is easy to perform and reproducible, and it is a sensitive and specific measure for evaluating fall risks [23, 33]. In our study, 11 seconds was selected as the threshold value for the TUG test based on receiver operator characteristics (ROC) analysis. Thirty-three percent of the subjects took more than eleven seconds to complete the test. We found low functional mobility to be a risk factor for both types of hip fractures. Similar results with a similar population were reported earlier for general hip fracture risks among subjects with slow TUG performances . In some studies, walking speed and repeated rising from a chair have been used as alternatives to the TUG test to assess mobility and neuromuscular function. In a study by Fox et al. , walking speed was associated with both hip fracture types, but the ability to complete five chair stands was not associated with either. In contrast, Cummings et al.  reported that subjects unable to rise repeatedly from a chair had a higher risk of hip fractures. Because functional mobility is easy to assess, it should be routinely evaluated to screen for individuals at high risk for fragility hip fractures.
The role of arterial hypertension as a risk factor for hip fractures is somewhat controversial. Hypertension alone has been shown to increase fracture risks by affecting calcium metabolism , as well as by increasing the risk of falling due to reduced baroreflex sensitivity or hypotension . In the present study, hypertension was found to be protective against hip fractures. This may be due to the use of thiazides as diuretic medication for hypertension. Thiazide diuretics have been reported to have positive effects, in terms of bone strength , by reducing urinary calcium excretion and helping to maintain calcium balance , and by inhibiting bone resorption by means of inducing metabolic alkalosis .
Multiple studies have indicated that smoking increases fracture risks in both women and men [41, 42]. Numerous reasons for increased fracture risks have been suggested. These reasons include direct toxic effects on the bones due to exposure to nicotine, reduction of calcium absorption, transient increases in cortisol levels after smoking, lower BMI, and an increased risk of falling in smokers, as well as lower estrogen levels and earlier menopause . In our population, there were only seven fracture subjects who declared at baseline that they smoked daily. The observed increase in trochanteric fracture risk may be due to the above-mentioned reasons, but the ultimate reason cannot be determined. Because the trochanteric region is rich in trabecular bone, the increased risk might occur because tobacco affects this metabolically active region. However, the small number of fractures limits the statistical power of this finding. Nevertheless, our results suggest an increased risk of trochanteric fractures among smokers and are in line with earlier studies.
Our results suggest that coffee consumption of more than 3 cups per day may prevent cervical hip fractures. However, excessive coffee drinking has been reported to be associated with an increased hip fracture risk [8, 21]. In a recent review by Higdon and Frei , the effects of coffee on bone density and hip fracture risk were discussed. Caffeine affects calcium absorption and leads to a slightly negative calcium balance in individuals with inadequate calcium intake. Moderate coffee consumption has some other health benefits , which together with our finding support moderate coffee consumption.
We found no differences in hip fracture risks between women who had received estrogen treatment or taken osteoporosis medication at baseline compared to women who did not receive estrogen treatment. One possible explanation for this result is that the new-generation drugs for osteoporosis were not available earlier in the study period, and very few women had taken medication for osteoporosis. Furthermore, population studies have observed a reduced risk of hip fractures with postmenopausal hormone use among sedentary women but not among physically active women [10, 27]. However, we did not observe this type of trend. This finding might be due to the low number of fractures, especially among women taking estrogen medication. The postal questionnaires also included questions concerning asthma and incontinence. These conditions, however, showed no effects on the hip fracture risk in this population.
The strengths of this study were its population-based, prospective nature and the long follow-up period. The target population was a homogenous, stable, and representative sample of older Finnish women, obtained from the National Population Register of Finland, which provides 100% coverage. This study also has some limitations. Because of the population-based nature of the present study, the results can be generalized to older Caucasian women. However, there might be some selection bias because the 459 women (27.3% of the total cohort of 1681 women ) who neither replied to the postal questionnaires nor participated in the clinical examination were more fragile, with a higher hip fracture rate (9.8% vs. 6.3%) and higher mortality (50.8% vs. 25.1%) than the participants. According to Finnish National Institute for Health and Welfare, in year 2009, 6085 hip fractures occurred in Finland. The age-standardized hip fracture incidence of 293 fractures /100,000 persons among Finnish women has recently been reported . Thus, the results may not be suitable for generalization to very frail or institutionalized women. Unfortunately, other health data were not available here for the non-participants. The number of hip fractures was limited. In 1222 women, we observed 49 cervical and 31 trochanteric fractures. It also should be noted that all of the clinical and questionnaire data, excluding the fracture data, were collected only at baseline and were not repeatedly collected during the follow-up. In addition, no hip or spine DXA data were available.