Stiffness index and self-reported walking speed
Women with a faster self-reported walking speed had a higher mean stiffness index than those with a slower self-reported walking speed after the adjustment for age, BMI, grip strength, comorbidities, current smoking, and current alcohol drinking. A previous study showed that self-reported walking speed is a good marker of measured walking speed. Self-reported walking speed is a useful measurement when the use of actual walking speed is not feasible. Cross-sectional studies have reported that calcaneus bone quality in elderly women is significantly associated with walking speed, stride length, and total number of steps.[17, 22] Increased walking speed caused a significant increase in activities of the lumbar erector spine, biceps femurs, and medial gastrocnemius; lumbar motion; and vertical ground reaction force in the loading response and mid-stance phases. Therefore, women with a faster self-reported walking speed may be placing an increased mechanical strain on bone due to an increased vertical ground reaction force and consequently gain bone mass.
Stiffness index and age
In this study, older age was associated with a lower stiffness index in postmenopausal women. This result supports that of a previous study that calcaneus stiffness index significantly decreased with aging. Gregg et al. reported that, in addition to age-related bone loss, menopause-related acceleration of a loss of stiffness index appears to occur in the perimenopausal period. This result suggests that measuring and maintaining bone mass is important in postmenopausal women.
Stiffness index and BMI
The present study showed that a higher BMI was associated with a higher stiffness index in postmenopausal women. A higher BMI was associated with a higher stiffness index, similar to previous reports. However, since obesity (a high BMI) is a major risk factor for cardiovascular disease and DM, maintaining a healthy BMI is recommended to support general health.
Stiffness index and grip strength
In this study, a stronger grip strength was associated with a higher stiffness index in postmenopausal women. Previous studies demonstrated a relationship between stronger grip strength and a higher stiffness index. Poor physical performance in elderly people may be reduced due to mechanical strain on bone resulting from reduced physical activity, leading to a bone mass reduction. These results suggest a strong role of maintaining muscular strength for preventing bone loss in healthy and functionally independent elderly people.
Bone mass and comorbidities (heart disease, lung disease, stroke, DM)
Patients with chronic heart failure had a lower reported bone mass than those without it. The prevalence of osteoporosis is reportedly higher in patients with chronic obstructive pulmonary disease than in healthy subjects. Bone mass losses were greatest among patients with severe lung disease. At 1 year of follow-up, a decreased bone strength index was reported in chronic stroke patients. Several factors reportedly affect bone quality (and mass) in patients with DM, mainly due to reduced bone turnover. Although our multiple linear regression analysis adjusted for such comorbidities, no significant difference was noted. Further studies, including other potential confounders, are needed to clarify the associations between comorbidities and bone mass.
Stiffness index and smoking and alcohol drinking
Several studies indicated that smoking and alcohol consumption influence bone loss, although others did not confirm these findings. In the present study, current smoking and current alcohol drinking was selected as explanatory variables on stiffness index in the multiple regression model. Few subjects were current smokers (2.1%) or current drinkers (11.9%). Thus, smoking and alcohol consumption appeared not to have a major influence on stiffness index in this study.
This study has several limitations. First, because actual walking speed was not measured, the relationship between actual walking speed and self-reported walking speed could not be examined directly in this subject. Second, because this study was conducted in a cross-sectional setting, these results do not show a causal relationship. Longitudinal studies are required to establish causal relationships between self-reported walking speed and bone mass. Third, the study subjects were community-dwelling residents who voluntarily attended a health examination, contributing to selection bias. It might suggest that the subject's awareness of health may be higher than that in non-attendants. Therefore, our results can’t be adapted to the general population. Fourth, the present results were obtained from only Japanese women; therefore, it is not possible to extrapolate the results to men or individuals of other ethnicities. Fifth, potential confounders that can affect self-reported walking speed and decreased bone mass, such as a sedentary lifestyle, vitamin D deficiency, low calcium intake, secondary hyperparathyroidism, peripheral nerve dysfunction, disorders of skeleton, vertebral osteoarthrosis, kyphosis, current medications, years after menopause, and renal insufficiency, were unavailable for participants in this study.