Osteoporosis is a skeletal disease characterized by reduced bone mass and disruption of bone architecture, leading to increased bone fragility and fracture risk [1
]. This condition is highly prevalent worldwide. It is estimated that osteoporosis affects 27.5 million adults in the European Union [2
] and 10.2 million people in the United States [3
]. Further, it is expected that one in three women and one in five men over the age of 50 years will suffer a fracture due to osteoporosis [4
]. These figures exhibit an important economic burden. In Europe, the costs associated with osteoporotic fractures hospitalization and aftercare are estimated at €37 billion annually [2
]. In Spain the number of individuals with osteoporosis is estimated at 2.5 million, generating a cost for the health care system of €3.5 billion per year [5
]. Moreover, these costs are expected to increase twofold by 2050 based on the expected demographic changes [6
]. This large burden on health systems demonstrates that osteoporosis is a major public health concern worldwide. It is therefore clearly of interest that effective osteoporosis prevention programs are developed.
Among the strategies that may help in preventing osteoporosis, regular exercise is recommended for a variety of health and fitness reasons. Evidence shows that different types of activities (e.g., aerobic exercise and strength training) have an effect on the bone mineral density (maintenance and stimulation) in both pre- and post-menopausal women [7
]. Bones are living tissues and have the capacity to adapt to physical activity by increasing their size, shape or density in order to better resist biomechanical demands [8
]. Specifically, exercise increases bone mineral density (BMD), bone mass and bone strength by stimulating osteogenic pathways and the activities of osteoblasts and osteocytes, as well as by inhibiting osteoclastogenesis and bone resorption [9
]. For bone mass to increase, bone tissue must be stimulated with loads exceeding a certain threshold, as the osteogenic response depends on the magnitude of mechanical forces applied [10
]. In this regard, previous research [11
] has highlighted the importance of reaching accelerations above 3.9 g during physical activity to elicit positive bone adaptations. Therefore, it is suggested that in order to develop effective protocols for osteoporosis prevention and to ensure skeletal integrity, exercise intensity must be objectively quantified and monitored (i.e., bone-loading forces).
To that end, new technologies such as accelerometry are increasingly being used to monitor exercise and assess mechanical loading in physical activities [10
]. An accelerometer is an electromechanical device that converts mechanical motion into an electrical signal [12
]. Therefore, these devices can provide objective measurements of movement (i.e., physical activity) and could assist in implementing effective osteoporosis prevention programs. Advances in commercially available accelerometry-based activity monitors represent an opportunity for a wider population to assess the intensity of different types of activities. Nevertheless, the cost of reliable accelerometers is still high and interpretation of the resulting data by the user remains a challenge [13
]. Moreover, there is a paucity of data regarding the validity of wearable accelerometer-based activity monitors aimed at preventing osteoporosis [10
]. Consequently, the aim of this study was to determine the validity of a wearable accelerometer-based activity monitor in order to assess its suitability for the implementation of population-wide osteoporosis prevention programs.
The aim of this study was to determine the validity of Muvone®
, a wearable accelerometer-based activity monitor, in order to assess its suitability for the implementation of population-wide osteoporosis prevention programs. Its validity has been tested against ActiGraph GT3X+, as ActiGraph activity monitors are widely used in physical activity research [14
] and their reliability to measure acceleration accurately has been previously reported [15
]. Measurements from both devices were positively and significantly correlated for all exercise tests (CMJ and treadmill bouts at all velocities), showing a very high to almost perfect correlation. Pearson correlation coefficients were also calculated selecting only measurements above 3.9 g, as this acceleration value has been previously described as a threshold for osteogenic physical activity [11
]. Bone adaptations display a dose- and intensity-dependent relationship with mechanical loading, and these authors found positive BMD changes at the proximal femur with an exercise dose of at least of 60 impacts per day exceeding the acceleration level of 3.9 g. Therefore, accelerometer-based activity monitors should provide accurate measurements above this level in order to be considered for the implementation of osteoporosis prevention programs. In the current study, for measurements over 3.9 g moderate to very high correlations between Muvone®
and GT3X+ were found for all tests (CMJ and treadmill bouts) and both accelerometer locations (hip and wrist) (Table 2
To assess the level of similarity in group estimates from both devices, the differences between outputs were assessed using t
< 0.05), and non-significant differences were found for all tests and locations (Table 4
Finally, absence of measurement bias (level of agreement between devices) was evidenced by Bland–Altman plots (Table 5
). Relative mean bias was below 6%, which translates into an absolute bias below 0.35 g for all exercise tests and accelerometer locations, an error value reasonably acceptable in an osteoporosis prevention exercise program.
In people affected by osteoporosis, the threshold between an effective stimulus for positive bone adaptations and a harmful stimulus for bone integrity could be reduced. Therefore, in order to ensure the effectiveness and safety of an osteoporosis prevention exercise program, accurate monitoring of mechanical loading is necessary. Traditionally, the effect of physical activity on bone health is studied by employing force platforms to measure ground reaction force. These devices are mostly limited to laboratory environments [10
]. Wearable accelerometer-based activity monitors provide the opportunity for a wider population to participate in osteoporosis prevention programs.
A limitation in this study was the array of physical activities tested. The exercise protocol included CMJ and treadmill bouts at different speeds. During normal daily life, many other activities take place that imply accelerations that would need further analysis. Besides, many activities involve arm movement that is dissociated from lower body movement (e.g., sweeping, carrying bags, etc.). Therefore, when assessing physical activity that may be beneficial for bone health, the relevant accelerometer location must be considered. Moreover, because this was a non-experimental study, causal associations cannot be made and thus we are not able to determine if an accurate quantification of the mechanical loading during physical activities can incur bone mineral density changes in premenopausal women. Finally, and although our sample was relatively homogeneous in age, it was small and thus our results might not be generalizable to other age groups. Despite these limitations, our data support the validity of both wrist-worn and hip-worn Muvone® accelerometers in premenopausal women.