Evidence shows that the ratio of the aged population is growing worldwide due to declining birth rates and increased life expectancy [1
]. Biologically, aging is an inevitable process that occurs throughout the life of species, with different evolution [2
]. In humans, this process implies a progressive deterioration of physiological systems in the last years of life [3
]. Aging is, therefore, a degenerative mechanism determined by genetic and environmental variables, in which lifestyle is strongly related to the development of the process [3
]. Therefore, certain daily habits such as physical exercise, physical activity, smoking, or stress can regulate this process positively or negatively.
As aging advances, the risks of fragility and disability increase [4
]. Although aging cannot be stopped, its effects can be countered by certain behaviors or lifestyles. Physical exercise and physical activity are undoubtedly health- and disease-related variables [5
]. Physical inactivity accentuates risk factors related to mortality and disease, such as cardiovascular disease, diabetes mellitus, or some cancers [5
It is important to differentiate between physical activity and physical exercise, both of which are related to life-long health. On the one hand, physical activity refers to any movement produced by the muscle system that involves an increase in energy consumption; activities such as shopping, gardening, house cleaning, etc. increase people’s physical activity. On the other hand, physical exercise is a planned, structured, and repetitive physical activity aimed at improving one or more components of one’s physical condition [7
]. Due to the risk of inactivity and the benefits of physical activity, the general physical activity recommendations of the American College of Sports Medicine (ACSM) are to increase physical activity through the habitual practice of well-scheduled physical exercise [7
There are currently many physical exercise recommendations for the health of adults [8
] and older people in particular [7
]. Following the ACSM recommendations, older people should perform aerobic, strength, and flexibility exercise and, if required, balance/neuromotor exercise [7
]. These different exercise modalities involve different activities during physical exercise sessions.
In sports, the importance of quantifying physical exercise is well known. The training load is measured to improve exercise prescription and to be more efficient in the training process, achieving better fitness goals [11
]. In prescribed health-related physical exercise, individualized and tailored programs are currently being proposed to improve the impact of physical exercise on health [12
]. This implies that physical exercise must be quantified by valid and reliable tools. Not only is accurate and objective information necessary, but such information must be accessible and, if possible, inexpensive to extend its use to the entire population.
Questionnaires and diaries have commonly been used to measure physical activity or physical exercise, but they present certain problems of subjectivity and bias [11
]. The Borg scale, used to measure the subjective perception of effort, is also commonly used to assess the intensity of exercise, but it does not always have a high relationship with the physiological changes produced during physical exercise [14
]. Heart-rate (HR) monitoring is a useful tool for measuring the intensity of aerobic physical exercise [11
], as HR and work intensity have a linear relationship when below the maximum oxygen consumption.
Accelerometers have been used in recent years to measure people’s overall physical activity over long periods of time (days, weeks), showing their usefulness in adolescents [15
], adults [16
], and older people [17
]. Accelerometry provides greater accuracy and precision than self-report measures or pedometers [18
Accelerometers allow interpreting the time, intensity, and frequency of physical activity (e.g., sedentary, light, moderate, or vigorous activity) or its combined subcomponents (e.g., moderate to vigorous physical activity, usually expressed as MVPA) during specific periods of time. Accelerometers are easy to transport and use and can analyze a large amount of data [19
Although accelerometry is considered a gold-standard measure to quantify people’s daily physical activity levels, this method has some limitations. This instrument cannot quantify the activity of upper body movements, aquatic activity or cycling, so certain activities are underestimated [20
]. Moreover, accelerometers do not provide information about physical activity behavior [20
]. Finally, the great variety of monitors, epoch lengths, and cut-off points selected are several factors that hinder comparisons between studies [21
Accelerometers are useful to quantify overall physical activity. However, as general physical activity recommendations promote physical exercise to increase daily physical activity and health [7
], it is necessary to ensure that the use of accelerometers will correctly quantify these scheduled and organized activities recommended for the general population.
The objective of this study is to evaluate the effectiveness of accelerometry to estimate two sessions of different physical exercise intensities in older people. For this purpose, heart-rate monitoring was used as a standard to correlate with accelerometer results.
The objective of this study was to evaluate the effectiveness of accelerometers in estimating physical activity during physical exercise sessions, as these sessions are generally recommended for increased activity and health [7
]. This prescription of physical exercise includes sessions with different exercise modalities and different intensities [7
], which should be as personalized as possible [12
], making the quantification of such exercise an important parameter in the management of training programs.
This study shows moderate correlations between HR and accelerometry in moderate-intensity aerobic sessions, but no correlation in low-intensity sessions based on balance.
There is also an almost non-existent correlation between HR in a physical exercise session and the energy expenditure calculated with accelerometers during the same session. The moderate correlation between HR and accelerometry during moderate-intensity aerobic sessions may allow the use of accelerometers to monitor these activities, at least for the five-minute intervals. However, with these correlation values, it should be kept in mind that, when using the accelerometer, considerable information is lost about the physical activity carried out during this type of session. Low-intensity exercises based on balance are completely underestimated with the use of accelerometers. Other physical exercise modalities that are recommended, such as resistance training sessions, are probably also underestimated by this device.
Accelerometers are often used to evaluate physical activity over extended periods of time, but not to quantify physical activity in particular physical exercise sessions [13
]. However, to improve these levels of physical activity and health, it is recommended to perform physical exercise programs with different training-session modalities [7
], and it should be taken into account that these activities will surely be underestimated when using the accelerometer to calculate individuals’ activity and energy expenditure.
In this study, two main limitations were found to consider accelerometers as useful for controlling exercise programs in older people. First, accelerometers do not present a high correlation with moderate aerobic sessions. Second, the accelerometer is completely inadequate to assess the intensity of low-intensity sessions based on balance and neuromotor exercises.
On the other hand, as Schrack et al. indicated in their study, we must take into account that activities such as walking are more intense for less functional subjects, [25
] and this can hardly be taken into account by accelerometry. In fact, if less functionality involves more effort and more energy expenditure when walking or performing any other physical activity, we should have specific cut-off points for older people concerning their functionality, and not simply because of their age.
Different cut-off points have been proposed to accurately measure the physical activity of older people. The cut-off points that Sanchez-Lozano [23
] used in this article show a low deviation in low-intensity activities, such as those performed in this study, but higher deviations in other measures [26
]. To our knowledge, there are no cut-off points for older people according to their functionality, and this would probably be necessary to gather more accurate information about physical activity programs. Likewise, it would be necessary to have different cut-off points for different types of physical exercise to more accurately estimate the physical activity carried out by older people when participating in organized physical exercise programs to improve their health.