The adverse effects of sedentary behaviour and benefits of physical activity on health have recently come to the fore. A number of studies supports the fact that physical activity can reduce effects of sedentary behaviours and the interventions to increase physical activity become widespread (Swartz, Rote, Cho, Welch & Strath, 2014; Aparicio-Ugarriza et al., 2015). Sedentary behaviour is associated with a wide spectrum of chronic disorders, including chronic obstructive pulmonary disease (Hill, Gardiner, Cavalheri, Jenkins & Healy, 2015), mental illness (Zhai, Zhang & Zhang, 2014; Hoare, Milton, Foster & Allender, 2016), some types of cancer (Cong et al., 2013, Schmid & Leitzmann, 2014), and cardiometabolic disorders and related mortality (Hamilton, Hamilton & Zderic, 2014; Peterson, Charlson, Wells & Altemus, 2014; Saunders, Chaput & Tremblay, 2014; Chau et al., 2015). For these reasons, it is of great importance to provide valid methods for the assessment of physical activity and to better understand the relationship between physical activity and health so that the success of interventions can be determined. The ultimate goal is identification of optimal physical activity to reduce health risks in general population (Ainsworth, Cahalin, Buman & Ross, 2015).
There are various subjective and objective methods to assess physical activity. Subjective methods, such as questionnaires, diaries, interviews, and direct observation, are inexpensive methods to assess physical activity in many situations and are commonly used in studies with a large number of participants. However, the assessment results might not be precise because they depend on subjective observation and interpretation. On the other hand, objective methods for the assessment of physical activities offer opportunity to obtain more accurate data about physical activity. These include direct and indirect calorimetry, physiological measurements, such as Doubly Labelled Water method, and mechanical and electronic monitors, including accelerometers, pedometers, and heart-rate monitors (Hills, Mokhtar & Byrne, 2014). Still, the precise quantification of physical activity can be difficult, and the choice of assessment method is influenced by different factors, such as age, gender, and body weight. Also, type of physical activity, objectivity of the data, and a cost-efficiency should be taken into consideration (Sylvia, Bernstein, Hubbard, Keating & Anderson, 2014).
The aim of this literature review is to provide an overview of advantages and disadvantages of motion sensors, as well as their reliability and validity in the assessment of physical activity. This review will also provide information about the use of motion sensors and possible interventions in clinical research studies.
Motion sensors in assessment physical activity
Subjective assessments of physical activity have been used for a long time because they are cost-effective, easily adaptable to different types of research, and generally accepted. They provided significant information regarding the relation between physical activity and health. However, they are not very precise and rely on subjective interpretations and consequently lead to underestimation or overestimation of physical activity. The introduction of objective methods to assess physical activity reduced human error and provided more accurate estimation of physical activity and energy expenditure (Ainsworth et al. 2015).
Wearable motion sensors (accelerometers and pedometers) are popular tools for objective assessment. Pedometers are used to measure steps and distance while accelerometers measure acceleration and movement (Strath et al., 2013). Pedometers are motion sensors that record movement in terms of steps taken. Early forms of pedometers used mechanical sensors that identified steps based on the force generated during walking. Nowadays, with the advancement of technologies, they use microelectromechanical systems to identify steps which considerably increased their accuracy. Most of them are hip-worn, but it is suggested that the more accurate position should be the ankle. Furthermore, some recent models also allow measurement of energy expenditure, acceleration, and sleep (Plasqui, Bonomi & Westerterp, 2013). Accelerometers provide information about type, frequency, intensity, and duration of physical activity, and, thus, they are commonly used in research studies. Similar to pedometers, they are typically hip-worn, but can also be fixed to ankles or wrists. It is proposed that the more accurate position to wear accelerometers is the lower back or hip, i.e. closely to the centre of the mass. They rely on microelectromechanical systems to record acceleration and objectively capture body movements. Thanks to the technology advancements, they can detect types of physical activity and energy expenditure. There are many commercially available accelerometers with different characteristics making the choice of the most suitable accelerometer very difficult. (Plasqui et al., 2013; Ainsworth et al., 2015)
Not only that the use of accelerometers increases in recent years, but with technology improvements, there is a tendency to insert them into smartphones as they are regularly used in everyday lives, especially among adolescents. It is proposed that designed application for mobile phones should be used with other objective assessment monitors, which will improve the quality of collected data (Dunton et al., 2014; Shoaib, Bosch, Incel, Scholten & Havinga, 2014). On the other hand, there is a high inaccuracy of smartphone pedometer applications, which suggests caution in interpretation of smartphone application data (Orr et al., 2015).
Advantages and disadvantages of motion sensors
The use of wearable motion sensors, such as pedometers and accelerometers, in physical activity assessment increases in research and clinical assessment. However, the choice of the most adequate monitor will depend on several factors: research goal, target population, physical activity characteristics, cost-efficiency, and required measurement precision (Ainsworth et al., 2015).
Pedometers are inexpensive and present a low burden for participants. Further, they can be used in studies with many participants and data obtained from pedometers are easily processed. But pedometers do not measure intensity or duration of physical activity and are not accurate for assessment of energy expenditure (Strath et al., 2013). Pedometers also fail to be accurate at slower walking speeds or when worn at pockets or wrists and they cannot detect sedentary activities, posture, and energy expenditure (Ainsworth et al., 2015).