\subsection{Personal Activity Monitoring Devices/Apps}

The simplest of self-monitoring devices are pedometers. Such devices have long been of interest to health promotion professionals for their ability to encourage more active lifestyles \cite{Bakeretal08,Copelton10}. It has been shown that their use can lead to increased walking \cite{Croteau04,ThomasWilliams06} and moderate weight loss \cite{Richardsonetal08}. However, health promoting physical activity interventions have usually used pedometers alongside other resource intensive support and promotion activities, such as classroom training and face-to-face motivational sessions \cite{Bravataetal07,Kangetal09}. Pedometers exist either as dedicated devices whose sole purpose is to measure step count, or as embedded features in other equipment such as mobile phones. This dedicated nature of pedometers also presents barriers to their use, particularly for those less committed to physical activity. Users have to be committed enough to fitness and lifestyle-change to both purchase the pedometer and then remember to wear or carry it. Furthermore, questions of fashion, design and convenience can deter people from using such devices \cite{Consolvoetal06}. 

In addition to collecting physical activity-related data, some personal monitoring devices also include feedback on performance. Examples include: i) the Nike+ iPod Sport Kit\footnote{\url{http://nikerunning.nike.com/nikeos/p/nikeplus/en_US2}} MP3 player, which tracks individual exercise levels and interrupts music to verbally report on progress; ii) the Pediluma \cite{Limetal11}, a shoe accessory that tracks wearers’ physical activity and produces a visualisation of activity levels by varying the intensity of LED-lit custom-built enclosures that contain Arduino boards; and iii)  systems such as foot pods\footnote{e.g. Garmin (\url{www.garmin.com}), Polar (\url{www.polarusa.com}) or Sigma Sport (\url{www.sigmasport.com})}, Jawbone UP\footnote{\url{https://jawbone.com/up}}, Misfit Shine\footnote{\url{http://www.misfitwearables.com/}} and FitBit\footnote{\url{http://www.fitbit.com}} monitor various activities throughout the day and synchronise the data almost in real time to a mobile app or a web-based interface, where they are used either in an online coaching system or as an input to a virtual competition with a computer-generated partner.

Mobile-phone based pedometer apps\footnote{e.g. MapMyWalk \url{http://www.mapmywalk.com/}} have the advantage of being embedded in equipment that people already own and keep attached to them. Smartphones offer various sensors to infer and record physical movements throughout the day and provide a convenient platform to give feedback about activity levels. Their in-built display and communication capabilities allow users not only to view their feedback but also to share it with others. As a result, smartphones are increasingly being used to address the problem of sedentary lifestyles \cite{Consolvoetalb08,Fogg98, KingTester99}. Some mobile apps also allow the diarisation of physical activity data in order to provide additional motivation for physical activity \cite{Slootmakeretal09}. That said, to accurately measure and validate physical activity levels, there needs to be a consistent monitor placement \cite{Trost2005}, which is highly unlikely when integrated into mobile devices. 
