Population physical activity levels remain low and are a major public health concern in high-income countries. The burden and cost of inactivity to society are high due to being associated with non-communicable chronic diseases such as cardiovascular disease, some cancers, diabetes, osteoporosis, and obesity ,. Given the magnitude of the problem, effective large-reach low-cost physical activity interventions are needed . Implementing interventions through the Internet has substantial potential, as Internet access in most high-income countries is very high. For example, 79% of Australians have access to broadband Internet , and it is estimated that in 2015 there will be over 2.8 billion Internet users worldwide . There are many advantages of using the Internet for health promotion and preventive medicine, including options for instantaneous interactivity, continued assessment and follow-up, individual tailoring, diverse delivery formats (e.g. (printable-) text, video, audio, e-mail or combination), anonymity, and high convenience ,. Moreover, Internet interventions can reach respondents nearly anywhere at any time through desktops, laptops and mobile devices ,.
However, achieving health behavior change through the Internet has proved to be considerably harder than anticipated . While the potential of Internet technologies to change health behaviors has not yet been fully understood, web-based interventions still hold much promise. Recent reviews and meta-analyses in the field of physical activity show small and short-term effects in web-based interventions ,. Long-term effects are lacking predominantly due to problems in attracting, engaging and retaining participants into web-based interventions ,. However, much progress has been made and knowledge of what works and what doesn't in web-based interventions has grown tremendously in recent years ,.
A number of strategies, frequently used in web-based behavior change interventions, are associated with positive health outcomes ,. This includes strategies derived from behavior change theories such as the use of self-monitoring , goal setting ,, modeling , social support ,-, and providing educational content . But also the use of repeated contacts with participants ,, regularly updated websites , individually tailored feedback -,, e-mails ,, and alternative delivery modes (such as smartphone applications ,) have shown to be effective components of web-based interventions. Furthermore, the level of `interactivity' of behavior change websites itself has also shown to be essential for the effectiveness of web-based interventions ,,.
In this respect the development and implementation of social media applications, which can be defined as a group of online applications that allow for the creation and exchange of user-generated content , have vastly increased the interactivity of many websites in recent years. Several types of social media-based websites can be distinguished; examples are: collaborative projects (e.g., Wikipedia), blogs or microblogs (e.g., Wordpress and Twitter), content communities (e.g., YouTube), and social networking sites (e.g., Facebook) . These tools are a part of what was, in 2004, termed Web 2.0: the utilisation of the World Wide Web as a platform where content is continuously modified by all users in a collaborative fashion . Whilst the evidence regarding the effectiveness of social media applications for physical activity interventions is still emerging -, there are several reasons illustrating why using social media applications for promoting behavior change might be important. Firstly, social media applications can reach very large audiences (e.g. Facebook has 1.1 billion users each month) and have high levels of user engagement and retention . The highly dynamic and flexible nature of social media applications, with continuously changing content, is likely partly responsible for this high popularity and engagement . Secondly, information can be delivered via existing contacts (`friends'), and this `word-of-mouth' influence is more powerful than traditional social marketing strategies ,. Thirdly, social media applications are characterized by user generated content and multidirectional communication flows in which users participate as both creators and consumers of web content . This active engagement and content generation is more influential compared to websites that are passive or reactive in nature and which don't allow users to generate new content . Finally, social media applications can link people to physical activity opportunities, highlight previously unknown preferences among individuals, enable role-modeling, allow normative comparisons, and provide instantaneous feedback and reinforcement .
In 2003 Doshi et al.  conducted an evaluation of freely available physical activity promotion websites and concluded that the use of theory-based strategies was low, and that websites provided little assessment, feedback or individually tailored assistance. Also in 2003, Evers et al.  assessed existing health behavior change websites and concluded that many did not include the basic requirements to achieve health behavior change. When Doshi et al.  and Evers et al.  conducted their analyses, the field of web-based interventions was relatively undeveloped compared to the current understanding. Over the subsequent years the knowledge of what website features and components are likely to change behavior and increase user engagement has been guided by a wealth of original studies, systematic reviews and meta-analyses ,,. Given the continued proliferation of un-evaluated physical activity promotion websites, the increased knowledge of effective web-based intervention components, and the large developments in Internet technology, it is timely to re-assess to what degree current physical activity promotion websites incorporate evidence-based behavior change techniques and innovative social media features. This will provide insight into the current implementation of useful intervention components and will inform future work surrounding the dissemination of effective web-based interventions.
The aim of this study was to evaluate to what extent freely available physical activity promotion websites apply techniques that have shown to be conducive for behavior change in intervention studies and to evaluate, for the first time, to what extent they incorporate social media applications which allow for enhanced user interactivity.