Regular physical activity is important to personal health and well-being, both for the individual and the society.
Encouraging people to exercise more is key to maintaining or regaining personal health but, unfortunately, difficult to
achieve in practice. One barrier to exercise is that lay people often are insufficiently knowledgeable about effective and
safe physical exercises. Maintaining an exercise regime over longer periods of time requires high levels of motivation. It is
well established that access to a personal trainer has a significant impact on both adherence and motivation to a program
of physical exercise [1], and the quality of the exercise undertaken [2]. They continuously monitor the exercises and both
provide individualized advice and motivate the trainee. Personal trainers also play an important role in rehabilitation, e.g.,
exercise programs for muscle recovery after surgery, for which there is a need for advice regarding effectiveness and safety.
Unfortunately, this is too expensive to provide over extended periods of time, and where financial factors are not a barrier,
personal privacy preferences can be (i.e., the perception of the potential for embarrassment).
Smartphones, being pervasive devices, are ideal to support and contribute to regular physical exercising. Apps for all
purposes have turned the phone to a multi-functional device, far beyond its classic domain of application. They transform
the phone to a platform for a variety of applications pervading everyday life: From reading news on the go, over locationbased
services, games, up to specialized leisure-time and ‘hobby’ apps for every flavor. Musicians can turn their phone into
a guitar tuner, gourmets into a wine guide, and so on. This trend is also visible in the sports domain. Increasing processing