It starts with an article by Richard M. Ryan and Edward L. Deci, Intrinsic and Extrinsic Motivation: Classic Definitions and New Directions. We're hearing a lot of talk about intrinsic motivation from
Kohn's criticism also applies to the second type of extrinsic motivation called introjection. Actions in this stage of extrinsic motivation are driven by social approval. They are important for the person's ego. Although it is less externally controlled than pure external regulation it can still involve a controlling relationship between the source of approval and the actor. Praise is an ego-stroking reward that fits into this category. Or losing weight to avoid being mocked. In these two stages are dependent on external pressure. If the pressure goes away, motivation to do the activity goes away with it. People who have once been motivated through external rewards may fall below their base level once the rewards stop coming. In encouraging physical activity, it should be clear to us that this kind of motivation cannot be healthy, even if it works as long as rewards last. Behaviorists are not wrong, operant conditioning does indeed work - but important matters like physical activity should not be approached with a "whatever works" attitude.
However, there are two more types of extrinsic motivation where the source is to an increasing degree internal. What they have in common is that the activity's goal truly matters to the actor. Although they might not enjoy doing the activity in itself, they understand the importance of doing the activity because it progresses them towards the goal. Even better, they identify with the goal and it becomes a part of their ideal self ("I want to be fit to stay healthy" versus "being fit is a part of who I want to be"). The activity becomes valued by the actor and they feel they are doing it for themselves. In developing persuasive technology, this is where we should by playing ball. Of course it only works if the actor actually values the goal - if they do not, then things will be a lot more complicated. However, I consider that situation to be beyond what we can do with just technology.
So there are two fields where we can operate. The most common approach taken in HCI research is to aid users that are in the better half of extrinsic motivation - they have already established a goal of increasing their physical activity. In this domain, persuasive applications are helpers. It is their job to improve the actor's feelings of competence. There are a lot of ways to go about this. Simple technologies like pedometers alone fulfill one specific need of competence: progress feedback. Technology can quantify activity, allowing the actor to see how well they are doing. It goes up from there. Technologies can help to pick suitable short term goals. They can nudge. They can help connect with like-minded people who are going through the same things. The wealth of persuasive technologies that can be employed once the actor is at least somewhat motivated towards the big goal is huge.
The other field is where exergames operate. It's kind of like a shortcut. Because physical activity is a very broad term, there are a lot of specific activities within it. Exergames in particular aim to create a new activity that the actor might find fun to do. It is possible to jump from amotivation straight into intrinsic motivation if a suitable activity can be found. Dance Dance Revolution has been mentioned to death. Other popular examples include Geocaching. It's not just new activities though. Simply finding an enjoyable sport can do the trick. Technology can step in and help a person to find an activity they might actually like. This is best achieved by making it as effortless as possible to try out new activities. Exergames, especially mobile games, can have a very low barrier of entry (e.g. $1 for a mobile game that allows you to start immediately, or even free-to-play).
I believe that both of these approaches are correct. The first approach is suitable for people who are prepared to get into exercising but need a little push to start and a lot of pull to keep going. The second approach can work wonders with people who do not care about exercise and don't have any fitness goals. It is worth notice that these people can also get started with an exergame and realize that they actually have the competence to become fit if they keep going, which in turn allows them to assume a healthy exercise routine. Or they might want to start doing other exercise to become better at their game or sport. At this point we can combine methods from the first approach to help their training.
To summarize, there are two clearly distinct fields to play in regarding motivating physical activity. Both have their uses, and both have their unique challenges. Research has been done in both fields, but we're still waiting for a physical activity revolution to happen. In this field it seems rather typical that research prototypes do not turn into products, and commercial products are not necessarily up to date with their psychological research. There is still a lot more space for both research and new products in this domain. Oh, and physical activity is just one thing, there's more to come.