Keeping the motivation to go to the gym can be difficult. Pact uses behavioural economics to address this. 

Loss aversion, the fear of losing money is 2.5 times more effective in changing behaviour than receiving a reward. One of the founders of PACT is an endocrinologist and he finds the incentive aspect of the app important, clearly having information about the benefits of exercise is not enough.

There's more about this in the following story in the FT: