What factors influence our decisions about what physical activities to engage in and how physically active to be?

social environment such as our peer group and teachers and other circumstances such as the availability of facilities.

Explanation:

If your peer group re the one that involve in so much sedentary lifestyles such as playing video games and online social interactions, you are likely going to follow that pattern. This is because as we grow older, the influence of social groups on our behavior becomes intense.  

Most athletes who started their career from their school systems have remarked that their coaches and personal trainers influenced their participation at those sporting activities. They have such influence because they are in the position to confirm ones competence in the games and that either makes us more interested in the activities or make our interests wane.

also, the availability of play spaces can determine the amount of activities we can engage in.

which sentences ? that question is incomplete