Answer:
This phenomenon is called social facilitation.
Explanation:
Social facilitation refers to one's tendency to improve his/her performance when other individuals are present. When the individual is alone, he/she has certain performance but having other individuals around tends to make this individual want to improve.
In this case, Cody is riding the bike at a leisurely pace but as soon as the gym gets less empty, she improves her performance by the sheer presence of others.