You are standing in a moving bus, facing forward, and you suddenly move forward as the bus comes to an immediate stop. what force caused you to move forward?

No force is causing you to move forward. 

Newton's first law states that an object at rest stays at rest or an object in motion stays in motion unless acted on by an unbalanced force.

The bus was moving forward and comes to a sudden stop. You were inside the bus that was moving forward, but no force has yet acted on you, which is why you moved forward. Some unbalanced force would need to act on you to keep you from moving forward.

Hope this helps and makes sense!

All are great precautions but you can still lift lighter and different muscles in between hard workdays. this keeps other muscles not usually used in shape.