1 mark for each force (3 marks max).
1 mark for explaining an appropriate effect of the force (3 marks max).

gravity;
effect: pulls the sprinter towards the ground as they drive out of the blocks / forces the sprinter's feet to return to the ground;
muscular force;
effect: the power the sprinter exerts to push himself out of the blocks quicker / continues to be applied through to the ground;
air resistance;
effect: the faster the sprinter accelerates there is greater air resistance which will slow the sprinter down / acts in opposite direction to motion to slow them down / acts against the runner to slow them down;
ground reaction force;
effect: the force from the ground to the sprinter that is equal and opposite to the action force / the harder the sprinter pushes the force downwards and backwards helps to move the sprinter upwards and forwards;