Potential energy is the energy that an object possesses due to its position. The potential energy of the object is directly proportional to its height.
The kinetic energy is the energy that an object possesses due to its motion. The kinetic energy of an object is directly proportional to the square of its velocity.
As the sprinter starts to accelerate forward, the sprinter starts to gain velocity but the height of the sprinter remains the same. Thus, the kinetic energy of the sprinter increases and the potential energy remains the same.