To solve this problem, we have to use Hooke's Law.
Where k is the constant of the spring, and x is the displacement we have to find.
According to the problem, the force is 7 times the runner's weight, which means
Where m = 65 kg and g = 9.8 m/s^2. Let's find this force.
Then, we use Hooke's Law to find the displacement.
But, we have to find the displacement in millimeters. So, let's divide by 1000.
Therefore, the runner's Achilles tendon will stretch 0.002 mm.