The force exerted on the ground during human locomotion is a periodic function of time. The following video illustrates the correspondence between different components of a human stride while running and the graph of force as a function of time.
Other examples can be found on the website of the Skeletal Biology Lab at Harvard University.
In this project of the textbook you are asked to construct a mathematical model for the force exerted on the ground as a function of time using periodic extensions of the cosine function. Problem 3 of this project then asks you to explore the function
The following animation allows you to explore the graph of this function for different values of q.
References
Biomechanics video was kindly supplied by Dr. Daniel E. Lieberman.