This paper designs a Discrete-time Sliding Mode Control (DSMC) for heart rate during treadmill exercise. The control objective is to make the runner's heart rate follow a heart rate reference profile set by specialists. The nonlinear model describing the relationship between treadmill's speed and heart rate is linearized around an operation point. The linear model is then discretized by using a Zero Order Hold (ZOH) in order to obtain a discrete-time model. This discrete-time model is then used to design a sliding mode controller. Simulation results will show that the designed controller is able to make the runner's heart rate follow the prescribed profile irrespective of the uncertainties in the model. On the other hand, the simulation results in this work are presented in a wide range of speeds, range that is not usually employed in previous studies for heart rate control.