PDHPE

From 8 to 9 minutes, the heart rate plateaus, as exercise has continued at a constant intensity for a period of time and the body is able to supply sufficient oxygen to the working muscles.
The heart rate (HR) will change only if the exercise intensity changes e.g. if it increases, the heart rate will increase, or if it decreases, the heart rate will fall. The heart rate will not plateau if the exercise is at an intensity that does not provide the working muscles with the oxygen they need. In this case, the heart rate will continue to rise until the oxygen needs are met or until fatigue causes exercise to cease.