A buddy of mine is a serious endurance athlete, and monitors his heart rate regularly during workouts. For the last week or two, he has been unable to get his heart rate anywhere near his normal range.
Usually his max HR is around 185, but during his last few workouts, even with sprints and two hours of running, he has been unable to break 140. He still feels the normal signs of exhaustion towards the end of his workouts and is following his normal routines. His resting heart rate has gone down as well. The HR monitor is still working fine.
Anyone have any ideas what could cause this?