Using heart rate during ultra endurance training sessions to monitor intensity is not reliable for a number of reasons. There are a multitude of factors that can cause heart rate to increase or decrease without a change in exercise intensity.
- Dehydration: Your blood volume (the amount of blood circulating through the body) decreases when you are dehydrated. The body compensates for this by increasing heart rate to get oxygen to working muscles.
- Increase in body temperature: Training in hot environments or long endurance efforts that increase core temperatures increases heart rate about 10 beats per minute for every 1 degree rise in internal temperature.
- Altitude: Decreases in barometric pressure at altitude reduces the amount of oxygen that is passed from the lungs to the blood. The decrease in oxygen causes the heart rate to increase to improve oxygenation to muscles during activity and at rest.
- Caffeine: Many of the sports gels, blocks, waffles and drinks contain caffeine and can raise the heart rate within 15 minutes of consumption. It can take up to 6 hours for caffeine to leave your system.
- Fatigue: Exercise induced fatigue can also lead to an increase in heart rate for the same level of intensity. This is primarily attributed to a decrease in muscular efficiency.
- Emotional responses: Stress, anxiety, anger and excitement can cause an increase in heart rate. This can be as subtle as listening to a song or podcast that elicits an emotional response to a slip on a sketchy section of trail.
- Equipment failure: While technology can be a wonderful tool, batteries die, blue tooth connections can experience interference and the contact between the device and the skin can slip during activity. The type of heart rate monitor can also vary greatly in reliability.
Heart rate can be a valuable training tool during shorter duration workouts to establish a feel for what intensity you should be training at, but as the workouts get longer in duration reliance on equipment should be replaced with how you feel and breathing to gauge intensity.