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Using heart rate monitors during resistance training may not be accurate.
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Heart rate monitors can be effective weight loss tools as they enable individuals to monitor calories burned during exercise. While heart rate monitors can be relatively accurate for calculating calories burned during exercise, they may not be as accurate when used during weightlifting.
Estimations and Calculations
Heart rate monitors provide estimations of calories burned based on the relationship between heart rate and oxygen consumption. When more oxygen is consumed, more calories are burned and heart rate increases. This relationship is a key factor in the estimation of caloric expenditure from heart rate. The estimation of calories burned from heart rate is based on the heart rate and oxygen consumption responses to endurance or aerobic exercises. While you can use a caloric monitor while weightlifting, you aren't likely to get an accurate result due to the mechanics of this kind of workout.
Heart Rate Relationships
The estimation of caloric expenditure from heart rate only applies to longer duration exercises in which a steady state or a matching of the heart rate and oxygen consumption values can be reached. This is only the case for cardio or aerobic exercises. The cardiovascular responses to cardio and weight training are very different. During cardio, heart rate increases in direct proportion to exercise intensity. However, in strength training exercises, heart rate response is higher for a given increase in oxygen consumption than in cardio.
Heart Rate Responses
Cardio and strength training have different effects on heart rate and metabolism. When muscles contract, a signal is sent to the brain which responds by increasing heart rate and blood flow to the stressed muscles. During cardio, the increase in blood flow to the muscles is counteracted by the repetitive contraction of the muscles which helps the blood return back toward the heart. This action is referred to as the "muscle pump," as the muscles are literally pumping blood back upward toward the heart. During weightlifting, blood flow is increased to the muscles but there is no muscle pump to assist it in getting back to the heart. This reduction in the return of blood to heart stimulates the brain to increase heart rate even more.
The lack of precision in estimating caloric burn from heart rate is not limited to weightlifting, but also applies to other exercises like yoga and Pilates in which there is not continual repeated contraction of skeletal muscles. Using heart rate monitors during these types of exercises will almost always overestimate the calories burned due to the higher heart rate response for a given increase in metabolism with these exercises. Using heart rate monitors to estimate calories burned during workouts should only be applied to cardio until there is a more accurate way of estimating calories for other types of exercise.