Your exercise heart rate is a good indicator of exercise intensity during low-to-moderate intensity rhythmic exercise. But when you exceed your anaerobic threshold, heart rate alone may not adequately reflect your work rate.
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Learning more about the relationship between heart rate and exercise intensity can help you train for optimal performance.
Energy Pathways and Oxygen Delivery
Energy production in your cells takes place both anaerobically, in the absence of oxygen, and aerobically, in the presence of oxygen. The energy pathways that dominate at any given time during exercise are influenced by your exercise intensity.
At low to moderate intensity, the energy needs for rhythmic activities like walking, running, cycling and swimming are met aerobically by adenosine triphosphate, or ATP, in cellular organelles called mitochondria, using glucose and fat. When oxygen demands are met, your heart rate remains at a steady state.
When intensity increases, as in a sudden sprint, your heart rate speeds up in order to increase oxygen delivery to the cells. But when energy needs exceed the capacity of mitochondrial production, energy is augmented anaerobically outside the mitochondria using glucose. The point at which anaerobic pathways kick in is called your anaerobic threshold.
Read more: Anaerobic Training Adaptations
Anaerobic Threshold Revisited
The concept of anaerobic threshold can be misleading, because it implies that aerobic energy is no longer contributing to energy production, when in fact it continues to make a significant contribution.
Authors of a 2008 article published in Physical Education and Sport explain that both aerobic and anaerobic energy pathways are always in use, but the intensity of exercise and availability of glucose dictates how much of each is used.
Len Kravitz, Ph.D., and Lance Dalleck, Ph.D., contend that the term "lactate threshold" is a more accurate way to describe the point at which lactate, a by-product of anaerobic metabolism, abruptly increases in the blood.
Blood Lactate Training
Prolonged exercise at intensities above a certain point causes an abrupt increase in blood lactate levels. When your body produces more lactate than it can reabsorb and manage on a continuous basis, you have exceeded your lactate threshold, and you will quickly fatigue.
According to triathlon and track coach Marty Gaal on the USA Triathlon website, well-trained athletes are typically able to exercise at just below lactate threshold for about 60 minutes, but when the lactate threshold is exceeded, exercise can only continue for five or six minutes.
Lactate Threshold Heart Rate Training
Lactate threshold heart rate training is a way of raising your lactate threshold by improving the ability of your heart and cells to produce aerobic energy at higher intensities.
Gaal explains that you can increase your lactate threshold by working at intensities as close as possible to your maximum heart rate for intermittent bouts of four to five minutes, interspersed with easy low-intensity bouts of three to five minutes, repeating the cycle four to five times.
Over time, you will increase your endurance at higher levels of exercise intensity, meaning you can go farther at a faster pace without fatiguing.
Using Perceived Exertion
Changes in blood lactate are detected in a lab by monitoring changes in respiratory gases, but you can estimate your lactate threshold based on how you feel during exercise.
Sports coach Brian Mac recommends using the Borg rating of perceived exertion, or RPE scale. The Borg scale is based on a range of six to 20, with six being very little perceived effort and 20 being so difficult you can't keep going. Lactate threshold is typically achieved at perceived exertion levels of 13 to 14.
Calculating Your Lactate Threshold Heart Rate
Another option is to determine your lactate threshold heart rate based on percent of heart rate reserve, or HRR, the difference between your max heart rate, or MHR, and resting heart rate, or RHR.
According to Kravitz and Dalleck, lactate threshold is reached at 80 to 90 percent of HRR in well-conditioned individuals. To use this method, take your resting pulse, and then subtract your age from 220 to get your max heart rate. Then, use this formula: [(MHR-RHR) × .90] + RHR = Lactate Threshold Heart Rate.
Read more: Examples of Aerobic & Anaerobic Activities