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[[File:HeartRateSimplified.pngsvg|right|thumb|500px|A simplified chart of heart rate against exercise intensity showing [[Maximum Heart Rate]], [[Heart Rate Reserve]], [[Resting Heart Rate]] , [[Orthostatic Heart Rate]], and [[Heart Rate Deflection]].]]The heart rate deflection point is a controversial way of establishing an athlete's [[Lactate Threshold]] (LT), sometimes called the Anaerobic Threshold (AT). The Lactate Threshold is where the body starts to rely more on producing energy anaerobically (literally without oxygen) rather than aerobically (literally with oxygen). Heart rate is generally considered to be linear with effort (power output) up to about 80% of [[Maximum Heart Rate]]<ref name="MeyerDungs2015"/>. The best way of measuring the Lactate Threshold is to take a MLSS test, but this requires the analysis of blood samples. This is obviously invasive, inconvenient, painful and somewhat expensive. In 1980, Italian sport scientist and medical doctor Francesco Conconi developed a test procedure for establishing an athlete Lactate Threshold based on heart rate. The idea behind the test is to have the athlete exercise of gradually increasing intensity while recording their heart rate. When the athlete starts to produce energy anaerobically, their heart rate will not go up proportionate to the intensity. This change from linear relationship between heart rate and intensity is called the Heart Rate Deflection (HR_d).