Multiple studies conducted at KER in a laboratory setting have shown that heart rate climbs linearly as speed increases. Heart rate and oxygen consumption maintain a similar linear relationship. Conversely, blood lactate accumulation, a physiologic sign of fatigue, does not rise linearly. Instead, lactate accumulation remains tolerable until heart rate reaches about 180 beats per minute (bpm), which is approximately 80% of maximum heart rate, and then lactate increases exponentially as heart rate nears a maximum of 220 bpm. Accumulation of lactate leads to fatigue, which in turn may precipitate time penalties and disobediences.
Heart rate is an effective, noninvasive measure of exercise intensity because it takes into account numerous factors: breed, body weight, speed, experience, fitness, shoeing, weight of rider, rider ability, footing, environmental conditions, and other influences.