The phrase "acid threshold" refers to the limit or point at which a substance becomes acidic enough to have a noticeable effect, cause damage, or trigger a particular reaction. Full definition
The average heart rate during that 20 to 30 minute effort would be approximately your lactic acid threshold heart rate. (bengreenfieldfitness.com)
To illustrate how it works in practice, Koenig offers the example of a professional hockey player whose profile shows that he has gene variants found in people who respond rapidly to lactic - acid threshold training. (inc.com)
A first time marathoner or triathlete (or even an intermediate level endurance athlete) will RARELY be performing at or near their lactic acid threshold for extended periods of time. (completehumanperformance.com)