Lactic Acid Threshold

The body responds to crossing the lactic acid threshold by releasing HGH to repair and strengthen muscle tissue. Crossing the lactic acid threshold is the defining mechanism of HIIT; aerobic exercise below it produces negligible hormonal benefit. Crossing into anaerobic territory causes a burning sensation in muscles that is the signal triggering HGH release. Resting too long between intervals allows lactic acid to dissipate, eliminating the cumulative staircase heart-rate effect. Residual lactic acid from each interval drives the next interval's peak heart rate higher.