"Billat 30-30's are intervals that require a hard effort for 30 seconds, followed by an easy recovery for 30 seconds. Over time, the body ends up playing 'catch-up', through heart rate, breath frequency, demand for oxygen at the muscular level, and a buildup of lactate in the system.  We can see this buildup through lactate testing, which is invasive and cumbersome, but we can also see it through SmO2 values, as muscles de-saturate and re-saturate. Lactate is getting processed back into energy, but sometimes the demand for oxygen is greater than the body's ability to provide it. "