Here’s a run from a Humon athlete where he changed his pace in the middle of the run. When he walked outside, he started running immediately and you can see his muscle oxygen level (SmO2) dropped by about 15%. This is due to the initial muscle contraction, causing less blood to be available to the muscles. As he continued running however, the blood flow started to increase and his SmO2 plateaus during the first 3 miles (up until about minute 25) where he maintained a 7:45 min/mi pace. You can see two areas where he hit the orange zone, indicating he was pushing himself, but he never went into the red zone.
After minute 25, he was pretty exhausted and began running slower. The plot below overlays his pace on top of the SmO2 plot and you can see as he slows down (seen by the increase in pace - in units min/mi), his SmO2 starts increasing since the oxygen demand in the muscles is not as high anymore. During this portion of the run he was in a green zone with some blue. When he stopped at minute 50, he recovered and this is seen by the large increase in SmO2 and the significantly slower pace.