In a standard test of maximum power output, an athlete begins work (running, cycling, rowing, whichever) and the intensity gradually but steadily increases until the athlete can no longer sustain the power required to continue the test.  As intensity increases, lactate production increases. 

The intensity level at which the rate of lactate production exceeds the rate of lactate removal is called the lactate threshold.  When an athlete reaches the lactate threshold, further increases in power output cause increases in blood lactate.  Initially, these increases are linear, meaning, small increases in power output produce small increases in blood lactate.

Once a certain intensity level is reached, the rate of lactate production accelerates.  At this point, small increases in power output result in large increases in lactate.   This point of accelerated blood lactate accumulation is called the lactate turn-point.   

The points are illustrated in the figure below.  


 The LT and LTP are athlete-specific; they vary based on how well-conditioned an athlete is.  In general, athletes with very high aerobic capacity have a higher LT, a higher LTP, and a greater "distance" (in power output) between them.  In this test, the lactate threshold occurs around 7 minutes, at an intensity of about 160 watts.   The lactate turn-point occurs at 10 minutes and 250 watts.  

The lactate threshold appears as a yellow line because it is a warning that an athlete has approached their maximum sustainable power output.   The lactate turn-point appears as a red line, because exceeding this power output will make it impossible for the athlete to continue.