|RER represents respiratory exchange ratio. This is the ratio of CO2 expired to O2 consumed
Predicted (or estimated) maximum heart rate is 220-Age. So, a predicted maximum heart rate for a 20-year-old student is 220-20=200 beats per min. Remeber, we look at heart rate in beats per min.
Plateau of VO2 means the following: A maximal aerobic test progressively gets harder for the sujbect. When the oxygen consumption (VO2) plateaus < 150 ml/min VO2 as the test continues to get harder it is a very good indicator that the subject is at or near their actual VO2max capacity.
Note the Rating of Perceived Exertion chart below. It shows an example of the %intensity with each number. The two anchors are 6 (which is very quiet seated rest) versus 20 (which is the point of exhaustion from working as hard as you can).
Although VO2peak varies from textbook to textbook, for PEP326L a VO2 peak test is when a subject gives a maximal effort in an aerobic capacity test but does NOT attain any of the criteria. It means the subject worked as hard as she/he could, but did not attain the VO2max criteria.