The Léger-Boucher Test was a medical examination used to evaluate a person's cardiovascular endurance. The test was continuous and progressive; it was easy at the onset and gets harder as the test progressed. (RIS Bouteina: "Distress Call")
The principle of the test is simple: to complete the greatest number of 2-minute tiers. They had to run back and forth a 20-meter course and had to complete their run at the blip. The first tier's speed was about 8 km/h and the expected speed increased by 1/2 of a km/h every minute. When the runner was unable to run anymore, it had to stop. The reached tier is his/her LB rating.
One's LB rating was simply the number of tiers a person has completed. Halves of a tier indicated that the person had to stop after the halfway mark of a tier but before the tier's end.