This is the FM 21-20 list of principles, put into an order that makes a good mnemonic (P-R-O-V-R-B-S).
The Army has recently transitioned from Physical Fitness Training (FM 21-20) to Physical Readiness Training (TC 3-22-20). However, board questions are sometimes still asked that reference the old manual. Learn as much about both as you can, since FM 21-20 has a long history and many elements of it can be found still present in PT sessions around the Army.
TC stands for Training Circular.
TC 3-22-20 gives only three principles, in section 2-8: precision, progression, and integration, and has this to say about them:
These principles ensure that Soldiers perform all PRT sessions, activities, drills, and exercises correctly, within the appropriate intensity and duration for optimal conditioning and injury control.
2-9. Precision is the strict adherence to optimal execution standards for PRT activities. Precision is based on the premise that the quality of the movement or form is just as important as the weight lifted, repetitions performed or speed of running. It is important not only for improving physical skills and abilities, but to decrease the likelihood of injury due to the development of faulty movement patterns. Adhering to precise execution standards in the conduct of all PRT activities ensures the development of body management and fundamental movement skills.
2-10. Progression is the systematic increase in the intensity, duration, volume, and difficulty of PRT activities. The proper progression of PRT activities allows the body to positively adapt to the stresses of training. When progression is violated by too rapid an increase in intensity, duration, volume or difficulty the Soldier is unable to adapt to the demands of training. The Soldier is then unable to recover, which leads to overtraining or the possibility of injury. Phased training ensures appropriate progression.
2-11. Integration uses multiple training activities to achieve balance and appropriate recovery between activities in the PRT program. Because most WTBDs [Warrior Tasks & Battle Drills] require a blend of strength, endurance, and mobility, PRT activities are designed to challenge all three components in an integrated manner. The principle of integration is evident when WTBDs and their component movements are incorporated in PRT. For example, CDs and CLs develop the strength, mobility, and physical skills needed to negotiate obstacles. Military movement drills (MMDs) improve running form and movement under direct or indirect fire. The guerrilla drill (GD) develops the strength and skill associated with casualty evacuation and combatives. The drills, exercises, and activities in this TC integrate essential Soldier tasks, making PRT a critical link in the chain of overall Soldier physical readiness.