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Preparing The Body's Core
From: FM 7-22 October 2012
Muscles work to initiate and control movement. Because movement is more apparent than the lack of it, the focus is most often on the movement muscles create. Less obvious is the "braking" force that muscles apply to movement. Without this braking force, nearly all movement would be extremely sloppy and potentially dangerous. Around the body’s core (trunk and pelvis), this braking action of the muscles becomes extremely important for two reasons. First, the spine and pelvis are the base of attachment for many muscles that power the arms and legs. Without a strong, stable base of support, using these muscles is like firing a cannon from a canoe. Second, the body’s center of gravity is within the trunk area. Keeping it there leads to balanced, skillful movement. This is the job of the trunk muscles that do this primarily by putting on the brakes. The ability to maintain balanced postures is often referred to as stabilization. The load on the Soldiers shown in Figure C-9 demands strength and stability from the body’s core.

Figure C-9. Soldiers moving under load
To promote stable postures during exercise, it is essential that Soldiers learn to prepare the trunk. A simple, two-part action prepares the trunk for exercise:
  • Set the hips. This is also referred to as the neutral position of the pelvis. This position is found by first tilting the pelvis forward (buttocks goes back, belly goes forward, and the inward curve of the low back is increased), Figure C-10a. Second, tilt the pelvis backward (the buttocks and belly draw inward as far as possible, flattening the curve of the low back) (Figure C-10b). Then settle in between these two extremes (Figure C-10c).
  • Tighten the abdominal muscles. Once the hips are set, tightening the abdominal muscles will ensure readiness of the muscles that control and protect the trunk. To contract the correct muscles, imagine drawing the gut straight inward as if preparing for a blow to the mid-section or trying to appear slimmer. Keep the hips set as the abdominals are tightened (Figure C-10c).
After setting the hips and tightening the abdominal muscles, the Soldier’s posture should appear balanced and ready for exercise. The Soldier should not associate these two actions with a stiff, awkward posture. The goal is not to eliminate all movement from the trunk, but to simply control the natural motion that will occur.

Figure C-10. Set the hips and tighten the abdominal muscles

From: FM 7-22 October 2012 

  (Page last modified Feb 2, 2013) is dedicated to providing all the information you need to conduct the Army Physical Readiness Training as an individual or with a unit.  Everything you need from FM 7-22 is right here.
Chapter 1 Approach
Chapter 2 System
Chapter 3 Leadership
Chapter 4 Types of Programs
Chapter 5 Planning Considerations
Chapter 6 Special Conditioning Programs
Chapter 7 Execution of Training
Chapter 8 Preparation and Recovery
Chapter 9 Strength and Mobility Activities
Chapter 10 Endurance and Mobility Activities
Appendix A Army Physical Fitness Test
Appendix B Climbing Bars
Appendix C Posture and Body Mechanics
Appendix D Environmental Considerations
Appendix E Obstacle Negotiation
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