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Effect of Gravity
From: FM 7-22 October 2012
Gravity molds body tissues. The body adapts to the stresses placed upon it. Gravity exerts a constant influence. When body segments are not aligned properly, such as when the head is too far forward, gravity works to further pull the head forward, placing undue stress on the structures of the neck and upper back. Over time, the neck adapts to faulty posture and natural neck movements may become restricted. Another example of this effect is seen among those who allow their shoulders to round forward. Gravity compounds this effect, limiting overhead range-of-motion as shown in Figure C-1. By simply pulling the shoulders back as shown in Figure C-2, the arms are free to move fully overhead. To maintain this optimal position, Soldiers need to regularly stretch the chest muscles that are prone to tightness, and strengthen the upper back muscles that promote proper carriage of the shoulder girdle. More importantly, they need greater awareness of the manner in which they carry the shoulder girdle while performing everyday tasks. Rounding of the shoulders is a common postural problem with many Soldiers, perhaps from emphasizing pushing exercises at the expense of pulling motions.


Figure C-1. Poor posture limits range of motion


Figure C-2. Good posture allows better range of motion

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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