GRAVITY

Gravity is perhaps the most important concept of physics when it comes to weight lifting. Without the effect of gravity acting on the weights there would not be any resistance force to counteract. More specifically, center of gravity pertains to all weight lifting activities. Without knowledge of proper hand, foot, and body placement, you increase your risk of injury and embarrassment. Lets now explore the importance of gravity and how it pertains to weight lifting.

Center of Gravity
Whether attempting the bench press, military press, squats, or simple stretches, center of gravity applies heavily to preforming an exercise properly. Take squats for example. Placing the bar too far in front or behind the body's midline (axis of center of gravity) will cause you to topple over. In contrast, by placing the bar about the axis of center of gravity, a proper squat can be accomplished.  

As you many have guessed, there are two systems and therefore two center of masses we need to consider. Both the center of gravity of the person and the center of gravity of the bar must be accounted for during the weight lifting process.

In weight lifting, the use of a bar with the addition of weights on either side is the object we want to lift. The unique thing about the bar with added weights is its perfect symmetry. Symmetry is a physicist's best friend when it comes to calculating the center of mass of an object. In reference to our weight bar, assuming the same amount of weights are on each side, the center of mass of the bar lies directly in the center. We can verify this by using the center of mass equation below, which after calculating, the COM indeed lies in the middle of bar.
 Squat

Bar
http://physics.stackexchange.com/questions/113627/olympic-weight-lifting-bar-plate-location-affect
 cm

Finally, lets consider the more difficult center of gravity of the person doing the lifting. This can be a little more difficult to determine since of course every individual has a different center of gravity depending on their physique. In addition, ones center of gravity changes consistently depending on the position of their body during the lift. However, the laws of physics are still the same, and since the human body is nearly symmetrical, we know the center of gravity is along the center line of the body.
By incorporating both the center of gravity of the bar/weights and the athlete, you can determine the best position to grip the bar to maintain a balanced body position about your center of gravity. As a result, proper lifting technique can be achieved for each exercise and in turn will reduce the amount of avoidable stress your body experiences while lifting.    

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