Start:
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Finish:
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As you pull yourself to the bar, have your head lean back as far away from the bar as possible and arch your spine throughout the movement. At the upper end of the movement, your hips and legs will be at about a 45-degree angle to the floor. You should keep pulling until your collarbone passes the bar and your sternum touches it. By the time you've completed the concentric portion of the movement, your head will be parallel to the floor.
Vince Gironda once said that Sit-ups do nothing in the formation of the abs.
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Picture of Vince Gironda(Classic old school bodybuilding physique)
T-nation.com and images.google.com