On the days you don't do sit ups, it doesn't mean you don't do something to strenghten your core. You need to do more than just old fashion military sit ups. Lay on your stomach and extend your arms and legs off of the ground and arch your back up as far as it will go. This begins to build your lower back as well as the upper abdominal region. You won't be doing it long, but its not the point when you first start. Do it to a count of 10, and don't put your feet or arms down until you count it out slowly.
Before you do sit ups are you stretching out your hamstrings and quads until they are nearly pre-exhausted? People who do not have a regular cardio regime and suddenly do compound movements like the sit up fall prey to getting cramps or straining the large leg muscles. The reason why is because your body will draw compensatory muscle groups to help you out with the action your making the body do. When you don't have strong core strength, you pull with the heels of your feet much harder, placing great tension on your easily damaged rear hamstrings. What generally follows your situps? The timed run!
When you do the sit ups, its basic movements that will suit you best. Don't waste any energy on getting up. It really is just getting past the 45 degree angle of your body which most people have the problem with. Exhale the air in your lungs to the point that you will not be fighting gravity, fatigue and a large lung full of air on your trip upward. It is very hard to bend your torso repeatedly with a full payload of air, it also can make you black out.
Get cracking young one!
"Kid, I've sat on the department ****ter longer than you've been a cop." (CatDoc to Chevy SS, the single greatest post ever on RP!)
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