Pullups, chins, and abs

I was wondering why the body tends to adduct the legs by contracting the abs when fatigue sets in during pullups and chins. I’ve noticed this with myself and with clients that I train. Logically, it shouldn’t help cuz your bodyweight is still the same and yet when the abs contract it seems to give a little extra umph in the pull.

Any reasons?

Pavel Tsatsouline has talked about this for a while…it has something to do with what he calls “tension-irradiation” (i think that’s what it’s called). Contracting the abs has an amplifying effect on your strength by somehow extra-priming your nervous system. Do this for a test: take a hand (grip strength) dynanometer and squeeze it as hard as you can using just your finger and hand strength. Now try it again, but contract your abs and squeeze your butt at the same time. Notice any difference? I did this test myself and saw a 20% strength increase when I flexed my abs and glutes. I now use this technique whenever I’m going for heavy weights (it’s a good idea to “stay tight” during lifts like squats and deads anyhow). Presumably this same effect would occur with the chins as well. Hope this helps.

I believe its possible that you would use the seratus muscles more to help you get up by changing position