I'm no expert but try doing very slow negatives. Negatives supposedly generate 120% of the tension that positives do. Also maybe try "static contractions" where you tense the muscles and hold both at the starting position and final position of the exercise.
Then of course just hanging off the bar in the up position. Maybe figure out how long you can do it total, then use half that time to do "reps" with maybe 2 minutes rest in between. You could start with likje 1 1/2 minute rests and increase them as you become fatigued. Since this is essentially an isometric exercise you can work the hell out of it. If you don't have a bar at home you can do this and also negatives on doors on your home. Of course since your palms are outward on the doorm, I don't know if that makes a huge dif. Just make sure you are as close a possible to the hinged side of the door. I weigh 240ish and have yet to break a door in the apartment, but if it does break don't blame me :). Oh and also it helps to put a towel atop the doorm, under your palms so it doesn't hurt so much.
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