NAC Trainer, Krista Williams shows us:

TRX Pull-ups-
They  are a great way to build upper body strength, and work toward unassisted pull-ups. With the TRX (shortened to arm’s reach when sitting) you use your feet for assistance as little as possible, working up to eventually being able to lift your feet off the ground. As an added benefit, you can work your core by tucking your knees into your chest at the top of each rep.
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