People who habitually think in terms of bodybuilding-brainwashedness will say you need to "isolate the muscles!" (eg. the arms, the back, etc). This is bullshit. The better your performance, the more work you do, the more power you exert -- this is better in terms of all the metrics that really matter. So if you can do more pull-ups with your knees up, you're getting more out of it. Period. Performance is the first and ultimate standard. The enhanced performance will actually feed a positive feedback loop which will make you better at the inferior and stupid "isolate the muscles!" posture... so you'll progress at both, better than if you stuck with the isolate-the-muscles guidance. This has been demonstrated time and again. On both theoretical grounds / in debate, and more importantly by examining results and performance, I can bury anyone who advocates the bodybuilding / isolationist approach.