Maybe the most important principle in skill acquisition is “train what you want to improve”. The pulling path in a muscle up is different from a regular pull up. And while getting strong in pull ups will have a positive impact on your muscle ups, they don’t train the specific strength required. The vertical pulling will turn the bar into an obstacle, and it won’t allow you to move around it. That’s why we aim to pull diagonally and generate space to clear the bar, when executing and training for the muscle up. Diagonal pull-ups have direct carry-over, because they mimic the muscle-up’s pulling path. They’re also easy to implement, as they have fewer technical criteria than the muscle up. You simply pull to your own level.
It’s important to note how the body angle is held through the entire motion. I’m not only pulling at an angle, but also descending at the same angle. A common mistake when doing diagonal pull ups is resetting at deadhang after every rep and losing the momentum. This requires practice, but momentum utilization is key in muscle ups, and diagonal pull is a way to start learning that as well.
Also, as a last thing, you can use the diagonal pull up as a test to see if you’re strong enough for the muscle up. When you gain the strength to put your lower chest at the height of the bar, you’re ready, as this is the lowest point of transition.