The toes-to-bar is often thought of as an abdominal exercise. While the abs are definitely used in the movement, the shoulder plays a large, if not the largest, role in enabling an athlete to do toes-to-bar.
Success in the toes-to-bar is rooted in the kip. The ability of the athlete to close the shoulder angle makes it easy for the athlete to then reach their toes to the bar and allows for efficient and consistent movement.
In the first photo (left), you’ll see Nicole is at the very back of her kip as her toes reach to meet the bar. By closing the angle of her shoulder to this degree (almost 90), she shortens the distance her feet and legs must travel to reach the pull-up bar.
In the second photo (right), you’ll see that Nicole barely closes her shoulder angle in her effort to reach her toes to the bar. This means that her abdominals and hip flexors must do more work to lift her legs to the bar. She may be able to sustain this for a number of reps, but eventually her legs will be too heavy for her abdominals to lift them that distance consistently.
If you look at the height of her hips in each photo you’ll see that in the first photo, her hips are much higher and behind the bar. In the second photo, she is mostly hanging under the bar with her hips to the left of the bar she is hanging from. In the second photo, she has to close her hip more to reach her toes to the bar – again using her abdominals and hip flexors a lot!
Development of toes-to-bar starts with a strong kip (and it helps kipping pull-ups a ton as well!). When you’re tired or trying to figure out what to work on, go back to the kip and try to close your shoulder angle as much as possible.