please don't get me wrong. there's nothing wrong with jumping rope. dance is a better tool for beam.
and if jumping rope were conducive to the "bodies core" then you have to wonder why the entire world of gymnastics, including the elite world, don't do it on a daily basis. again, dance is the better choice.
when we're at the ranch nobody has jumped rope...ever...they climb the rope. more conducive for active training of the "bodies core". but a whole lot of dance is done.
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I don't disagree that dance is good tool for beam, but for me they are too kind of different things. In order for the girls to be confident on beam it helps to develop a library of movement and do different things, especially at the lower levels. Even if they are only developing mental confidence and no physiological effects...beam is just not very difficult physically, really, it's more confidence for the girls in learning to trust their bodies. However I do think from a physical standpoint if this drill is introduced and supervised correctly, it is helpful, because it is important to be able to get quickly to forced arch and have a stable releve on the beam during movement. Core conditioning and full body strength is good for beam too, but I'm skeptical that at developmental levels climbing rope is a specific enough exercise for the kind of footwork that is necessary to do good punching action on beam series, entrance to leaps, jumps, turning, etc.
There are other dance specific exercises for these things but again I think the concept of creating a library of movement at the developmental level is important. There is no reason not to use different exercises and approaches to build coordination, confidence, and focus. This is not a magic drill that is going to teach BHS BHS on beam, clearly. But it's one tool along the way that can be used.