The Pareto principle (also known as the 80-20 rule, the law of the vital few and the principle of factor sparsity) states that, for many phenomena, 80% of the consequences stem from 20% of the causes.
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Mathematically, where something is shared among a sufficiently large set of participants, there will always be a number k between 50 and 100 such that k% is taken by (100 ? k)% of the participants; however, k may vary from 50 in the case of equal distribution to nearly 100 in the case of a tiny number of participants taking almost all of the resources. There is nothing special about the number 80, but many systems will have k somewhere around this region of intermediate imbalance in distribution.
I would assume that this also holds for music-related body movement and mapping to musical sound.