In  a large study involving 494 12-16 year old swimmers of Portuguese national level Luis Rama and colleagues investigated the relationship with performance of a number of a large number of variables including anthropometric variables, training variables, physiological variables including power, gliding ability, buoyancy, swim tests including max velocity and mean velocity over 30m. The best predictors were the mean velocity over the 30m test (T30) plus height and maximal elbow extension strength  for the older swimmers and the glide after the start for younger swimmers. Many other variables had small but statistically significant relationships with performance.

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Rama, L., Santos, J., Gomes, P., and Alves, F. (2006). Determinant factors related to performance in young swimmers. In J.P. Vilas-Boas, F. Alves, A. Marques (eds.) Xth International Symposium on Biomechanics and Medicine in Swimming, University of Porto, Porto, June, 2006. pp.246-249.