There is no rule which would predict precisely which tissues may be involved in determining the apparent volume of distribution. It is frequently speculated that fat soluble drugs will have larger volumes in obese people and this is quite reasonable. Hard evidence is harder to come by.
Insurance companies seem to have used similar criteria for life insurances.
If your subject is practising sports, and if everything else indicates that he in not unhealthy, there should be no reason for not accepting him as a subject in a bioequivalence study. The only problem you might have is to justify your decision to the more bureaucratically inclined people that seem to be infiltrating companies and regulatory bodies alike. Unfortunately, the correlations still remain unexplained so I can't help you out further than to let you know that you may have to take muscle weight into account. Coz as far as i know in bioeq. So, any probable faults about a person is corrected in this sort of study.