For peak conditioning as we've defined it, it's just what the doctor ordered. Although the consequences of cross-training have not been thoroughly investigated, a number of studies have been done on how various activities relate to others. Following are some of the conclusions to date. It's never wrong to be strong. Research on strength training consistently shows that it won't interfere with aerobic development.
Strength training affects muscles locally but has minimal effect in terms of oxygen uptake.
And the effects that strength training does have on the heart may improve cardiovascular function. One study, for example, finds that weight training on its own can lower resting heart rate and blood pressure. The caveat is that strength training won't necessarily improve performance in aerobic sports. For example, weights are often used to develop the upper-body strength required for competitive swimming, but research fails to find consistent evidence that strength in the gym translates to strength in the water. One study, for example, found that when weight training was added to swim training, the weight lifters' stroke strength improved no more than that of men who just stuck with swimming.