"If you want or need to add cardio to your strength training, itís best to perform them on separate days, if your schedule allows.
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In this study, strength gains were similar when doing cardio immediately after strength training versus on a separate day. However, the group doing their cardio on a separate day experienced more than two-fold greater hypertrophy than the group doing their cardio immediately following lifting.

If you integrate aerobic training into a training program focusing on hypertrophy, youíll probably gain more muscle if you do your lifting and cardio on separate days.
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Even though the presently reviewed study was performed with untrained lifters, other evidence suggested that even in trained lifters, separating strength training and cardiovascular training by at least six hours may eliminate the deleterious effects of cardiovascular training on strength gains.
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You can still make gains if you do your lifting and cardio in the same session, but your progress will probably be a bit slower than it would otherwise be if you could perform your lifting and cardio on separate days."