Optimal Resistance Training Frequency

I found a couple of good studies that compare lifting frequency on a basis of equal volume.  This study compares an equal volume of sets across two frequencies: 1 day per week or 3 days per week.

Though the 1 day per week did gain strength, this strength gain was only 62% of the strength gain seen in the 3 days group.  But the biggest difference came in terms of muscle mass.  The 1 day per week group had no change in muscle mass, while the 3 days group increased lean mass by over 10 pounds!  It is amazing that this change in frequency from 1 to 3 days (with the exact same total volume) can have such an impact on muscle mass.

Other research shows that 1 day per week is not enough to build new muscle.  This study shows that 1 day a week was only enough to preserve strength and mass.  Subjects lifted three times per week to build mass and then shifted to 1 day per week to maintain.

Yet while 1 day per week is not enough, that does not mean that 3 days per week is necessarily optimal.  In this study, an equal volume of resistance training was spread over either 2 or 3 days per week.  The result?  Both groups increased mass and strength at exactly the same rate.  So if 2 days is just as effective as 3 days per week, and 1 day per week is not enough, then it appears lifting twice a week could be the optimal training frequency.