Split, do not stack
Six days only helps if you rotate muscle groups so each has recovery time. Push pull legs twice, or an upper lower plus arms setup, lets you train often while no muscle gets hit two days running.
Yes, if you split the volume so no muscle is overloaded and you eat, sleep, and program recovery in.
Six days a week works well as long as you spread volume so each muscle still gets 48 hours to recover, for example a push pull legs run twice through. The risk is not the six days, it is hammering the same muscles daily or piling on total volume you cannot recover from. Keep hard sets per muscle around 10 to 20 weekly, eat enough, and sleep 7 to 9 hours or the extra days backfire.
Six days only helps if you rotate muscle groups so each has recovery time. Push pull legs twice, or an upper lower plus arms setup, lets you train often while no muscle gets hit two days running.
More days can quietly balloon your weekly sets past what you recover from. Keep each muscle in the 10 to 20 hard set range across the week rather than adding sets just because you have the extra day.
Six sessions demand more food and sleep than three. If you are not eating enough protein and calories and sleeping 7 to 9 hours, the added frequency turns into accumulated fatigue and stalled progress.
Yes, if you split the volume so no muscle is overloaded and you eat, sleep, and program recovery in.