Reply To: Twice-daily training for hypertrophy and strength
Impossible to say. If you look at the model of Bulgarian weightlifting, they were often training 3-5x per day most days per week! Of course, they also trained everyone to death and whoever survived was the Olympic champion, so take that with a grain of salt. It also depends on what makes up each session (i.e. plyometrics and sprinting in the morning and weights at night vs. two weights sessions vs. one weights session and one energy systems / cardio session) and if you are keeping relative volumes similar to 1x per day training, or if 2x per day training represents a large volume increase. Too many factors to give you a concrete answer unfortunately.
As for why it works, it's because it reinforces proper quality of each work set, even if the total set volume is the same due to greater recovery. Think of it like taking an EXTRA long break between sets. The 4-6 hours of rest allows for a little bit of glycogen re-synthesis if the diet is in check, and CNS restoration. It is also possible that more frequent spikes in muscle protein synthesis are responsible. MPS stays elevated for 24-36 hours following a resistance training session, so by adding a second session to a given day, you are lengthening this window.
I realize I'm opening another can of worms here, but I've seen great success training the same body part / movement pattern for 2-4 days in a row before taking an extended (3-5 day) break before hitting that cycle again. The theory here is that you are elevating MPS over several consecutive days in a row. Just some food for thought.
I will leave you with this.....these days I leave 2x per day training for strength and hypertrophy purposes as an "ace in the hole" for a spurt of fast progress or as a plateau buster. I'm talking 6-10 weeks per year, tops. I think the rest of the year you should be able to make great progress in strength and hypertrophy training 1x per day, 3-6x per week. I have done it for much longer. I once did 2x per day weight training for myself for nearly 2 whole years, and while I made progress, I make just as much progress now doing once-daily weight training sessions and saving twice per day for the occasional plateau buster as opposed to doing it year round. Use it wisely!