It’s a pretty hot topic right now and there are many mixed views with regards how often you should train a muscle in order to get the most benefit from your training.
In my opinion a muscle should be trained as often as you can recover from the stimulus placed on it. So, by this I mean, if you’re a novice lifter with a low training age (the amount in years you’ve been training consistently for) and you try and train your legs 3x per week, the chances are your workouts will suck as you won’t have recovered from your last session. So in your case, a more realistic training frequency per muscle group may be twice per week or maybe even just the once in order to make progress.
On the opposite end of the scale, if you have a relatively experienced training age (5+ years) and you already train a muscle group once per week and have seen great changes, in order to make further progress, introducing another session may produce further gains so long as you can recover effectively.
Another variable to then consider is whether the athlete in question is natural or assisted. So let’s assume we’re not dealing with a novice lifter, A natural athlete in my opinion requires more training frequency in order to grow and develop at a faster rate. This is due in large to protein synthesis. See, when we train our rate at which we synthesise protein is elevated for 48-72 hours before returning to baseline. Once it’s returned to baseline, we need to elevate it again in order to keep anabolism high. Keeping protein synthesis high is the key to continued muscle tissue development. This means we will now probably train each muscle group every 3 or so days to get the best return. If you for example, trained a muscle group once per week, by the time protein synthesis has returned to baseline after 3 days, you now need to wait another 4 days before you train it again which isn’t ideal. An assisted lifter on the other hand may be absolutely fine training a muscle group once per week as protein synthesis will remain elevated for longer whilst aided.
Now, your muscles will only take so much hammer per session. To make the numbers simple, if you’ve stimulated your muscle after 5 working sets and progressed from the previous week, that’s job done, go home. There is no need to hit a further 4 sets through triple drops, rest pauses, isometric holds just because it’s looked at as more ‘hardcore’. This now may have no further positive impact yet may fatigue you further for sessions to follow.
So, does this mean you would now complete 5 working sets one day and then 3 days later another 5 working sets because training a muscle 2x per week is more beneficial? If you double your training load immediately, the chances are over time you won’t recover from that efficiently enough to maintain progression as prolonged periods of training with too much volume (total work done) can have negative effects. Rather, complete 3 working sets in one session followed by a further 3 working sets 3 or so days later. That’s a total of 6 working sets in the week in comparison to just 5, and you’ve elevated protein synthesis twice in that week.
To conclude, assisted or unassisted, you can train a muscle group once per week and make great progress, absolutely. However, providing you can recover efficiently, I personally feel it is more beneficial to maintain a higher training frequency for greater long term benefits. Studies have shown that training a muscle group up to 3x per week can produce great results with little negative side effects so long as you can recovery from it. Most would be able to tolerate 2x per week which would be my preferred middle ground.
Look at it this way, training once per week = 52 opportunities to progress each year. Training a muscle 2x per week = 104 opportunities to progress each year.