This is a somewhat lengthy article written by Charles Poliquin, one of the premier strenght coaches of Northern America. He explains his view on training frequency and has a short paragraph devoted on training twice a day.
It is not exactly the same as what I do or my trainers from Germany taught me but it is close. Hope this helps.
FREQUENCY OF TRAINING
by Charles Poliquin
Frequency of training is measured as the number of training sessions for a given muscle group or lift per unit of time. A certain level frequency must be given in the administration of a training stimulus in order to maintain or build upon a previous training stimulus. Optimal recovery time between training sessions is important in maximizing adaptive processes. As a rule of thumb, one would think that as the intensity of the training stimulus increases, there would be a decrease in need for training frequency.
However, in the practice of strength training in the sporting world, especially with elite athletes, there are conflicting schools of thought on training frequency. For example, Rich Weil, World Record holder in the bench press, recommended one session per week per muscle group (Weil, 1985) while at the other end of the spectrum 9 to 12 weekly sessions are common on leading successful weightlifting teams. For example, Tara Nott, America's first Olympic Gold Medalist in weightlifting has regularly squatted 9 times a week to achieve her sporting standard. Some weightlifting National Teams have done as much as 4 to 5 training sessions for the hip and knee extensors on a daily basis. And to rely on the scientific literature for an answer is rather useless, as the data is coming from untrained, unmotivated university subjects.
There are three main schools of thought on frequency:
School 1: Train three times a week. Bompa espouses that concept so did Chuck Sipes a very strong bodybuilder from the sixties. It is the most used system in the World.
School 2: Train at least 6 days a week the lift or its variations that you want to improve. Again, some very successful individuals like Mel Hennessey and Bulgarian lifters have endorsed this training concept.
School 3: Train hard, come back once you can lift more. In other words, wait for supercompensation to take place. So training frequency per muscle group is once every 3 to 10 days per lifts. Fred Hatfield is a strong proponent of this system. So was Mike MacDonald, one of the most successful bench pressers of all time. Terry Todd related to me that he would test how he felt in the bench press muscles with just using a broomstick for resistance. If it felt odd he would take an extra day off, or whatever how many offs he felt it would take.
Here are my own observations on training frequency:
Of all the loading parameters, I think training frequency is the one that is most influenced by individual genetic differences, regardless of drug use or not. I believe that it is the loading parameter that one must experiment with most to find out what works out best for them. I have seen very strong individuals get strong on once every 10 days to 10 times a week. In both extremes there where individuals using recovery agents and some not.
Frequency of training will vary the person's level of qualification. In the immediate start of strength training is definitely a must. As the person gets stronger, genetic differences become more important.
The principle of training economy has to be considered: how much time can you actually devote to training?
Provided that the training intensity and volume are challenging, a frequency of once every 5 days works for most individuals, most of the time. This is how I train 70% of my clients with appreciable results. Of course, you will here arguments like my uncle Bob bench pressed once only every equinox, and he can bench 600 lbs, or on the other side, you will not make gains unless you train at a frequency per week that represents the last 4 digits of you social security number.
The choice of training method influences recovery. For example, the more eccentric overload, the more need for recovery. Squatting 4 sets of 6 with chains is more demanding that squatting 8 sets of 3 explosively, even though the total reps are the same.
All factors being equal, for strength development, frequency is more important than it is for hypertrophy development.
If you can afford dedicating it the time, I believe that training twice a day for the same body part ( if you can afford the training time) is the system that works best. The morning workout facilitates the evening workout. Then again, I make the individual train that body part 5 days later. The trainee will train 3 days out of five twice a day for 10 days, then go on to once a day for 5 days.
You have to consider the entire training system. For example, Louie Simmons has an extremely successful system with variations of loads throughout the week. His system works well when you do it in its entirety. So you can't mix a Louie Simmons bench press cycle with a Finnish deadlifting routine and a Russian squatting system. Always give a training system a fair try only in its original design. Combining training systems can lead to failure.
Training more than 3 times a week for a improving a lift is excellent to go through a plateau. In this area, there is scientific literature to back up this concept. But there is also a need to lower training volume once every three weeks for males and every three weeks for females.
Multiple sessions a week is for individuals who want to achieve Olympic standards. In personal communications with Chinese, Bulgarian and former East German weightlifting coaches, all of them stipulated that it takes about 3 years of incremental training to develop the work tolerance for such workloads.
In summary, training frequency will be determined by your training goals, your gender, the choice of training methods, magnitude and intensity of training load and most important your genetic make-up. In other words, you have to find out what works best for you. Take for example my assistant Chad Ikei, he bench pressed a World Record of 316 lbs at a bodyweight of 112.5 lbs at age 19. At that time he was bench pressing twice a week. Later on, when he was on the US weightlifting team, he trained the hip and knee extensors at a frequency of 8 to 12 times a week and yet his best performance was 13th in the World, and was National Record holder in the snatch, clean and jerk and total.
