I have always been "anti-periodization", until now...
My thoughts, were that one should be able to design a program that produces both optimal stimulation, and suits our recovery needs. I now realize, not everyone can. If you are one that can, they read no further. However, if you find yourself stuck in the dilema of balancing the difference in recovery between the muscular systems and some other system (CNS, motivation, endocrine, joints, tendons, etc.), then a periodized plan may be just what you need.
Periodization is based on the idea that the body operates on 2 main factors (fitness and fatigue). Also, and most importantly, that there are many systems in the body, each with it's own level of needs for stimulation, and needs for recovery. An example would be, the amount of stimulation the muscles need to induce an 'alarm reaction' and stimulate growth, is simutaniously, too much for the CNS to handle long term. Another scenario could be that the load and volume required for strength gains, might be too much for the tendons and joints to handle long term.
Choice one: Train for optimal stimulation all the time. Your joints and tendons might get very sore, and you will probably get injured. Your CNS and endocrine system will wear down and you'll feel burned out and stop making progress.
Choice two: Train for optimal recovery of all systems all the time. Your CNS, joints, and tendons will all be recovered. You also probably won't gain as your training is never severe enough to stimulate a reaction in the body.
Choice three: Work with the recovery times. Train severe enough, as long as you can, then make a temporary adjustment to accomodate the other systems. Research shows that the adaptations you earned from your training, will last approximately 3 times longer than the fatigue you accumulate. This means, you have no worries about losing any gains during the time it takes for the other systems recover and rest. Then, you can go back to the severe training again and move ahead.
Heavy Duty: Why not just rest longer between workouts? The problem with this, is when you lower everything down to where all systems recover, you end up training very sub-optimal. Your basing your training on the slowest to recover system. If this is very protracted, it then might be very sub-optimal. After a few weeks of this, your systems are recovered, you could go back to the more frequent intense training, but you don't. Your forever stuck at the lower volume/frequency.
Gironda (Train 21 rest 7): This is a good method and makes sense. If one is not interested in keeping up with the 'practice and performance' of lifts, then a week off could work very well. Obviously, everything is rested during that week so there is no question that all will have a chance to recover. The only problem could be is that one week might not be enough for some systems. In that case, it might be better to at least do some kind of training so your not idle for more than a week.
DC training: This is the only structured form of 'periodized H.I.T., which is why it can work so well. You push hard for a few weeks, then take 2-3 weeks to recover. If one plans it right and uses a volume, intensity, and a frequency that fits them, instead of a 'cookie cutter' approach, it has great merit and fits the stimulation and recovery needs.
General periodization: Customizing would seem to be the absolute best method, as it can be taylored to your exact needs. You set up two or three programs, each with it's intentions. You perform them when and how it fits you best. A rough example might be as the following.
Let's say a person finds that to gain best, they need to train each muscle twice a week, with either a very intense set or several heavy sets. They also find that after 2 weeks, motivation drops, and after 4 weeks, even their joints start to hurt. They decide on this,
Weeks 1 and 2: Each muscle 2x per week, 1 set to failure or 4 heavy sets
Weeks 3 and 4: Each muscle 1x per week, 1 set to failure + extenders, or 4-5 heavy sets (this keeps their system recovered)
Weeks 5 and 6: Each muscle 1x per week, 1 high rep set to failure, or 5 light CFT sets (lets the joints rest)
Week 7: Total break from training
Week 8: Restart
This might work for them. Weeks 1 and 2 are their maximum gain weeks. Weeks 3 and 4 let them keep gaining strength, but at a lower frequency to avoid systematic burnout. Weeks 5 and 6 allow them to continue to stimulate strength/endurance while letting their system continue to rest and also give their joints a break. This is just a hypothetical idea, one person's setup could be completely different than anothers. As long as you alternate a maximum gain period, with some kind of recovery period (whether it's time off, a lower frequency, lighter weights, etc.) and it allows you to recharge, all is good.
The main point of periodization, is that a person's systems have different recovery rates, and we do not have to base our training on the slowest to recover. We merely need to give it time to recover, then go back to the more optimal training. It doesn't have to be an either/or between do-able and optimal, it can be both!