Training frequency is comprised of two components. They are:
How often you can train a muscle/muscle group depends largely on how much training volume and intensity you work it with when you do train it. As was mentioned in the article entitled Training To Failure: A Look Inside, some trainers believe that for maximum training stimulus all work sets must be taken to the point of momentary concentric failure (a training style that has come to be called high intensity training - HIT). It is usually also often a part of this philosophy of training that only one work set per exercise should be performed. After the workout the trainee should wait for an extended period (to ensure full recovery and supercompensation) before training that bodypart again.
On the other side of the coin there are the people who believe that sets should not be taken to failure, and muscles/muscle groups trained more frequently. The fact that the trainee isn't training to failure places less stress on the body's 'recovery systems', allowing the trainee to train the muscles more often.
HITers believe that the surest way to guarantee that the muscle has been stimulated to strengthen/grow is to train to failure; if sets aren't taken to failure then one can't really be certain that a training response will be stimulated. More traditional trainers believe that training to failure consistently is "too much" for the body to handle and only leads to "burnout".
I believe the sources of confusion here - although they are seldom identified - are the nervous system and joint capsules.
The nervous system and muscular systems may indeed require different recovery times after heavy work. In particular, firing the type IIB fibers at maximal frequencies (utilizing weights above ~85% of your 1 rep maximum) presents quite an effort on the part of the nervous system. In fact, this type of work results in a post-exercise period of nervous system inhibition which often requires 7 or more days for complete recovery. Training the type IIB fibers intensely, in the same pattern (i.e. the same exercise), before this nervous system recovery has taken place will not stress those type IIB fibers maximally because the nervous will not have the capability to fire them frequently enough to produce maximum tensions. The additional workout will not provide worthwhile stress to the muscle but will drive the nervous system further into the recovery zone, requiring even more recovery time. For this reason, an extended rest period between training sessions of the exercise in question is required or, if a second session is performed, a different loading pattern (i.e. set and rep count and weight on the bar or an exercise variation of the basic lift) must be used.
From the Muscular Fatigue During Weight Training article:
In order for a muscle fiber to 'twitch' the central nervous system (CNS) must send a nerve impulse to the controlling motor unit. The innervating nerve cannot maintain its capacity to transmit this signal, with optimum frequency, speed and power for extended periods of time. Eventually, decreasing concentrations of available substrates such as sodium, potassium, calcium, neurotransmitters, enzymes, etc., as well as the build-up of fatigue by-products, will cause fiber twitch rate and muscle contraction to become slower and weaker, respectively. If maximum discharge rates are continually demanded the nerve cell will assume a state of inhibition to protect itself from further stimuli.
Following this, a recovery period of up to seven days or more may be necessary before maximum muscle contraction can be achieved again.
On the other hand, significant research data indicates that anabolic processes within a muscle may be fully completed within 48 hours of bodybuilding type work (~80% of 1RM) - protein synthesis has returned to baseline within the muscle - though this does not necessarily imply such things as complete recovery of connective tissues in and around the muscles and joint capsules. The nervous system, once stressed with ~80 (or greater) of 1RM weights, may indeed take longer than the course of these processes to recover if the set(s) was/were taken to failure or close to it, or work volume was high enough.
It must also be acknowledged that many free-weight compound exercises place significant strain on the connective tissues and joint capsules, causing them to deform if heavy weights are used or safely "aligned" joint positions are compromised during training - such as frequently occurs when "cheating" or failure is reached. This deformation results in proprioceptors signalling the central nervous system, relaying information about the compromised position of the joint. When this occurs the central nervous system will inhibit the prime mover muscles of the motion, resulting in decreased muscle contraction force ...and, essentially, ending the productivity of that set. IF the tendons, ligaments, bursae and supporting muscles of the joint are not given sufficient time to recover from the exercise, then working that particular exercise (or sufficiently similar ones) will not be productive until complete recovery has occured because joint deformation will result in nervous system inhibition.
This, along with yet-to-be understood workings of the brain and nervous system, are the primary reasons why heavy free-weight compound exercises such as Deadlifts, Squats and Bench Presses, typically, cannot be trained heavily every 48 hours. Extended rest periods are normally required after heavy and intense training (85% of 1RM and sets taken to failure or close to it) on these exercises.
Lastly, from the series The Neuromuscular System:
One definite thing that we can learn from the 48 to 72 hour anabolic period is that it is not wise to train a bodypart more frequently than this (once every second or third day). In practice, hard training, drug-free weight trainers will often find that the nervous system and joints require longer periods than this between sessions involving the same exercises - even when sets are not taken to the point of failure (but, of course, they must be intense enough to stimulate supercompensation - 'difficult' sets) and sets are kept to a minimum (to minimize glycogen and enzyme depletion).
From Making A Strength/Size Routine Part III: Sets And Reps And How To Perform Them we have the guideline of doing no more than 3 'work' sets per exercise if hypertrophy is the goal and from Making A Strength/Size Routine Part I: Exercise Selection we have the guideline of placing the vast majority of our efforts into the free-weight compound exercises. By limiting specialized isolation work we have automatically cut down on some of the load that the body's nervous and glycogen/enzyme replenishment systems have to bear. This limited workload represents a reduced systemic strain as compared to routines filled with isolation movements, allowing us to train the compound exercises more intensely and more often. So, since I can work within the bounds of few isolation movements, a concentration on free-weight compound exercises, and 2-3 'hard' sets per exercise, I can then make some generalizations.
If you train your exercises to failure you will require more recovery time (longer breaks between hitting the bodyparts) than if you don't train to failure. How do you know what is the right balance? Some good guidelines to follow are these:
For categorizing purposes, large-boned trainees are trainees whose wrist circumferences (in inches or cm) are greater than 10.5% of their heights (in inches or cm). Their ankle circumferences will be larger than 13.0% of their heights. Small-boned trainees will have wrist and ankle circumferences less than 10.5% and 13.0% of their heights, respectively. It is somewhat common for trainees to be small-boned in their upper bodies, but big-boned in their lower bodies, or vice-versa. These trainees will usually find that their upper and lower bodies react to training at significantly different rates.
All weight training sessions take their toll on the body's recovery 'systems'. Just because you trained legs yesterday doesn't mean that training shoulders today is a harmless thing to do. The body has depleted glycogen, enzyme, ATP, etc. stores that must be replenished. Training shoulders today may interfere with the recovery processes that are going on in the legs (and central nervous system) due to yesterday's workout. For strength and size, an everyday workout scheme may produce results for genetically gifted individuals and anabolic drug users, but for a genetically typical drug-free weight trainer they are usually a mistake. NOTE: The exceptions to this rule may be more frequent sub-maximal work, as it doesn't produce significant metabolic stress, nervous system fatigue and/or joint/connective tissue stresses.
For general strength/size training, experience has taught many a drug-free lifter that he should spend more time out of the gym than in. This means that for typical trainees three weight training sessions per week is the maximum. Training days should always be separated by a rest day (for example, train on Monday, Wednesday and Friday if three days-per-week training is chosen). As was covered earlier, a typical drug-free lifter cannot make optimum muscle and strength gains by training the same exercises intensely three times per week. Therefore, if the trainee chooses to perform three full-body workouts per week, different exercises should be performed on each of the week's training days or a heavy/light/medium scheme of training must be imposed over the training days - this will be covered further in future articles. (NOTE: Beginners are a special case that should be considered separately). Some, more genetically gifted, individuals are able to prosper on four or more days-per-week strength/size split routines. In general, however, such frequent training does not foster optimum strength gains and is typically adopted to allow advanced bodybuilders to include more "detail" work in their routines or achieve quicker fat loss.
Here is a general summary of what has been covered in this series so far, and guidelines for effective weight training routine construction:
The above recommendations are only general guidelines ...but guidelines that will produce consistent results for the vast majority of drug-free trainees. The progress of thousands of drug-free trainees throughout the years has verified the efficiency of the above approach. However, if you follow the guidelines for awhile and are not gaining strength at each workout (as measured by fractional strength increases), don't worry, you just have to start fine-tuning things. By systematically eliminating all possible reasons for any lack of progress you will find the routine that actually produces results for YOU, and not just a routine filled with the promises of a drug-using, genetically gifted champion. The above guidelines provide the proper place for you to start. Continue reading the rest of the Making A Strength/Size Routine series and especially the up-coming article, Making A Strength/Size Routine Part VI: Balancing The Balancing Act, for further advice on constructing the best possible routines for you.