General Guidelines to HIT BRIEF, HARD work done INFREQUENTLY.

When you're in the gym you want to focus your energies on only performing work that is productive, i.e., growth producing. In good form, you push yourself as far as you can go on every set. Now, by training this way you simply CANNOT do the marathon 2-3 hour workouts the "champs" say they do in the muscle mags.
HIT can be summed up in the following GENERAL guidelines. These guidelines - or ones very similar - have formed the basis of strength training programs for years:

1. TRAIN WITH A HIGH LEVEL OF INTENSITY.
Intensity is defined as "a percentage of momentary ability". In other words, intensity relates to the degree of "inroad" or muscular fatigue, made into muscle at any given instant. Research, going back almost 100 years now to studies done by German scientists, has conclusively shown that intensity is the SINGLE MOST IMPORTANT FACTOR in obtaining results from strength training.
It has been shown that the HARDER that you train (intensity), the GREATER the adaptive response. A high level of intensity is characterized by performing an exercise to the point of concentric (positive) MUSCULAR FAILURE, i.e., you've exhausted your muscles to the extent that the weight cannot be moved for any more repetitions. Failure to reach a desirable level of intensity - or muscular fatigue - will result in little or no gains in functional strength or muscular size as low intensity workouts do very little or nothing in the way of stimulating muscle size/strength.
Evidence for this "threshold" is suggested in the literature by the OVERLOAD PRINCIPLE (Enoka, 1988; Fox and Mathews, 1981; Hochschuler, Cotler and Guyer, 1993; Jones, 1988; Wilmore 1982).
Essentially this principle states that in order to increase muscular size and strength, a muscle must be stressed - or "overloaded" with a workload that is beyond its present capacity. Your intensity of effort must be great enough to exceed this threshold level so that a sufficient amount of muscular fatigue is produced.

2. FOLLOW THE "DOUBLE PROGRESSION" TECHNIQUE IN REGARDS TO REPETITIONS AND WEIGHT.
For a muscle to increase in size and strength it must be forced to do PROGRESSIVELY HARDER WORK. Your muscles must be overloaded with a workload that is increased steadily and systematically throughout the course of your program. This is often referred to as PROGRESSIVE OVERLOAD. Therefore every time you work out you should attempt to increase either the weight you use or the repetitions you perform relative to your previous workout. This can be viewed as a "double progressive" technique (resistance and repetitions). Challenging your muscles in this manner will force them to adapt to the imposed demands (or stress).

3. PERFORM 1 TO 3 SETS OF EACH EXERCISE.
In order for a muscle to increase in size/strength it must be fatigued or OVERLOADED in order for an adaptive response to occur. It really doesn't matter whether you fatigue your muscles in one set or several sets - as long as your muscles experience a certain level of exhaustion. When performing multiple sets, the cumulative effect of each successive set makes deeper inroads into your muscle thereby creating muscular fatigue; when performing a single set to failure, the cumulative effect of each successive repetition makes deeper inroads into your muscle thereby creating muscular fatigue. Numerous research studies have shown that there are NO significant differences when performing either one, two or three sets of an exercise, provided, of course, that one is done with an appropriate level of intensity (i.e. to the point of concentric muscular
failure).

4. REACH CONCENTRIC MUSCULAR FAILURE WITHIN A PRESCRIBED NUMBER OF REPETITIONS.
As stated above, research shows that our level of intensity is the most important factor in determining your results from strength training - the HARDER you train, the BETTER your response.
As muscle hypertrophy is an adaptive response by the body to stress, you should always strive to go as far as you can go on that "impossible" rep. Every centimeter matters. Your "impossible" rep should last between 10-15 seconds. One could even call this an "isometric rep". If concentric muscular failure occurs before you reach the lower level of the repetition range, the weight is too heavy and should be reduced for your next workout. If the upper level of the repetition range is exceeded before you experience muscular exhaustion, the weight is too light and should be increased for your next workout by five percent or less.
The GENERAL recommendation is 8-12 repetitions But this can vary from individual to individual, and from body part to body part. In many cases people have been known to benefit from higher reps for their lower body (12-15), while lower reps for the upper body (6-8). The most important thing to remember here is that it the *number* of repetitions isn't the key factor - TIME is. One can perform a set of 10 reps in as low as 10-15 seconds, or a set of only 1 rep in 60 seconds. So how many seconds per repetition? The general guideline is a 6 second repetition consisting of a 2 second lifting (concentric) phase, followed by a 4 second lowering (eccentric) phase. The emphasis
is placed on the lowering, or NEGATIVE, as research has shown this to be the most productive part of the rep.

The lowering of the weight should also be emphasized because it makes the exercise more efficient: the same muscles that are used to raise the weight concentrically are also used to lower it eccentrically. The only difference is that when you raise a weight, your muscles are shortening against tension and when you lower a weight, your muscles are lengthening against tension. So, by emphasizing the lowering of the weight, each repetition becomes more efficient and each set becomes more productive. Because a muscle under tension lengthens as you lower it, lowering the weight in a controlled manner also ensures that the exercised muscle is being stretched properly and safely. Thus in a 8-12 rep scheme with the above guidelines, each set should take you between 48-72 seconds until you reach concentric muscular failure.