Metabolic Conditioning (aka Cardio)
Of all the misconceptions out there, this is easily the biggest and the worst. From fitness mags to the average guy in the gym, "everybody knows" that cardio is what women have to do, and lots of it.
In order of importance on the list of things that actually make a difference in changing the body, aerobic endurance cardio is dead last. As far as I'm concerned, spending hour after hour on the treadmill is all but worthless if the remaining pieces of the puzzle aren't in place.
This does not mean that there is no use any time ever for low-intensity cardio. What it means is that you need to be smart about it. Smart beats hard for the sake of hard every single time.
Let's go over a little cellular energetics so that things make a little more sense.
At the cellular level, the body has two basic processes for obtaining energy, the anaerobic (oxygen independent) pathway and the aerobic (oxygen dependent) pathway.
The pathways are intensity dependent, with oxidative metabolism providing energy at rest and anaerobic metabolism providing energy as the intensity of movement increases.
The difference is the availability of energy. Anaerobic processes can readily provide energy over short time periods, whereas aerobic oxidation is slower from a metabolic standpoint. Intense activity uses up energy faster than aerobic metabolism can provide it. Anaerobic metabolism can provide energy quickly, but only in finite amounts before it needs recharging.
Over very short, very intense intervals, the primary energy substrate is ATP/CP, the so-called alactic or phosphagen anaerobic system. ATP, short for Adenosine Triphosphate, is essentially the energy currency of the cell, directly fueling all processes that require energy. The entire energetic system of the cell is designed around the creation and management of ATP. CP, creatine phosphate, is a short-term reserve of ATP, a compound that can almost immediately restore ATP levels.
The ATP/CP pathway is depleted after roughly 30 seconds of continuous activity, which means that very high intensity activity as a rule will not last longer than this. Athletes such as short-distance sprinters, powerlifters, and Olympic weightlifters are alactic-anaerobic dominant in their events.
The next step down is the glycolytic-anaerobic pathway. This involves the breakdown of muscular glycogen stores to provide a supply of ATP to the working muscle. Glycogen is simply blood glucose stored as a supply of energy, and "glycolytic" refers to the breaking down of glycogen. The by-product of this is lactic acid (or lactate), which results in the "burn" you feel when doing continuous exercise. Glycolytic-anaerobic work can sustain continuous work for around 90 seconds to two minutes, give or take. Most athletes competing in non-endurance competitions, such as most team sports, will fall into this category, as do strongmen competitors and the weight training of a great many bodybuilders.
Finally we have oxidative metabolism, which is where fatty acids in the blood are directly oxidized ("burned" if you will) for energy. This is the slowest pathway, and as such it is not able to sustain activity of any meaningful intensity, at least as compared to what the anaerobic pathways can handle. Aerobic/oxidative metabolism is operative at rest, or during low intensity activities of long duration.
It's important to note that these are just generalizations, and in reality some measure of all three are working at any given time. It's just a matter of which pathway is dominant in any given activity.
It might be very tempting to look at aerobic exercise, with the fact that it burns fatty acids exclusively, and think, wow, that burns a lot of fat! You'd be both right and wrong. Yes, oxidative metabolism burns fat almost exclusively. The problem is, it doesn't burn a lot of it. Have you ever looked at the calorie usage indicators on any cardio equipment? The amount you burn is ridiculously low. There's around 3500 calories in a pound of fat. You'll see numbers like 300-400 calories per hour for low intensity work. That's not a lot of fat being burned, especially not in return for an hour of your life wasted on a treadmill.
If anything, aerobic exercise, at least for physique training purposes, should be used as an adjunct to your diet. If you're burning 300-400 extra calories a day, then that's potentially 300-400 more calories you don't have to cut out of the diet. Of course, if you hate low-intensity cardio as much as most do, cutting your diet might end up being the easier route.
Look at something like a 200 meter sprint on the other hand. The activity itself is dominantly glycolytic, with a comparatively smaller ratio of fatty acids used. However, something interesting happens when you deplete glycogen stores in the muscle.
There's a molecule in cells called AMPK. Without going into detail, AMPK is a sort of "thermostat" for the energy status of a cell. When energy is low, AMPK activates. It has two general effects, to decrease energy expenditure and to increase energy scavenging. The biggest relevant effect is that the body stops using up glycogen for fuel, storing it instead from blood glucose, while increasing the use of fatty acids for energy. This is known as improving insulin sensitivity, and AMPK increases this when activated.
That has interesting implications. It means that even though the activity itself doesn't use much in the way of fatty acids, the interval following the activity can potentially use up a ton of them. You're also more likely to burn up a ton more calories from a session of hard anaerobic-interval training than you are even with an hour of low-intensity aerobic work. A higher energy expenditure coupled with a preferential increase in the use of fatty acids is a good combination.
Anaerobic workouts are performed as intervals, which are alternated periods of work and rest. This takes advantage of the fact that anaerobic output has a limited time frame to operate. By stressing the glycolytic pathway, then resting briefly to allow recovery, you're training that pathway to become more efficient.
High-Intensity Interval (or Intermittent) Training
This is likely the most commonly known variant of interval training. HIIT as it is widely known involves the use of intervals of maximal intensity, which means that each work set is taken to the limit of your ability to handle.
In research, HIIT has shown itself to be devastatingly effective at changing body composition. It's also quite good at improving cardiovascular elements such as VO2 max, due to the intensity of work performed.
However, HIIT doesn't differ much from high intensity strength training in regard to recovery. As a result, HIIT has to be used judiciously, and planned into a weekly program accordingly. Do too much, too often, and you'll risk burn out.
The idea behind all interval training, including HIIT, is to sequence periods of high-intensity work with low-intensity work or rest. In practice, you'll see things like 30 seconds high, 30 seconds low, or 60 seconds high, 90 seconds rest, and so on.
A high-intensity interval might have different meanings, but in most cases it means that either i) the work intervals are maximal or ii) the rest intervals are short so as to prevent complete recovery in between sets.
Tempo Training
Tempo training would be best described as "moderate intensity interval training". Tempo runs have been used by sprinters as a means of achieving a high volume of work, as opposed to the maximal efforts of HIIT. I've used these workouts to great success among figure girls after getting the idea from Charlie Francis' sprinting workouts, especially once the pre-contest diet becomes restrictive and recovery becomes a premium.
Tempo runs are performed at around 75% of maximum. What this means is that if your best time for a 100 meter sprint is 11 seconds, you'd want your work sets to take around 15 seconds (75% of 9m/s is around 6.7-6.8m/s).
If you aren't out running sprints on the track and have no idea of how your maximal performance compares, have no fear. There's a pretty easy way around that.
On whatever cardio machine you're using, simply divide your work:rest ratio into fairly equal increments. Say 30s:30s, or 45s:45s, whatever you prefer, and alternate between them.
The idea is to avoid complete recovery between each work interval in a set, and focus on the overall volume of work. Just remember to keep your work interval intense, but not all-out.
Beginner Level
Set 1: Work, Rest, Work, Rest, Work
Take a 60-90 second rest interval between each set.
Repeat 4-6 times.
Intermediate Level
Set 1: Work, Rest, Work, Rest, Work
Set 2: Work, Rest, Work, Rest, Work
Set 3: Work, Rest, Work, Work, Rest, Work
Set 4: Work, Rest, Work, Rest, Work
Rest interval of 60-90 seconds between each set
Repeat 2-3 times
Advanced Level
Set 1: Work, Rest, Work, Rest, Work
Set 2: Work, Rest, Work, Work, Rest, Work
Set 3: Work, Rest, Work, Work, Rest, Work, Work
Set 4: Work, Rest, Work, Work, Rest, Work, Work
Set 5: Work, Rest, Work, Rest, Work
Rest interval of 60-90 seconds between each set.
Repeat 2-3 times