The SAID Principle
The SAID principle is one of the most important basic concepts in sport science. It is an acronym which stands for Specific Adaptation to Imposed Demand. It means that when the body is placed under some form of stress, it starts to make adaptations that will allow the body to get better at withstanding that specific form of stress in the future. The adaptation process does not occur by any one mechanism - it is a general tendency of the body which is played out in innumerable separate mechanisms.
While it is almost impossible to understand and account for all these separate mechanisms in devising a training program, it is easy to remember the general SAID principle - it means that the body is always trying to get better at exactly what you practice.
Adaptation is Specific
Let's take some simple examples. If you place mechanical stress on the bones of the body by shock or impact, this will set in motion simple physiological processes that will thicken and harden the bones in the exact area of stress. For example, the place where your heel bone strikes the ground will be very hard and dense. The dominant arm of a tennis player will have larger bones than the opposite arm. Martial artists can toughen their shins and forearms into steel weapons through repeated shock training of the bone. The same thing happens with tendons and ligaments, which thicken and strengthen in response to mechanical stresses such as resistance training. Stress to muscles will cause them to get bigger, and so on.
The SAID principle also refers to adaptations that are far more sophisticated and complex, such as learning new motor skills. As you practice physical skills, there are numerous physical changes to the structure of the brain as a result. For example, if you spend hours practicing the piano, the part of your brain that controls hand coordination will actually grow larger. The neurons responsible for the coordinated finger actions will develop better and faster lines of communication between themselves. And your memories of hand skills will be placed into parts of the brain where they can be accessed and executed automatically, without any degree of conscious effort or thought.
So if you want to get better at dealing with some some form of stress such as hitting a tennis ball or running 26 miles, start exposing yourself to the stress in question and then hope that the body makes some favorable adaptations. There are two major limitations to keep in mind. First is that the training stress must be the right amount and second, the stress must be sufficiently specific to ensure "transfer" or "carryover" to your sport or activity. Let's look at these issues in turn.
The Right Amount of Stress
Stress in the right amount simply means not too much and not too little. If there isn't sufficient stress, there will be no adaptation, and if there is too much stress, you will cause injury or burnout. If you want to strengthen your arm bones, tapping them with your finger won't help, and a whack with a hammer will just break them. If you have been biking for years without improvement in your speed or endurance, then maybe you are not exposing yourself to sufficient stress to encourage the body to build the adaptations that will allow biking success. On the other hand, maybe your failure to improve is because each workout is too stressful, and therefore the body is failing to fully recover before the next workout and is instead just progressing into chronic injury. The basic rule about getting better at anything is to keep progressing the level of difficulty of the training without getting hurt or overtired. Very simple concept in theory, but it can be hard to apply in practice. Performance tends to plateau when the difference between too much and too little is so small that we can't find it. The greatest athletes in the world are those people who are able to expose themselves to the greatest amount of stress without injuring themselves. At some point even they will reach a point where further stress will only cause injury instead of adaptation. Most of us reach this point much sooner.
Carryover of Training to Sport
The carryover issue is a little more complex. Remember that that the S in SAID stands for specific. This means that the body only makes adaptations to withstand the specific stress it encounters - it has no interest wasting time making changes that don't directly address the issue. For example, if you train your right arm, the right arm will get stronger, not the left. If you practice the piano, you will get better at the piano, not horseshoes. But if you practice the piano will you get better at the oboe? Maybe a little. In other words, there is a some carryover or transfer from piano to oboe. There's probably a lot of carryover from piano to organ. How much does your training program in the gym carryover to the sport you are training for? The answer as confirmed by almost any study on this issue is - probably nowhere near as much as you would imagine.
Let's use some examples. What about trying to become a better soccer player by using a swiss ball or other unstable surface to train your "balance." Study after study shows that training on an unstable surface confers no measurable performance benefits on the field of play that could not be obtained by general exercise. In fact, people who train balance on a swiss ball are no better than anyone else at balancing with one foot on the ground! Why? It turns out that the mechanism by which the body balances on an unstable surface (called the "righting reflex") is a totally separate mechanism from that which allows you to balance on a stable surface (called the "tilting reflex." But you don't even need to remember all that, just remember the SAID principle - if you want to get better at soccer, play soccer, don't try to balance on a ball, that's an entirely different skill.
What about using passive stretching as means to improve your "flexibility" in soccer and prevent a hamstring pull during a sprint or kick? Studies have shown repeatedly that pregame stretching does absolutely nothing to prevent injuries, and in fact makes you slower and less explosive for a short period after the stretch. Part of the reason is that passively stretching your hamstring on the ground is a completely different activity from actively kicking the leg out in front of you during a sprint or kick. In other words, stretching is not a specific preparation for soccer, and therefore violates the SAID principle. By the way, studies also show that you can effectively prevent injuries on the soccer field by a pregame warm up of the specific skills to be used on the field - like cutting, sprinting and kicking. The SAID principle in action again.
What about cross training - can you train your aerobic capacity for cycling by running or vice versa? Most studies show that there is some small amount of carryover here, but again not as much as you would probably expect. Sports scientist Matthew Wright estimates that the aerobic benefits that could be derived from 100 hours of endurance running might translate into the equivalent effect of 10 hours of endurance training for cycling. So why not just get on the cycle for ten hours? The carryover of cycling to running is even weaker, because running is a complex activity that relies to a much greater extent on skills of coordination, and bicycling is a much simpler activity. For example, consider Lance Armstrong, the greatest biker of all time and co-owner of the highest VO2max ever measured. He recently completed a marathon in about three hours - an excellent time for an amateur, but nowhere near where his performance would be if his aerobic capacity from cycling had a strong carryover to running. He said the race was one of the hardest things he ever did. More proof of the SAID principle.
So, in summary, remember to keep training simple - if you want to get better at X, do X as hard as possible without getting hurt or overtrained. Be very skeptical of the carryover or transferability of "functional training" or even training that purports to be "sport specific." Chances are, it's not.