Cross Training and Periodization
Cross training maximizes fitness gains for all who use it for many reasons. Traditionally, when someone trains for a specific event, they use a practice known as specificity. This relates to the fact that you train for what you do. For example, a baseball pitcher is going to train his arm and shoulder muscles so that they can become stronger and faster so that they can have maximum throwing power and control.
The downside of specificity training is that often times you create muscular imbalances. Muscular imbalance, if used over long periods of time can lead to injuries for a few reasons. Firstly, certain parts of your body can become faster or stronger than other parts, putting much greater stresses on certain tissues and joints. The other reason is that unused muscles over time begin to atrophy (break down) as your body basically follows a "use it or lose it" principle.
So back to the baseball pitcher for a second. Throwing a baseball involves significant angular velocities at the shoulder, not to mention tremendous arm extension power at the elbow. The muscles involved here are all of the muscles that encapsulate the shoulder joint, but also the triceps to extend the arm. Antagonist muscles act as a braking mechanism to try and prevent injury. In the throwing a baseball example, if the triceps are extending the arm at a high velocity, the biceps work as a brake in order to make sure the arm does not extend further than it should. So a pitcher training only his triceps and not his biceps is increasing his/her likelihood of injury. That is just one small example of how cross training can aid all types of athletes.
More and more studies are being conducted and are finding that distance runners are benefiting from forms of cross training like plyometrics and heavy leg-resistance routines. For a while, it was generally thought that adding muscular size would create slow muscles, so distance runners were avoiding heavy resistance training to their legs in fear of creating slow legs. Not the case, as a properly designed strength program has been proven to shave minutes off of their times!
For the average Joe that is just looking to stay in shape, cross training should be your primary source of training. You should partake in a wide variety of exercise movements, sports, intensities, etc. to train as many different body parts, energy systems and interests as possible. It is when you begin to train for a specific event that you should begin to look at specificity.
This is how the pros do it. A proper training program should be broken up into cycles, also known as periodization. The shortest stage of periodization can last a few days to a few weeks and it is called a "microcycle". After that is a "mesocycle" which can run you anywhere from several weeks to several months. Finally, the "macrocycle" can run from several months to several years! Macrocycles are usually used by Olympic athletes that plan out a 4-year plan between Olympics.
The whole goal in peiodization, especially for athletes, is that you want your body to "peak" in performance at the right time, which is right at competition. For example, let's take a look at a professional football player. As soon as the season ends in January/February, he will probably take a few weeks rest to let his body recuperate after the grueling season. He will then enter a training program that initially starts out with heavy cross training. This will involve total-body resistance training, aerobic and anaerobic conditioning using such things as Plyometrics, power weightlifting, etc.
As the season draws nearer and nearer, his training begins to become more specific and more power-oriented, not to mention training camp will start and his skill-specific training will start to take more and more of his time. All of his training leading up to the season gets more and more specific as he nears closer to first day opening kickoff, his cross-training initial stage is what was one of the most crucial stages of his development because it allowed him to develop a strong, athletic base. The principles and training modalities used at the beginning using total-body conditioning creates a strong basis of fitness to then build on with power moves and skill-specific training.
If you are a runner, try some heavy, leg resistance training. If you are a hockey player, play some soccer in the off-season, etc. By training in multiple athletic movements and events will make you a more well-rounded and injury-resistant person!