There are, however, several universally accepted scientific training principles that must be followed in order to improve conditioning and athletic performance.

Because every athlete is different, each person's response to training will vary. A proper training program should be modified to take individual differences into account. Some considerations:

• Large muscle groups heal slower than smaller muscle groups.
• Fast or explosive movements require more recovery time than slow movements.
• Fast twitch muscle fibers recover quicker than slow twitch muscle fibers.
• Women generally need more recovery time than men.
• Older athletes generally need more recovery time than younger athletes.
• The heavier the load lifted, the longer it will take the muscles to recover.

These are just some of the many differences in athletes. All of these differences will factor into an athlete's training routine. Coaches should also be aware of these differences, and not expect all the athletes on a team to perform the exact same routines.

The Principle of Overload

The principle of overload states that a greater than normal stress or load on the body is required for training adaptation to take place. The body will adapt to this stimulus. Once the body has adapted then a different stimulus is required to continue the change. In order for a muscle (including the heart) to increase strength, it must be gradually stressed by working against a load greater than it is used to. To increase endurance, muscles must work for a longer period of time than they are used to. If this stress is removed or decreased there will be a decrease in that particular component of fitness. A normal amount of exercise will maintain the current fitness level.

The Principle of Progression

The principle of progression implies that there is an optimal level of overload that should be achieved, and an optimal time frame for this overload to occur. Overload should not be increased too slowly or improvement is unlikely. Overload that is increased too rapidly will result in injury or muscle damage.

Beginners can train progressively by starting near threshold levels and gradually increasing in frequency, intensity, and time within the target zone. Exercising above the target zone is counterproductive and can be dangerous. For example, the 'weekend athlete' who trains vigorously only a couple of times per week, does not train often enough, and so violates the principle of progression.

Many athletes violate this principle by failing to train above threshold levels and in the exercise target zone. Clearly, it is possible to do too little and too much exercise to develop optimal fitness.

The Principle of Progression also makes us realize the need for proper rest and recovery. Continual stress on the body and constant overload results in exhaustion and injury. You should not train hard all the time! Doing so will lead to overtraining and a great deal of physical and psychological damage will result.