4. SAID Principle
Your muscles and their respective subcellular components will adapt in highly specific ways to the demands (adaptive stress) you impose upon them in your training. This applies as well to various bodily systems and tissues other than your muscles. This is the SAID Principle, the acronym for “Specific Adaptation to Imposed Demands.” If your training objectives include becoming more explosive, then you have to train explosively. If you desire greater limit strength (primarily from an increase in the cross section of myofibrils), you must use heavier weights than if you were training, for example, local muscular endurance (capillarization and mitochondrial adaptations). If your objectives include deriving cardiovascular benefits, then you must tax the heart muscle as well as the oxygen-using abilities of the working muscles.
In fact, the SAID principle is so uncompromising in its highly researched tenet of training “specifically” that problems frequently arise if one possesses more than one training objective at a time. The specific training required for one will frequently detract from the expected gains in the other. For example, training for aerobic strength endurance (aerobic power) will severely limit the level of limit strength one can attain. Similarly, stressing one’s ATP/CP energy system calls for different training methods than does training one’s glycolytic (lactic acid) or aerobic (oxidative) energy systems.
Your specific adaptive responses to exercise can change dramatically over time. This is particularly true as you age. But it is also true if you have successfully improved your body’s recovery abilities. Clearly, this can be accomplished through the use of (illegal and often dangerous) drugs or through the use of certain nutritional supplements. Simply, with improved recovery ability, your body has become a different body, so the adaptation mechanisms have changed.