One of the first things I learned as an undergraduate student while studying exercise science was the SAID Principle, which stands for Specific Adaptations to Imposed Demands. SAID is the basis for all training programs, regardless of your fitness goals. In simpler terms, SAID means that your body will respond and adapt to the stress (exercise) you place upon it. It is important to note that not all training produces the same adaptations. To illustrate, I’ll contrast someone who primarily trains for sprinting a 100-meter dash and someone who trains for a marathon.

A world-class male sprinter can cover 100 meters in less than 10 seconds, while an elite marathoner can cover a 26.2 mile course in roughly 2 hours and 5 minutes. Both of these achievements are incredibly impressive and require years of hard work, dedication, favorable genetics and the right type of training.

I’ll start with the most striking difference between the two: physical appearance. A sprinter’s body is built for power and speed. A sprinter has a chiseled physique defined by robust musculature because of how he trains. Training for sprints consists of short intervals of hard, intense work on the track and in the weight room. While a sprinter’s training may not be as long as that of a marathoner, the effort that goes into his program is incredibly intense and has the specific purpose of helping him get from one end of the track to the other faster than anyone else. A sprinter’s style of training emulates exactly what he will be doing during competition: working as hard as possible for a brief period of time.

Now let’s compare this to an elite marathoner, who runs at a consistent pace for more than two hours. An endurance athlete is trim and does not carry much extra muscle mass, other than what is needed to perform his event. He may run many miles a day at a set pace and increase the speed of his pace over many days to accomplish a desired time. This also emulates what will be required of him during competition.

The physiological changes that occur in the bodies of these two athletes are quite different. One major difference between the two is how their muscles respond.  The sprinter develops a surplus of what is called fast-twitch muscle fiber. Simply put, fast-twitch fibers allow him to produce extraordinary amounts of power and speed for a short period of time. Fast-twitch fibers are great for sprinting but poor for endurance. If you asked a top-level sprinter to accompany you to your next 5K event, he would probably look at you like you had three heads. A sprinter would perform horribly at an endurance event, simply because he has not trained to do anything other than run hard and fast. Even though he is loaded with muscle, his fast-twitch fibers are not built to handle endurance activities.

On the other hand, the marathoner develops many slow-twitch muscle fibers, which allow him to run at a sub-maximal pace for an extended period of time. This too is a double-edged sword, since he sacrifices speed and power at the expense of being able to perform for hours on end.

The good news is you don’t have to be an amazing athlete to reap the benefits of SAID. If your goal is to get stronger, then you need to perform resistance exercises. For more flexibility, be sure to stretch or consider adding yoga to your exercise plan. The main point is to make sure you are training for what you want to achieve.