The type of demand placed on the body dictates the type of adaptation that will occur.
S - Specific
A - Adaptation
I - Imposed
D - Demands
So, what does the SAID rule mean? Well, it's the type of demand placed on the body which dictates the type of adaptation that will occur. For example: If you’re stuck sitting curled over your keyboard everyday, your posture will start to adapt to the specific demand you’re placing on it.
During the pandemic specifically, we will see an increase in back pain due to poor posture when working from home.
It's important to understand your goal, because it's what helps dictate the specificity of your training. The type of training will dictate the type of result that you're after or that you desire.
Breaking down your goals a bit more will help you to create that specific adaptation. For example, if you had an injured hamstring, we'd want to make sure the hamstring not only heals properly, but becomes stronger to avoid injuries in the future.
If you wanted to improve your aerobic capacity, you'd be looking to increase your VO₂ Max, which is basically the capacity of your lungs to take rigorous activity for a long period of time. You'd also want to improve your muscular endurance to ensure that they able to run for the amount of time that your goal is.