Se and Si aren't people, they're mental processes. Whenever you're pushing yourself or making effort you're using Se. When you're paying attention to your state of comfort and physical needs, you're using Si. It's really not that complicated.
Roughly yes, if it's something you can do naturally it wouldn't use (much) Se. But getting better athletically almost always entails pushing yourself somehow.when I think of Si in athletics, I think of mastering your body by recognizing its natural strengths and honing them, gauging its limitations, finding ways to circumvent those limitations via your natural strengths, making your body do what you want it to do without burning yourself out, regardless of extraneous pressures, because at the end of the day, your self-esteem lies in your ability to read these cues, and to adjust to them accordingly. essentially there is no need to feel threatened by competition and/or "powerful" opponents if you're confident in your own talent(s) and in control of yourself.
Commonly? I've never heard that before in my life.(I think it's important to note that quick reflexes are commonly attributed to Ne-egos [specifically alpha NTs] so they don't buckle under "pressure" in the way that it's been portrayed here, instead it's more like an aversion to excessive physical exertion, not falling prey to threatening circumstances or people)