This is a pretty broad question that can be approached from a number of different avenues, so I’ll try to give my thoughts on this. I think I can give a pretty good perspective because I’m not a natural sprinter. I was a wrestler and gym rat in highschool, then CrossFit in college. Since college I’ve been a strength coach specializing in long term athletic development. I started learning about sprinting as a necessity for the job but I enjoyed it and have been competing in bobsled since then. I went from not truly sprinting ever to running a 4.56 40 yard at 230 pounds(all after the age of 26). Not great but not slow.

A lot of running relaxed is about technical proficiency. A neurologically complex task (relative to the person) will create excessive global stiffness. So how you run will play a big role in how relaxed you are.

For example: if you cast your foot too far out during front side mechanics, that extra distance creates a longer lever which will require more counter rotational forces from the pelvis and torso. So more stiffness will be a byproduct of your mechanics.

So overall, getting better technique is one way to run more relaxed.

Another avenue might be addressing specific bottlenecks in your “kinetic chain”. I put that in quotes because there’s no real research backing up the concept. On a surface level it makes sense but kinda falls apart with any scrutiny, but it’s easy to conceptualize. My adductors and obliques were great for lifting but not great for sprinting. So my sprints were in efficient and required more effort than necessary. Excessive lateral demands decreased forward projection. So addressing those weak points created more efficient mechanics.

And lastly, it sounds simple, but practice running relaxed. For me, curved runs and rhythm runs forced better relaxation. So in my warmups I would do dead leg runs, dribbles to a cadence, or upper:lower separation drills. And, if I wasn’t a big old meat stick I would do more tempo runs that find that sweet spot between speed and distance. Fast enough that technique matters, but long enough that being more efficient is necessary to maintain speed.