One example would be Francis Obikwelu:
At the start he pumps his arms violently, then after about 20 meters he gets nearly erect and relaxes visibly, and when reaching top-speed he gets completely erect, holding his rythm and relaxation.
Is this relaxation at 20 meters a conscious effort do you think?
If so is there a certain body-type that necessitates such an abrupt change?
Obikwelu was a great 200m guy.
Is that just a general observation or a cryptic answer to my question?
Don’t confuse Dcw23 with Clemson 
Thor i know exactly what you are talking about but with all fairly tall sprinters you will find a trend of relaxed running when in the upright position.carl lewis was a prime example of relaxed running making it look easy but in actual fact he was running at full speed.FO is a very relaxed and smooth sprinter.
let FO be a lesson for you by means of–in order to reach top speed you must be in a relaxed and rhythmic state.use him as a model and anaylise his form in greater detail and you will understand form better.great mechanical sprinters will always make it look easy and effortless
Taller sprinters have a higher COM as a result greater momentum needs to be generated in the arms to maintain stability. Human stability is not well researched in respect to application of mechanical principles more research is needed in the area of human stability : its important to note the answer i have given ignores neuromuscular factors .