(this is going to sound strange i know)
ds1 (2.5yo) has always had a thing about wheels and balls. nothing unusual there i know, but over time we've seen him get more and more involved when he watches them move
it started with him tensing his arm muscles e.g. when watching a ball roll down steps
it has progressed so that now he spends much of his time - in fact most of his play time - either with all his muscles tensed (so he's shaking slightly) and sucking air in through his mouth, or, jumping up and down over and over again
you can see it's when he's concentrating, or imagining soemthing
e.g. he just walked into this room, put his train on the desk, stepped back, sucked in air and jumped several times while frowning at his train. picked it up and walked out normally
he's very skinny and i can't help but wonder if his jumping has anything to do with it. he must jump thousands of times a day