Firstly I established the position of the character; In order to see the number on the paper attached to the shirt I left it hanging down - this works since it gives the impression that this is a fun runner with a numbered label on their shirt.
I initially drafted out a rough estimate as to where the knee behind the 81 sign would be, to make it easier to judge the proportion on both legs. I sketched out where the knees are with 3 lines so that the audience have a better understanding of what's happening.
I think the proportions look OK but it moves slightly too slowly for it to be someone starting a race. For these tests I have doubled up the frames, however I will test out a faster speed to see if that works better.