Who moved the goals?

At MoMA today I found this girl in quiet amusement, bewilderment, contemplation of a painting by Umberto Boccioni.

A card besides the painting helpfully states … "In this work a soccer player dematerializes into a luminous and flickering atmosphere, save for his firmly sculpted calf, at centre."

Of course it is! There is so much I just don't understand about modern art.