In case panel four has you confused:

Piro speaks:
Mumu is whapping shigeo on the shoulder. Her hand is at the top of it's arc and her fingers are splayed back. Maybe your fingers dont do that, but i know many people's who can. It's perfectly valid to exaggerate slightly even in realistic drawings to emphisize motion (artists do it all the time). If i could draw hands slighly better this wouldnt even be a question, and it would be clearer if her left arm was visible.