Why "HFS?"  One explanation may be the English "Highly Flexible Sole" - translated as "very flexible sole", English experts can listen to other explanations in the following video. Or at least look how nice the shoes are!  -light running shoe  -binding on shoelaces  -breathable mesh upper  -silk lining for moisture wicking made of material "37.5"  -removable insole  -The FeelTrue® sole is 5.5 mm  -suitable for normal to wider foot  -vegan    Size chart: