IAAF Competition Rules, Rule 143
2. Athletes may compete barefoot or with footwear on one or both feet.
The purpose of shoes for competition is to give protection and
stability to the feet and a firm grip on the ground. Such shoes,
however, must not be constructed so as to give an athlete any unfair
additional assistance, including by the incorporation of any
technology which will give the wearer any unfair advantage. A shoe
strap over the instep is permitted. All types of competition shoes must
be approved by IAAF.