Typically refers to two muscles located in the anus (although the term “sphincter” can refer to any circular muscle), the internal anal sphincter and external anal sphincter.

The external anal sphincter can be voluntarily controlled, and in order to engage in penetrative anal play, individuals need to relax and expand this muscle. Some toys are designed to strengthen the external sphincter, and some desensitizing lubricants are designed to numb the muscle in order to allow for easier relaxation and penetration. The internal anal sphincter cannot be voluntarily controlled.