How many electrical muscle stimulators have received FDA clearance for toning, strengthening and firming abdominal muscles?
1 (Slendertone Flex)

Why does FDA regulate electrical muscle stimulators?
Electrical muscle stimulators are considered devices under the Federal Food, Drug, and Cosmetic Act. Under this law and the agency's regulations, the FDA is responsible for regulating the sale of all electrical muscle stimulators in the United States. Therefore, firms must comply with appropriate FDA premarket regulatory requirements before they can legally sell their stimulators. Most electrical muscle stimulators (EMS devices) that have been reviewed by FDA are intended for use in physical therapy and rehabilitation under the direction of a health care professional. If a company wants to sell EMS devices directly to consumers, the company needs to show FDA it can be used safely and effectively in that setting.

To read more, visit http://www.fda.gov/cdrh/consumer/ems.html