Transcutaneous electrical nerve stimulation -- TENS -- machines are a common form of pain relief, especially for musculoskeletal pain. Mild muscle soreness and skin irritation may occur with excessive use.

Electrical muscle stimulation is a process through which low-level electrical currents are used to stimulate muscles, forcing them to contract.

Slendertone is an electrical muscle stimulation device (EMS). This means that a mild electrical current produced by the device forces involuntary contractions from your muscles. Advertisements for commercial EMS systems, including Slendertone, claim these devices will help tone and strengthen muscles.

Electrical stimulators work similarly to the nervous system -- in both cases, an electrical signal causes a specific muscle to contract. A key difference is that the body generates electrical signals internally, while electrical stimulators generate the electrical signal outside of the body.

There are quite a few muscle stimulating devices on the market that promise its users tighter and firmer abs. Unfortunately, the FDA explains that only one brand may be able to fulfill such a promise.

Electrical muscle stimulation -- often called "e-stim" or "TENS" -- is a type of treatment often used in physical therapy or other rehabilitation settings. The two primary uses for this treatment are pain relief and muscle re-education. In most settings, there is a machine that provides an electrical current.