A treadmill key is a safety feature that the vast majority of modern motorized treadmills employ. It works in a similar manner to the safety key on a jet ski or other machine where it is possible the user might fall off. The key itself allows the treadmill to operate and is on one end of a cord, the other end of which is attached to the user. Removal of the key indicates that there may have been an accident and triggers the treadmill's safety feature that stops the belt from moving further.
While different brands of treadmills require a specific size and shape of treadmill key, the basic function of the key is the same. Most treadmill keys clip onto the user's clothing, but some may have a bracelet at one end that can be worn around the wrist. The treadmill requires constant contact with the treadmill key to remain powered on, and if the key is removed, the treadmill will immediately stop moving and turn off to prevent injuring the user in case he or she has stumbled and fallen off of the belt.
The manner in which the treadmill key attaches to the treadmill can vary depending on the brand and model. Some are shaped like a small, thin card that is inserted into a slot on the treadmill, and when combined with the press of a "power" or "start" button, turn the treadmill on and allow the belt to move. Other brands and models use a magnetic treadmill key, which is easier to pull off of the magnetic pad on the treadmill's interface and may be more sensitive to movements, including accidental falls, that pull the cord to the side. This type is slightly less convenient for the user because it is easier to accidentally knock the magnet off of the pad, interrupting the workout, but it also keeps the user safer from injuries when falling to the side of the treadmill. These are also typically less likely to break and need replacement than the card type.
Unfortunately, many users do not feel that it is necessary to wear the treadmill key or feel that it is too inconvenient to tie around the wrist or clip onto a pocket or sleeve. If the user falls without wearing the treadmill key, there will be no signal to stop the treadmill's motor, and the belt will keep moving. This can be dangerous if a foot or other body part or piece of clothing has gotten caught under the belt or wedged under the machine. Also, tripping and falling when the belt is moving at high speeds can send the user shooting backward off the end of the machine into a wall or whatever is placed behind the treadmill.