How the Nike + iPod Works

Over the last few years, warnings about the health risks associated with obesity have become increasingly dire. At the same time, computers, game consoles and other electronic devices have become increasingly present in people's homes. So it's not surprising that many new gadgets, from the Nintendo Wii to the ExerStation console controller, combine technology with fitness.

The Nike + iPod Sport Kit is similar. Essentially, it combines a portable music player with a pedometer -- two devices that runners have used for years. But the Sport Kit is considerably more advanced than an ordinary pedometer. It uses circuitry, radio waves and software to track and report on a person's workout.

The Kit has two components -- a sensor and a receiver, both of which are about an inch (2.5 centimeters) long. The sensor fits into a small space under the insole of a Nike+ shoe. The receiver plugs into an iPod Nano.

The Nano is not included, but it is required for the system to work. It provides battery power for the receiver and a user interface for the workout software. Runners use their Nano's click wheel to control the software, which is accessible through the "Nike+ iPod" menu. The workout software lets people:

Create workout playlists
See how far and how fast they've run as well as how many calories they've burned
View statistics about past workouts
Set workout goals

All iPod Nanos shipped after July 13, 2006 come with the workout software already installed. Older Nanos can automatically download the software using the iPod Update feature in iTunes.

In addition to providing power and a user interface, the Nano tells runners how the workout is progressing. A computerized voice describes how far they've run, how quickly and how far away the destination is.

The Nano's flash drive also provides storage space for workout data. When synched, the Nano transfers that data to a PC or Mac. The computer's iTunes software can automatically upload the data to a Nike+ account. At the Nike+ Web site, runners can view workout statistics and send challenges to other runners. People can also use the site's MapIt feature to map and share their routes.

**This user-submitted route begins near the** **HowStuffWorks office.**

All of this data comes from the interaction between two devices -- the Nike + iPod sensor and receiver. We'll look at them in more detail next.

The Nike + iPod Sensor

Just like cell phones and televisions, the Nike + iPod sensor and receiver communicate using radio waves. The sensor detects every step a runner takes and broadcasts this information to the receiver. The receiver routes the information to the iPod Nano, which relays it to the runner, either on the screen or through the headphones.

Piezoelectric materials can create electrical impulses when they

change shape.

The sensor fits into a hole under the insole of a Nike+ running shoe. It detects the runner's footfalls through its piezoelectric accelerometer. Piezoelectric materials produce electrical current when they change shape, or they change shape when exposed to electricity. Piezoelectric transducers, often used in speakers, rapidly change shape when they come in contact with electrical current. Piezoelectric sensors, on the other hand, use quartz, silicon or manmade crystals that produce electricity when squeezed, moved or bent.

These generator-like crystals are often microscopic. Depending on how the crystals are cut, they usually produce an electrical charge when compressed in a specific direction or along a specific plane. For these reasons, piezoelectric sensors can be very small and very accurate. In addition to the Nike+ sensor, tiny piezoelectric sensors provide the motion-sensing capabilities for the PlayStation 3 and Nintendo Wii controllers.

**The Nike+ sensor's accelerometer rests on top of its battery. A transmitter broadcasts signals to the accompanying receiver.**

The accelerometer in the Nike+ sensor detects when a person's foot is on the ground. When someone is standing still or walking slowly, his feet spend more time touching the earth than in the air. But when jogging or sprinting, his feet spend less and less time on the ground. The faster he runs, the less time his feet spend in contact with the surface under them. Because of this basic trait of walking and running, a processor can use equations to convert contact time into running speed. The process is the most accurate when runners calibrate their sensors by running a pre-set distance so get a baseline reading.

The accelerometer also acts as the sensor's on/off switch. When the shoes aren't moving, the accelerometer has no footsteps to report -- it stops sending data. In the absence of the accelerometer's output, the sensor eventually puts itself to sleep. But when a runner puts his shoes on and takes a few steps, the sensor generates electrical pulses, and the sensor resumes operation. The sensor also has a physical switch which can turn the unit off; the unit stays off until someone presses the switch again. This preserves the battery's life (the 1,000-hour battery can't be replaced) when someone isn't running regularly or is running in different shoes.

**An on/off switch, processor and antenna are located on the underside of the sensor.**

The Nike+ iPod sensor sends information to the receiver using a built-in transmitter and antenna. It broadcasts its data at a radio frequency of 2.4 gigahertz using a proprietary protocol. In addition to transmitting data about a person's running stride, it transmits a unique code that it uses to identify itself. We'll look at how the receiver uses this information next.

Nike+ Shoes

In addition to the Sport Kit and the iPod Nano, the Nike+ system requires compatible Nike shoes -- specifically, shoes from the Nike+ Line. Only Nike+ shoes have the necessary space under the insole to accommodate the transmitter. Other manufacturers, however, have developed pouches that hold the transmitter and lace to the top of a person's shoe. These manufacturers claim that the transmitter still gives accurate readings when in the pouch. However, the angle at which the sensor rests on top of a person's foot is slightly different from its angle inside a person's shoe. Depending on how the piezoelectric crystals align within the sensor, this could affect the overall accuracy of the system.

The Nike + iPod Receiver

Since its storage, software and battery power reside on the iPod Nano, the Sport Kit's receiver is a little simpler than the transmitter. It plugs into the Nano's dock connection with a multi-pin connector -- these pins carry data and power back and forth between the two devices. The receiver itself has only a few components, including a processor, a receiver, an antenna and several resistors and capacitors.

The first time someone uses the Nike + iPod system, the sensor links to the receiver. The receiver learns to recognize that particular sensor's identification code. For this reason, multiple people can run together without their Nike + iPod sensors interfering with one another. If several people in one family use the same iPod but different sensors, the receiver can learn to recognize each sensor, and the Nano can store different settings for each user.

**The Nike+ receiver's antenna and receiver detect the signals from the transmitter.**

This system is flexible, and the 60-foot (18.2-meter) range of the transmitter makes it unlikely that the two parts of the system will lose contact with one another. However, the sensor's relatively long range has raised privacy concerns about the Nike+ system.

In November of 2006, members of the University of Washington's Computer Science and Engineering department released a paper detailing how the Nike + iPod Sport Kit could be used for nefarious purposes. The paper pointed out that the transmitter sends data regardless of whether the receiver is nearby or even plugged into an iPod. According to the paper's authors, this constant transmission could be used to invade a person's privacy.

The paper proposed several scenarios in which a Sport Kit could be used to track a person. The scenarios had a few technical requirements. In order to successfully track someone using the Sport Kit, a perpetrator must:

Determine the identification number of the victim's sensor in order to track the right person.
Conceal receivers and antennae, ensuring that the victim will pass within 60 feet (18.2 meters) of them
Develop a method for retrieving and analyzing the sensor's data

Several media outlets picked up on the story, and some asserted that the Sport Kit was an easy-to-hack RFID system that threatened people's privacy. However:

The Sport Kit is not technically an RFID system - its range is much greater than the range of an RFID tag, and it operates on a different radio frequency.
Although the sensor transmits a unique identification code, it does not transmit a person's name or other identifying information.
Deploying a surveillance system designed to track a person's Nike + iPod equipment would work only when the victim was wearing his running shoes with the sensor turned on. It would also require an extensive network of antennae.
Easier location-tracking methods, such as GPS receivers, are readily available.

However, the Nike + Sport kit currently faces a different issue -- in February of 2007, electronics manufacturer PhatRat Technology sued Apple and Nike for patent infringement. PhatRat claims that the Sport Kit infringes on its wireless performance-tracking technology.