One correspondent said that what we’re building here is essentially a Holter monitor…a device that records, over time, the ECG or other medical data for a single person. That’s exactly right, except for 2 differences:
1. Our goal is not just to record ECG, but as many other biophysical variables as possible
2. Unlike a Holter monitor, the iPhone can send the data on the fly to a remote database
3. The iPhone serves as the diary of activity performed
From Wikipedia:
In medicine, a Holter monitor (also called an ambulatory electrocardiography device), named after its inventor, Dr. Norman J. Holter, is a portable device for continuously monitoring the electrical activity of the heart for 24 hours or more. Its extended recording period is sometimes useful for observing occasional cardiac arrhythmias that would be difficult to identify in a shorter period of time. For patients having more transient symptoms, a cardiac event monitor which can be worn for a month or more can be used.\
Much like standard electrocardiography (ECG), the Holter monitor records electrical signals from the heart via a series of electrodes attached to the chest. The number and position of electrodes varies by model, but most Holter monitors employ from three to eight. These electrodes are connected to a small piece of equipment that is attached to the patient's belt, and is responsible for keeping a log of the heart's electrical activity throughout the recording period.