iRelief is an app that provides a breath control training for the user to practice controlled training techniques, while providing bio-feedback on the user's training data. Using photoplethysmographic techniques and the iPhone's camera, iRelief is able to measure the heart rate and calculate Heart Rate Variability (HRV) just by placing the user's fingertip on the camera lens. The HRV is a measure taken by the beat-to-beat interval of the user's heartbeat, which based on existing research, increases as we inhale and decreases as we exhale.
pdr (personal data recorder) and pdx (personal data expert) are applications that collect and evaluate mostly numeric personal data. The intended use case is for logging individual medical data (blood sugar, blood pressure, body temperature, weight, heart rate, and medications), but they can also be used for technical, sports, fitness, weather, environmental, or financial data. They work on a continuous flow of numbers in time that you want to monitor. Every data item can also be commented by text. pdr can use Twitter and email accounts for data input, which makes mobile phones usable for data input. It is one of the few free applications for diabetics.