Health disclaimer

Not a medical device

Terrifit is not registered as a medical device with the MHRA, the FDA, or any other regulator, and it is not intended to diagnose, treat, cure or prevent any disease. Nothing it shows you is a substitute for a clinician.

Where the numbers come from

Fitness age is estimated from resting heart rate against population norms, adjusted for BMI and activity, using the approach behind the HUNT Fitness Study and the Uth–Sørensen–Overgaard–Pedersen estimate of VO₂ max. It is an estimate with a wide error bar, not a measurement of your cardiovascular age.

Sleep quality combines duration, overnight restoration and regularity. True sleep efficiency needs time-in-bed, which most sources do not record, so it is left out rather than guessed at.

Load — labelled 'load', not 'stress' — is derived from suppressed heart-rate variability alongside an elevated resting heart rate. It cannot tell a hard week at work from a hard week of training or the start of a cold. All three move it the same way.

Body battery models a reservoir charged by sleep and drained by the day's load, from daily aggregates rather than continuous sampling.

What it cannot do

It cannot detect atrial fibrillation, sleep apnoea, infection, or any other condition. A low recovery score is not a diagnosis and a high one is not a clean bill of health.

Consumer optical heart-rate sensors are less accurate during movement, on darker skin tones at high intensity, and in cold conditions. Treat single readings sceptically and trends seriously.

Before you start training

If you are pregnant, have a heart condition, are recovering from surgery or illness, or have not exercised in a long time, speak to a clinician before starting any programme. Stop and get help if you get chest pain, unusual breathlessness, or feel faint.