Letz
All studies

Read the guide · Measurement

What the app cannot measure

A health app that only highlights what it gets right is misleading. Estimating calories from photos carries large and systematic error; watches measure steps and heart rate far better than they measure calories and sleep; and no training load model predicts individual injury with useful precision. Letz reports the confidence level of each reading, uses ranges rather than exact numbers where error is high, and never presents injury risk as a probability.

Rules and studies

  1. 01Limitations of Image-Based Dietary AssessmentPhotos can make logging easier, but energy and portion sizes carry significant error; user review remains necessary.The photo generates a draft with a probable range that the user confirms or corrects; nothing is recorded without review.
  2. 02Limitations of Smartwatches and WearablesSmartwatches help monitor trends, but energy expenditure and other metrics can carry error and vary by device and activity.Exercise calories from wearables appear as context and are never added to the target; the adaptive TDEE already absorbs real expenditure.
  3. 03Training load does not predict individual injuryRecent training load can provide context for workout decisions, but simple load ratios do not support reliable individual injury prediction.Letz displays 7-day and 28-day load as a trend and suggests gradual progression; it never displays injury risk.
  4. 04Training response varies widely between individualsFollowing the same program, gains in strength and muscle mass range from negligible to substantial; study averages do not predict an individual's response.This is why Letz pairs general guidelines with individual data: parameters start within the literature-backed range and are adjusted based on logged history.