The health data sources and science behind Wellness Project

TDEE (Total Daily Energy Expenditure)

The app estimates daily calorie burn using one of three methods, selected automatically based on available data:

• Mifflin-St Jeor equation (primary): BMR derived from weight, height, age, and sex, scaled by an activity multiplier from step count and logged workout intensity. Reference: Mifflin MD et al., "A new predictive equation for resting energy expenditure in healthy individuals," American Journal of Clinical Nutrition, 1990.
• Katch-McArdle equation (lean-mass path): Used when recent body composition data is available. BMR derived from lean mass rather than total weight. Reference: McArdle WD, Katch FI, Katch VL, Exercise Physiology: Nutrition, Energy, and Human Performance.
• Adaptive TDEE (back-calculation): When sufficient logging history exists, TDEE is back-calculated from the relationship between observed caloric intake and smoothed weight trend using exponentially weighted moving average and regression analysis. Conceptual basis: Hall KD et al., "Quantification of the effect of energy imbalance on bodyweight," The Lancet, 2011; Thomas DM et al., "Time to correctly predict the amount of weight loss with dieting," Journal of the Academy of Nutrition and Dietetics, 2014.

NSI (Normalized Strength Index)

NSI scores a strength training set relative to population standards for the user's bodyweight, age, and sex.

• 1RM estimation: Derived from a validated predictive equation using lifted weight and repetitions performed. Reference: Lander J, "Maximums based on reps," National Strength and Conditioning Association Journal, 1985; Brzycki M, "Strength testing: predicting a one-rep max from reps-to-fatigue," Journal of Physical Education, Recreation & Dance, 1993.
• Population strength standards: Per-exercise benchmarks interpolated by bodyweight and adjusted by age factor, derived from published normative strength data and allometric scaling research. References: Haff GG & Triplett NT (eds.), Essentials of Strength Training and Conditioning, 4th ed., NSCA; Jaric S, "Muscle strength testing: use of normalisation for body size," Sports Medicine, 2002.
• Age adjustment: Strength standards are scaled by age to reflect the documented decline in peak force production across the lifespan. Reference: Pearson SJ et al., "Muscle function and ageing," Scandinavian Journal of Medicine & Science in Sports, 2002.

Fit Score (Daily Composite 0-100)

The daily Fit Score combines training performance, sleep, nutrition, recovery, activity, and subjective wellbeing into a single 0-100 wellness index. Components re-normalize when data is missing for a given day. Underlying methodologies for each domain are documented in the individual metric sections on this page.

Sleep Score (0-100)

When stage data is available from a connected wearable, the score uses four components. When only duration is available, the duration component scales to 100%.

• Duration: Scored against recommended nightly sleep duration for adults. Reference: Hirshkowitz M et al., "National Sleep Foundation's sleep time duration recommendations," Sleep Health, 2015.
• Deep sleep: Scored against published normative deep sleep values for adults. Reference: Ohayon M et al., "Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals," Sleep, 2004.
• REM sleep: Scored against published normative REM values for adults. Reference: Ohayon M et al., "Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals," Sleep, 2004; Carskadon MA & Dement WC in Principles and Practice of Sleep Medicine.
• Awakenings: Scored based on clinical sleep continuity guidelines. Reference: American Academy of Sleep Medicine, International Classification of Sleep Disorders.

Recovery Score (0-100)

Recovery Score combines physiological and subjective signals into a single readiness index.

• HRV: RMSSD values compared to the user's personal rolling baseline. Because HRV is highly individual, personal baseline is weighted more heavily than population norms. Reference: Plews DJ et al., "Training adaptation and heart rate variability in elite endurance athletes," Sports Medicine, 2013; Shaffer F & Ginsberg JP, "An overview of heart rate variability metrics and norms," Frontiers in Public Health, 2017.
• Resting heart rate: Compared to the user's personal baseline and population health norms. Lower RHR correlates with greater cardiovascular fitness and recovery. Reference: American Heart Association guidelines on resting heart rate as a cardiovascular risk marker.
• Sleep: Hours logged from wearable or manual entry. Reference: AASM 7-9 hour recommendation for adults.
• Wellbeing: Self-reported energy, mood, soreness, and stress. Soreness and stress are inverted (higher soreness/stress = lower recovery).

Heart Rate Variability (HRV)

The app records and displays RMSSD (root mean square of successive RR interval differences) in milliseconds, the most validated short-term HRV metric for athletic monitoring.

• Measurement: Sourced from Apple Health, Fitbit, Oura, or Google Health Connect. Each provider uses their own validated capture method (overnight average or morning reading).
• Interpretation: Compared to the user's own rolling baseline, not population averages, because absolute HRV values vary dramatically between individuals. Reference: Plews DJ et al., "Comparison of heart-rate-variability recording with smartphone photoplethysmography, Polar H7 chest strap, and electrocardiography," International Journal of Sports Physiology and Performance, 2014.
• Readiness signal: A meaningful drop below personal baseline (especially combined with elevated RHR) is used as a recovery flag. Reference: Buchheit M, "Monitoring training status with HR measures: do all roads lead to Rome?" Frontiers in Physiology, 2014.