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Sources

Updated October 3, 2026

How Liftkin calculates its scores, targets, and estimates, and the published research behind each one. The same sources are in the app under Home, Sources, and next to every score.

Readiness, HRV, and resting heart rate

Readiness compares last night’s sleep, heart rate variability (HRV), and resting heart rate with your own 30 day baseline, so you are measured against yourself. It suggests how hard to train today; it does not measure your health.

  • Plews et al., 2013. Training adaptation and heart rate variability in elite endurance athletes. Sports Medicine 43(9):773–781.
  • Kiviniemi et al., 2007. Endurance training guided individually by daily heart rate variability measurements. European Journal of Applied Physiology 101(6):743–751.
  • Buchheit, 2014. Monitoring training status with HR measures: do all roads lead to Rome? Frontiers in Physiology 5:73.

Training adjustments and effort

On a Push day main lifts go about 2.5 percent heavier. On an Easy day they use about 90 percent of your usual weight and stop with about 3 reps in reserve (RPE 7). Recover days swap in mobility and easy cardio.

  • American College of Sports Medicine, 2009. Progression models in resistance training for healthy adults. Medicine & Science in Sports & Exercise 41(3):687–708.
  • Zourdos et al., 2016. Novel resistance training–specific rating of perceived exertion scale measuring repetitions in reserve. Journal of Strength and Conditioning Research 30(1):267–275.
  • Helms et al., 2016. Application of the repetitions in reserve–based rating of perceived exertion scale for resistance training. Strength & Conditioning Journal 38(4):42–49.

Training load

Your recent training is compared with your own 4 week average. A big jump above your usual load lowers readiness a little, and a couple of rest days raise it.

  • Gabbett, 2016. The training–injury prevention paradox: should athletes be training smarter and harder? British Journal of Sports Medicine 50(5):273–280.

Estimated one rep max

Liftkin estimates your one rep max from each set with the Epley formula: weight × (1 + reps ÷ 30). Strength progress and PRs use this estimate. It is most accurate for sets of about 10 reps or fewer.

  • LeSuer et al., 1997. The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. Journal of Strength and Conditioning Research 11(4):211–213.

Sleep goal and Sleep score

The default goal is 8 hours, within the 7 or more hours adults are advised to sleep. The Sleep score is 50 percent how close your nights come to your goal, 30 percent a consistent bedtime, and 20 percent your trend versus the last 4 weeks.

  • Watson et al., 2015. Recommended amount of sleep for a healthy adult (American Academy of Sleep Medicine and Sleep Research Society). Journal of Clinical Sleep Medicine 11(6):591–592.
  • Hirshkowitz et al., 2015. National Sleep Foundation’s sleep time duration recommendations. Sleep Health 1(1):40–43.
  • Windred et al., 2023. Sleep regularity is a stronger predictor of mortality risk than sleep duration. SLEEP 47(1).

Calorie and protein targets

Maintenance calories are estimated from bodyweight (14 to 16 kcal per lb, by experience), then lowered 20 percent to lose fat or raised 5 to 10 percent to get stronger or build muscle. Protein is 0.8 to 1 g per lb (about 1.6 to 2.2 g per kg), fat 0.3 g per lb, and carbs fill the rest. These are starting estimates to adjust as your weight trends.

  • Morton et al., 2018. Protein supplementation and resistance training–induced gains in muscle mass and strength: a systematic review and meta-analysis. British Journal of Sports Medicine 52(6):376–384.
  • Jäger et al., 2017. International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition 14:20.
  • Helms et al., 2014. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition 11:20.
  • Hall et al., 2011. Quantification of the effect of energy imbalance on bodyweight. The Lancet 378(9793):826–837.

Daily steps

The default goal is 8,000 steps a day, and you can set your own. In a large analysis, benefits rose with more daily steps and leveled off at about 6,000 to 8,000 steps for adults 60 and older and 8,000 to 10,000 for younger adults.

  • Paluch et al., 2022. Daily steps and all-cause mortality: a meta-analysis of 15 international cohorts. The Lancet Public Health 7(3):e219–e228.

Food and nutrition data

Food search and barcodes use Open Food Facts. Meal photo scans check the foods they find against USDA FoodData Central. AI estimates can be off, so check portions before saving.