BELIEVR - Science
The Science Behind BELIEVR
Training smarter starts with understanding what your body is doing and why. Every target, zone, and recommendation in BELIEVR is grounded in established exercise science and decades of endurance coaching research. Here's how it works.
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Heart Rate Zones
Your heart rate is one of the most reliable indicators of exercise intensity. BELIEVR uses a five-zone model to guide your training across different effort levels.
How We Calculate Your Zones
If you haven't provided your own heart rate data, we estimate your maximum heart rate using the widely-used age-predicted formula:
Max HR = 220 - your age
This formula, first published by Fox, Naughton, and Haskell (1971), provides a practical starting point for most athletes. Your lactate threshold heart rate — the point where your body shifts from primarily aerobic to anaerobic energy production — defaults to approximately 88% of your max HR, consistent with values observed across trained endurance populations.
From these two anchor points, we derive five training zones:
Custom Zones
If you know your actual max HR or threshold HR — from a lab test, a field test, or your own training data — you can enter those values directly in Settings. BELIEVR will recalculate all your zones accordingly. You can also set fully custom zone boundaries if your coach or testing protocol uses a different model.
References:
Fox, S.M., Naughton, J.P., & Haskell, W.L. (1971). Physical activity and the prevention of coronary heart disease. Annals of Clinical Research, 3, 404–432.
Karvonen, M.J., Kentala, E., & Mustala, O. (1957). The effects of training on heart rate. Annales Medicinae Experimentalis et Biologiae Fenniae, 35, 307–315.
American College of Sports Medicine. (2021). ACSM's Guidelines for Exercise Testing and Prescription (11th ed.). Wolters Kluwer.
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Pace & Power Targets
Every run, bike, and swim session in your plan comes with personalised intensity targets. These are derived from your fitness data using methods proven in competitive endurance training.
Running Pace
Your easy run pace is the foundation for all run targets. BELIEVR determines it through the following priority:
Your custom easy pace — if you've entered it directly
Your recent 5K time — easy pace is approximately 30% slower than 5K race pace
Your recent 10K time — easy pace is approximately 25% slower than 10K race pace
Experience-level defaults — based on your training background and sex
From your easy pace, we derive tempo, threshold, and interval targets using percentage-based offsets that reflect the physiological relationship between training intensities. This approach draws on the work of Jack Daniels, whose VDOT system has been the standard in distance running coaching for over three decades.
Cycling Power
Bike sessions use Functional Threshold Power (FTP) — the highest power output you can sustain for approximately one hour. If you haven't set your FTP, we use experience-based defaults. Power zones follow the same five-zone model as heart rate, with thresholds calibrated to your FTP.
Swim Pace
Swim targets are based on Critical Swim Speed (CSS) — your sustainable threshold pace per 100 metres. CSS is the swimming equivalent of lactate threshold and is widely used in competitive swim training to set interval targets.
Custom Values Always Win
If you provide your own pace, power, or speed data, it always takes priority over any estimate. The defaults exist to get you started safely — as your data improves, your targets become more precise.
References:
Daniels, J. (2013). Daniels' Running Formula (3rd ed.). Human Kinetics.
Allen, H. & Coggan, A. (2010). Training and Racing with a Power Meter (2nd ed.). VeloPress.
Swimsmooth. Critical Swim Speed Protocol. Based on research by Wakayoshi et al. (1992).
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Estimated Distance
Sessions in BELIEVR are time-based, not distance-based. When you see an estimated distance on a session, it's calculated from your pace settings and the planned duration.
This estimate is for planning purposes only — to help you anticipate your route or pool lane count. What matters is completing the specified duration at your target intensity. Pace will naturally vary day to day based on terrain, weather, fatigue, and how your body feels. Trust the time and effort targets; the distance will follow.
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Nutrition Guidance
For longer and higher-intensity sessions, BELIEVR provides hydration and fuelling recommendations tailored to the workout's duration and demands.
Hydration
Fluid intake recommendations are based on general guidelines from the American College of Sports Medicine, which suggest 400–800 ml per hour during exercise, adjusted for intensity and conditions. BELIEVR scales this to each session's demands — shorter or lower-intensity sessions may need little or no additional hydration, while long endurance efforts receive higher targets.
Carbohydrate Fuelling
During exercise lasting beyond 60–90 minutes, your body benefits from exogenous carbohydrate to maintain performance. BELIEVR's recommendations follow current sports nutrition consensus:
Under 60 minutes: Generally no additional fuelling needed
60–150 minutes: Up to 30–60 g of carbohydrate per hour
Over 150 minutes: Up to 60–90 g per hour (using mixed glucose-fructose sources)
These are starting points. Individual tolerance, gut training, and race-day strategy all influence what works best for you.
References:
American College of Sports Medicine. (2007). Exercise and Fluid Replacement: Position Stand. Medicine & Science in Sports & Exercise, 39(2), 377–390.
Thomas, D.T., Erdman, K.A., & Burke, L.M. (2016). Position of the Academy of Nutrition and Dietetics: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics, 116(3), 501–528.
Jeukendrup, A.E. (2014). A Step Towards Personalized Sports Nutrition. Sports Medicine, 44(Suppl 1), 25–33.
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Training Plan Design
Your BELIEVR training plan follows a periodised structure — progressively building fitness through cycles of increasing load followed by recovery. This principle of progressive overload with planned adaptation is the foundation of all effective endurance training.
Sessions are sequenced to balance the three triathlon disciplines across your available training days, respecting the recovery demands of each sport and the interference effects between them. The plan adapts to your life — when your availability changes, session priorities and intensities adjust to preserve the training stimulus that matters most for your goals.
References:
Bompa, T.O. & Buzzichelli, C. (2018). Periodization: Theory and Methodology of Training (6th ed.). Human Kinetics.
Friel, J. (2016). The Triathlete's Training Bible (4th ed.). VeloPress.
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A Note on Health Information
BELIEVR provides training guidance for informational and educational purposes. It is not a medical device and is not intended to diagnose, treat, cure, or prevent any medical condition.
The recommendations in this app are based on established exercise science and general guidelines for healthy adults engaged in endurance training. They do not constitute medical advice. Individual physiology varies — what works for most athletes may not be appropriate for everyone.
Always consult your physician or a qualified healthcare professional before beginning a new exercise program, especially if you have a pre-existing health condition, are pregnant, or have concerns about your ability to exercise safely.
Heart rate data from wearable devices can be affected by fit, skin type, movement, and environmental factors. Treat all sensor-derived metrics as estimates, not clinical measurements.
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Last updated: April 2026
