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The Science
How the recommendations are built
Every suggestion Sundee Fundee makes is driven by a layered model that reads your body's current state, your biology, and your injury history before it prescribes a single set or rep. Here is how each layer works.
The Readiness Score
Before each session, the app computes a composite readiness score from four inputs pulled directly from Apple Health and your in-app logs:
- HRV trend. A single-night HRV reading is noisy. The model tracks a rolling 7-day HRV trend and flags meaningful drops from your personal baseline rather than comparing you to a population average.
- Resting heart rate delta. An elevated resting HR the morning after a hard session is one of the clearest signals of incomplete cardiovascular recovery. The model weights this against your own recent baseline, not a fixed threshold.
- Sleep quality. Duration alone is a weak predictor. The app uses Apple Health sleep stages to weight deep and REM sleep more heavily than light sleep, because hormonal and neuromuscular recovery are tied to sleep architecture, not just total hours.
- Subjective feel. A brief daily check-in captures soreness, motivation, and energy. Self-report adds signal that wearables cannot measure — mood, stress, and perceived readiness are meaningful predictors of performance even when objective metrics look fine.
The four inputs are weighted and combined into a single 1–100 score. High-readiness days unlock heavier loading and optional PR attempts. Low-readiness days trigger automatic deloads or active recovery substitutions — not because the calendar says so, but because the data does.
Cycle-Phase Adaptation
Female physiology changes substantially across the menstrual cycle. Ignoring those changes means leaving performance on the table in some phases and accumulating unnecessary fatigue in others. Sundee Fundee maps programming to four distinct phases:
Recommended Training Intensity by Cycle Phase
Follicular phase(days 1–13)▶
Rising estrogen improves neuromuscular recruitment and blunts perceived exertion. This is the phase best suited for intensity work — heavier loads, PR attempts, and higher volume blocks. The app increases working weight targets and flags this as a prime window for strength testing.
Ovulatory phase(days 14–16)▶
Peak estrogen and a short LH surge create the highest strength potential of the cycle, but also the highest ligament laxity — particularly in the ACL. The app maintains high intensity while substituting or flagging exercises with known elevated injury risk during this window.
Luteal phase(days 17–28)▶
Progesterone rises and core temperature increases slightly. Fatigue accumulates faster and perceived effort goes up for equivalent loads. Volume is reduced by 10–20%, rest periods are extended, and hypertrophy-focused rep ranges take priority over absolute load.
Menstrual phase(days 1–5)▶
For athletes who experience significant symptoms, the app reduces intensity further and prioritizes movement quality over load. For athletes who feel strong early in menstruation, the readiness score will reflect that and normal programming applies — the system adapts to you, not a textbook average.
Cycle tracking is optional. Athletes who prefer not to use it train on the readiness-only model, which still outperforms a static calendar split.
Injury-Aware Programming
When you flag an injury or limitation in the app, the programming layer does not simply remove affected exercises and leave gaps in your training week. Instead, it applies a three-step substitution model:
- 1.Identify the movement pattern. The app maps each exercise to a primary movement pattern (horizontal push, hip hinge, vertical pull, etc.) and the primary muscles loaded. Flagging an injury tags both the pattern and the joint or muscle group involved.
- 2.Find the highest-fidelity substitute. The substitution library prioritizes exercises that preserve the same movement pattern with reduced load on the injured structure — for example, replacing barbell back squat with split squat variations for a hip flexor issue, rather than just removing all quad work entirely.
- 3.Maintain systemic training stimulus. Volume and intensity for unaffected muscle groups are kept intact or slightly increased to compensate for reduced mechanical loading elsewhere. The goal is continued progress, not a maintenance holding pattern while you heal.
Before — Static Program
MonBarbell Back Squat5×5 @ 80%
WedDeadlift4×3 @ 85%
FriBarbell Back Squat5×5 @ 82%
Hip flexor flagged — but program runs unchanged
After — Sundee Fundee Adapts
MonSplit Squat (DB)4×8 @ RPE 7
WedDeadlift4×3 @ 85%
FriLeg Press + Step-ups3×10 / 3×8
Hip flexor flagged — squat pattern preserved, load redistributed
Why recovery beats the calendar
Most training programs are built around the week as the fundamental unit of programming. Monday is chest day because Monday is chest day. The problem is that your body does not recover on a seven-day schedule. Two hard sessions back to back can shift your recovery curve by 24–48 hours, and stacking another heavy day on top is how acute fatigue turns into overreaching.
Readiness-gated programming treats your recovery state as the primary input and the calendar as secondary. A session will not be loaded at high intensity on a low-readiness day regardless of what day of the week it is. Over a training block, this produces more high-quality sessions, less junk volume, and fewer injuries — not because you trained more, but because you trained when your body was prepared to absorb it.
For a deeper look at the research behind this approach, see the full article on recovery-gated programming.
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