What HRV actually measures
Heart rate variability (HRV) is the variation in time between consecutive heartbeats. A heart beating at 60 bpm doesn't beat exactly once per second — the intervals between beats fluctuate slightly, and the magnitude of that fluctuation is HRV.
Higher variability generally signals that the autonomic nervous system is well-regulated, parasympathetic (rest-and-digest) activity is dominant, and the body is recovered and ready to handle physiological stress. Lower variability tends to indicate sympathetic (fight-or-flight) dominance — a state associated with accumulated fatigue, illness, psychological stress, or inadequate sleep.
The specific metric most commonly used is RMSSD — the root mean square of successive differences between beat intervals. It's the standard in athletic recovery monitoring because it's sensitive to short-term autonomic changes and relatively easy to measure accurately with modern wearables.
HRV is highly individual. A reading that signals excellent recovery for one person may indicate stress for another. Trends over time matter far more than absolute numbers — which is why the recovery score is normalized to your personal baseline, not a population average.
Why HRV alone isn't enough
HRV is the most sensitive autonomic marker available, but it's also noisy. A single poor reading can result from something as mundane as sleeping in an unfamiliar environment, consuming alcohol the night before, or measuring at an inconsistent time. Using HRV alone as a readiness gate produces too many false signals in both directions.
Mass in Motion uses HRV as one of five inputs — each capturing a different dimension of recovery state — and weights them into a composite score that's more stable and more actionable than any single metric alone.
The five inputs to the recovery score
RMSSD from overnight or morning measurement. Compared against your personal rolling baseline — not a fixed threshold. A reading 10% below your recent average carries more weight than a reading 10% below a population norm.
Morning resting HR elevated above your baseline is a reliable secondary signal of accumulated fatigue, illness onset, or inadequate recovery. Moves more slowly than HRV but with less noise — useful for confirming multi-day trends.
Total sleep time and a quality signal from your wearable, where available. Sleep is the primary recovery mechanism — the algorithm weights recent nights more heavily than older nights using an exponential decay model.
Acute-to-chronic workload ratio for both run and lift streams. A load spike carries forward into the recovery score — because physiological fatigue from yesterday's session is real today, regardless of what the biometric sensors show.
Caffeine and nicotine have measurable half-lives that affect sleep architecture when consumed too close to bedtime. Mass in Motion models the pharmacokinetic decay of both — if your last dose is still partially active at your target sleep time, the recovery score reflects the likely sleep quality impact. The app flags the cutoff time before it becomes a problem, not after.
How the score is used
The 0–100 score maps to a coaching recommendation, not just a color. Instead of "yellow means moderate," the coach note names the specific limiting factor — "HRV is trending down and sleep was short" — and tells you what it means for today's session specifically.
A low score doesn't automatically cancel a session. For most planned sessions, the algorithm adjusts prescribed intensity rather than removing the session entirely — because consistent training at managed effort produces better long-term adaptation than frequent skipped days.
Wearable integrations
Mass in Motion pulls biometric data from:
- Apple Health (iOS) — HRV, resting heart rate, sleep analysis, step count
- WHOOP — cloud sync via OAuth2, raw biometrics including HRV and sleep staging
Manual entry is always an option. Athletes without a wearable can log sleep duration and a morning feeling rating — the algorithm uses whatever data is available and increases the score's confidence weighting as more inputs become populated over time.
Recovery scores become meaningfully reliable after approximately 14 days of data. During the baseline-building period, the app shows progress toward that threshold — rather than presenting a number that carries false confidence before the baseline is established.
The stimulant model in detail
Caffeine has an average half-life of approximately five to six hours in most adults — meaning a 200 mg dose consumed at 3 PM still has roughly 50 mg active at 9 PM. Nicotine has a shorter half-life but similar sleep-disrupting properties when consumed in the hours before sleep.
Mass in Motion lets you log intake time and estimated dose. The algorithm calculates projected plasma concentration at your target bedtime and warns you if a significant amount will still be circulating when you plan to sleep. The warning appears before the next dose, not the morning after a poor night — so you can make an informed decision rather than discovering the impact the next day.
Further reading: How training load ratio is calculated · Hybrid athlete training overview · Race-based running pace zones