What is acute-to-chronic workload ratio?
Acute-to-chronic workload ratio (ACWR) compares what you've done recently against what you've been doing consistently. Specifically:
- Acute load — the training load from the past seven days
- Chronic load — the rolling average load over the past 28 days, representing your fitness base
The ratio of acute to chronic is the number that matters. A ratio near 1.0 means you're training at roughly the level your body is adapted to. A ratio significantly above 1.0 means you've spiked load beyond your current fitness base — and injury risk rises accordingly.
How training load is calculated
The most practical method for calculating session load is session-RPE: rate your overall effort for the session on a 1–10 scale, then multiply by the session duration in minutes. A 60-minute run at a 6/10 effort produces a load of 360 arbitrary units.
This method — validated extensively in team sport and endurance research — captures both the intensity and the volume of a session in a single number. It works across modalities, which makes it particularly useful for hybrid athletes tracking both running and lifting.
Wearable heart rate data is useful but not always available or accurate during lifting. RPE-based load captures the athlete's lived experience of fatigue regardless of the activity type — making it the most consistent cross-modality load metric available.
Why simple rolling averages miss the point
A standard seven-day rolling average treats a workout from six days ago the same as a workout from yesterday. That's not how the body works — recent load matters more than older load in predicting fatigue and injury risk.
Mass in Motion uses exponentially weighted moving averages (EWMA) instead. EWMA applies a decay factor so that recent sessions contribute more to the load estimate than older ones. The result is a more responsive acute load number and a smoother, more stable chronic load baseline — which produces a more accurate ratio.
ACWR for hybrid athletes: why you need two streams
For athletes doing only one type of training, a single ACWR number is sufficient. Hybrid athletes need more granularity.
A heavy lower-body session and a threshold run both generate load — but through entirely different physiological systems. A posterior chain that's 72 hours into recovery from a max-effort squat session isn't prepared for a quality run effort, regardless of what the combined ACWR says.
Mass in Motion tracks three load streams simultaneously:
- Run load only — captures run-specific fatigue and aerobic adaptation stress
- Lift load only — captures muscular and CNS fatigue from resistance training
- Combined load — total athletic output, used for overall recovery scoring
A spike in either individual stream triggers a flag even if the combined ratio looks healthy — because the body doesn't recover in combined averages, it recovers system by system.
Athletes who track only combined load can miss dangerous spikes in a single discipline. Three consecutive heavy lower sessions in a week can push lift ACWR above 1.5 while combined ACWR stays below 1.3 — especially if run volume is low.
The deload trigger model
ACWR alone isn't sufficient to determine when a deload is needed — it's one of eight factors in the Mass in Motion deload trigger model. The full set includes:
- Run ACWR spike above threshold
- Combined ACWR spike above threshold
- Sustained HRV decline over a rolling window
- Consecutive sessions with elevated perceived effort relative to prescribed intensity
- Resting heart rate trend upward over multiple days
- Sleep quality degradation sustained over several nights
- Scheduled block length — a standard deload every four weeks as a baseline
- Consecutive failed sets or missed rep targets within a lifting block
When enough factors align, the algorithm surfaces a proactive deload modal — rather than silently reducing load without explanation. You see why the deload is being suggested and confirm or defer it.
What a deload actually is
A deload is not a week off. It's a deliberate reduction in training load — typically 40–60% of normal volume and intensity — designed to allow accumulated fatigue to dissipate while preserving the fitness built during the preceding block.
The recovery supercompensation that follows a well-timed deload is often where genuine performance gains surface. Athletes who skip deloads frequently plateau or regress — not because they need more volume, but because they never give their body the quiet it needs to consolidate adaptation.
Fitness is built during training. Adaptation happens during recovery. A deload is part of the training block, not a pause from it.
How Mass in Motion applies ACWR
The algorithm calibrates ACWR nightly using session data logged throughout the day — both from in-app workout logging and from wearable integrations where available. The ratio feeds into the daily recovery score and governs tomorrow's prescribed session intensity.
When ACWR is healthy, the algorithm schedules normally. When it spikes — in either individual stream or combined — the next session's intensity prescription adjusts, and the deload model is re-evaluated.
Athletes see their load status in the recovery card on the Today screen, with plain-language context explaining what the number means for their training day.
Further reading: Hybrid athlete training overview · How the daily recovery score is built · Race-based running pace zones