Most training apps hand you a plan and ask you to trust it. You get a workout for tomorrow, but no explanation for why that workout, why that distance, why today instead of Thursday. The algorithm is a black box. You’re supposed to follow along and hope it knows what it’s doing.
Pacewright works differently. Every workout decision is based on published, peer-reviewed research — and we tell you exactly what that research says and how it applies to your specific situation. This article walks through the actual principles that drive your training plan, with the real numbers.
Three Foundations
Your plan is built on three principles that decades of sports science have consistently supported. None of them are proprietary. None of them are secret. They’re well-established findings that Pacewright applies to your individual training.
1. Polarized Intensity Distribution
In 2010, Stephen Seiler reviewed decades of research on how the best endurance athletes distribute their training intensity. The finding was consistent across runners, cyclists, cross-country skiers, and rowers: roughly 80% of training sessions should be at low intensity, with only about 20% at moderate-to-hard effort.
This is the polarized training model, and it’s counterintuitive. If you want to race faster, shouldn’t more of your runs be fast? The research says no. Athletes who spend most of their time running easy — genuinely easy, not “kind of easy” — consistently outperform athletes who spend more time in the moderate zone. The easy running builds your aerobic engine without accumulating excessive fatigue, and the hard sessions provide the specific stimulus your body needs to improve speed and lactate clearance.
A 2014 study by Stöggl and Sperlich compared polarized training directly against threshold training, high-intensity training, and high-volume training over 9 weeks. The polarized group saw the greatest improvements in VO2max and time to exhaustion.
How Pacewright applies this: Your plan targets roughly an 80/20 split between easy and hard effort. When you log a run, the app tracks where it fell on the intensity spectrum and adjusts future workouts to keep the distribution in the right range. If you’ve been running too many moderate efforts — the “gray zone” that feels productive but isn’t — you’ll see your next run shift to either genuinely easy or genuinely hard.
2. Training Load Management (ACWR)
Here’s a pattern that shows up over and over in injury research: runners don’t get hurt because they’re running too much in absolute terms. They get hurt because they’re running too much relative to what they’ve been doing recently.
Your acute:chronic workload ratio — ACWR — captures this. It compares your training load from the past 7 days (acute) against your average load over the past 28 days (chronic). Think of it as a speedometer for how fast your training is ramping up.
Tim Gabbett’s 2016 research in the British Journal of Sports Medicine established practical thresholds. When ACWR stays between 0.8 and 1.3, injury risk is relatively low — you’re training at or near your established capacity. When it climbs above 1.3, risk starts to increase. Above 1.5, and the research shows a sharp jump in injury rates.
A note on honesty: the evidence for ACWR’s predictive power specifically in recreational running populations is mixed. Some studies show strong correlations between load spikes and injury; others are less clear. Pacewright uses ACWR as a guardrail, not a crystal ball. It’s one signal among several, and it’s better at catching dangerous spikes than at predicting exactly when an injury will occur.
How Pacewright applies this: Every time your plan updates, Pacewright calculates your current ACWR. The thresholds are:
- Green (0.8–1.3): Proceed as planned. You’re training within your established capacity.
- Yellow (1.3–1.5): Caution. The app may reduce tomorrow’s workout or swap a hard session for an easy one.
- Red (above 1.5 or below 0.7): Intervene. Volume gets cut, intensity drops, and your plan explains exactly what triggered the change.
You can see your current ACWR in the app at any time. No hidden math.
3. Progressive Overload With Mileage-Dependent Caps
Your body adapts to stress — but only if you increase that stress gradually and give yourself time to recover. This is progressive overload, and it’s the oldest principle in exercise science. Banister’s foundational 1991 model showed that fitness and fatigue respond to training load on different timescales: fitness builds slowly and persists, while fatigue accumulates quickly and dissipates quickly. The art is in managing the balance.
The common advice is “don’t increase your weekly mileage by more than 10%.” It’s a reasonable starting point, but it’s also a blunt instrument. 10% of 10 miles is 1 mile — a trivially small increase. 10% of 60 miles is 6 miles — a substantial jump that can push a high-mileage runner into risky territory.
Pacewright uses mileage-dependent volume caps that scale with where you are:
| Your Weekly Mileage | Maximum Weekly Increase |
|---|---|
| Under 15 miles | 20% |
| 15–30 miles | 15% |
| 30–50 miles | 10% |
| Over 50 miles | 5% |
There’s a stability bonus too: if you’ve been running at the same volume for 3 or more weeks, you earn an additional 25% headroom on your cap. Consistency is rewarded because a stable training base handles increases better than a volatile one.
And there’s a single-session spike guard: no individual run will exceed 110% of your longest run in the past 30 days. Even if your weekly cap allows more volume, Pacewright won’t load it all into one run.
What Happens When Your Plan Updates
When Pacewright generates your next workout, it’s not pulling from a library of pre-built plans. It’s making a real-time decision based on your current state. Here’s what it actually considers:
Your training load history. What you’ve been running this week, this month, and how that compares to your established baseline. This drives the ACWR calculation and the volume cap check.
Your goal event. A marathon training plan looks fundamentally different from a 5K plan. The principle of specificity means your workouts need to target the energy systems and distances that match what you’re training for. A marathon plan emphasizes long aerobic runs and sustained tempo work. A 5K plan includes more interval work and shorter, faster sessions.
Your intensity distribution. Has your recent running been too heavily weighted toward moderate effort? Too many hard days in a row? The plan adjusts to keep the polarized distribution in the right range.
The weather forecast. Heat and humidity affect your body’s ability to perform. A dew point above 60°F measurably degrades running performance. Pacewright adjusts target paces based on current conditions so you don’t chase a pace that’s unrealistic for the weather.
Your schedule. Rest days, available time, the structure of your week — your plan fits your life, not the other way around.
What happened recently. If you missed a workout, Pacewright doesn’t just shove it into tomorrow. Some workouts get rescheduled, some get dropped, and some are never “made up” because doing so would spike your load. There are specific rules for this, and you can read them in our article on missed workout handling.
Every Decision Comes With a Reason
This is the part that matters most, and it’s the part no other training app does.
Every time Pacewright modifies your plan, you see three things:
- What changed. “Today’s run is now 4 miles instead of 5.”
- Why it changed. “Your ACWR is 1.4, which is in the caution zone.”
- What data drove the decision. “Based on your training load from the last 7 days compared to your 28-day average.”
This isn’t a decorative feature. It’s the core of how Pacewright works. If you disagree with a decision, you have enough information to understand what the app is seeing and why it’s responding the way it is. You’re not guessing. You’re not just trusting a brand. You’re looking at the same numbers the algorithm is looking at.
The training science isn’t ours. It belongs to the researchers who published it. What we built is a system that applies it to your specific situation and then tells you exactly what it did and why. That’s it. No secrets, no proprietary magic, no black box.
Your plan is built on real research, and we show our work.