The runner who goes from 20 miles per week to 40 miles per week in a month is more likely to get injured than the runner who maintains 50 miles per week for years. This feels counterintuitive — surely more running means more injury risk? — but the research points to a different culprit: the spike.[1]
Training load spikes occur when your recent workload is significantly higher than what your body is accustomed to. Your cardiovascular system adapts in days. Your muscles adapt in weeks. Your tendons, ligaments, and bones adapt in months. When training increases faster than the slowest-adapting structures can handle, something breaks.
The Acute:Chronic Workload Ratio
The best predictor of injury risk isn’t absolute training volume — it’s the ratio of your recent training to your longer-term baseline. Pacewright uses the Acute:Chronic Workload Ratio (ACWR) to quantify this.
Acute workload: Your training load over the past 7 days. Chronic workload: Your average training load over the past 28 days. ACWR = Acute ÷ Chronic
| ACWR | Zone | What It Means |
|---|---|---|
| Below 0.7 | Red (undertraining) | Training too low; detraining risk |
| 0.8 - 1.3 | Green (sweet spot) | Progressive overload with manageable risk |
| 1.3 - 1.5 | Yellow (caution) | Elevated injury risk; proceed carefully |
| Above 1.5 | Red (danger) | Significantly elevated injury risk |
The sweet spot — 0.8 to 1.3 — means your recent training is between 80% and 130% of your baseline. You’re doing enough to improve but not so much more than usual that your body can’t keep up.
How Spikes Happen
Comeback spikes. You took two weeks off (life, illness, travel). You come back and immediately resume your previous volume. Your chronic workload has dropped from the time off, but your acute workload jumps to pre-break levels. ACWR spikes above 1.5. Injury risk soars.
Motivation spikes. You feel great. You sign up for a race 6 weeks out. You double your training overnight. Your body wasn’t consulted.
Catch-up spikes. You missed 3 runs this week. You try to make them up on the weekend. Saturday and Sunday get triple their normal load.
Single-run spikes. Your longest run in a month was 6 miles. This week’s long run is 12. Even if your weekly volume is reasonable, the single run represents a massive spike in acute per-session load.
How Pacewright Prevents Spikes
Four interlocking systems:
ACWR monitoring. Every workout is checked against your acute:chronic ratio before it’s prescribed. If adding the workout would push your ACWR above 1.3, it gets modified. Above 1.5, it gets replaced with an easy day.
Volume caps. Weekly mileage increases are capped based on your current level — 20% for runners under 15 miles per week, down to 5% for runners over 50 miles per week. This prevents the “double your volume” spike.
Spike guardrail. No individual run can exceed 110% of your longest run in the past 30 days. This prevents the “one abnormally long run” spike.
Return-to-training ramps. After time off, the algorithm doesn’t resume at your previous level. It starts at 50-60% and takes 4 weeks to ramp back to full training.
The Paradox
Here’s what makes training load spikes tricky: the runners who do the most training have the lowest injury rates — as long as they got there gradually.[1] A 50-mile-per-week runner who built to that volume over months is better protected than a 20-mile-per-week runner, because their chronic workload is higher. Their tissues are adapted to handle the load.
The danger isn’t high training volume. It’s high training volume that you haven’t prepared for. The path matters as much as the destination.