Too Much Too Soon: Training Load and Injury

If you look at the circumstances around most running injuries, a common pattern emerges: the person recently increased their mileage, started a new program, returned from a break, or changed their training surface. Training load errors are the leading cause of overuse injury, and understanding how to manage them is the most practical thing a runner can do.

The 10% rule and its limitations

The 10% rule - don't increase weekly mileage by more than 10% - is a useful starting point but oversimplifies a complex relationship. It doesn't account for intensity, which matters as much as volume. Adding a hard tempo run to a week where total mileage stays the same is still a significant load increase.

Acute:chronic workload ratio

A more useful framework is the acute:chronic workload ratio (ACWR), developed in team sport research but applicable to running. The concept is simple: your current week's load (acute) compared to your average load over the previous four weeks (chronic). A ratio between 0.8 and 1.3 represents a 'sweet spot' for adaptation without elevated injury risk. Ratios above 1.5 are associated with sharply higher injury incidence.

Tracking your own ACWR: a worked example

This doesn't require an app - a notebook or spreadsheet works fine. Record your total weekly distance (or time, for mixed training), then calculate your 4-week rolling average as you go.

• Week 1: 20km, Week 2: 22km, Week 3: 24km, Week 4: 26km → chronic average = 23km
• Week 5 acute load 26km ÷ chronic 23km = ACWR 1.13 - inside the 0.8-1.3 sweet spot
• Week 5 acute load jumps to 34km ÷ chronic 23km = ACWR 1.48 - approaching the danger zone above 1.5

We see this exact pattern often with clients training on the Bondi to Bronte coastal path: a runner comfortable at 20-25km a week decides to add the full clifftop loop plus the stairs at Tamarama on top of their existing sessions, jumping straight to 35km in a single week because the weather's good and the path is right there. The stairs and undulating terrain also add a load spike beyond what the distance alone suggests, since eccentric loading on the calves and Achilles is higher on descents than flat running. That combination - a distance jump plus a terrain change in the same week - is a classic setup for Achilles or calf strain.

Red weeks and green weeks

A practical way to implement this: after every two to three weeks of progressive loading, schedule a recovery week where total volume drops by 20-30%. This resets the nervous system, consolidates adaptation, and prevents the cumulative load from becoming an injury-level stimulus.

Rest between sessions

Weekly totals are only half the picture; how you space the hard sessions matters too. After a hard session performance temporarily drops, and with adequate rest the body recovers and adapts to a slightly higher level. Train hard again too soon and you're training in a fatigued state. As a rough guide to recovery time by session type:

• Easy aerobic session (60-70% max heart rate): 12-24 hours
• Threshold or tempo run: 36-48 hours
• VO2max intervals: 48-72 hours
• Long run (>90 min): 48-72 hours
• Heavy strength session: 48-72 hours depending on volume

Easy sessions on adjacent days are fine as long as they stay genuinely easy, which most people fail to do. Recovery rate also varies with age, sleep, nutrition, and life stress. Persistently elevated resting heart rate, low motivation, disturbed sleep, and performance that isn't improving are the most reliable signs you need more rest.

Other load variables that matter

• New shoes: can change load distribution enough to cause injury in the first 2-3 weeks
• Trail to road or vice versa: surface change alters impact mechanics
• Returning from illness: fitness recovers faster than tissue capacity after a break
• Race tapers followed by return to training: tissues are sensitised after a race

Does footwear prevent injury?

Running shoe marketing has long promised injury prevention through motion control, cushioning, stability, and now carbon plates. No specific shoe design has been consistently shown to prevent running injuries in large, well-designed trials. Runners assigned shoes by foot type (motion control for overpronators, neutral for neutral feet) did not have fewer injuries than those given whatever felt comfortable, and comfort is the strongest predictor of whether a shoe suits you. Where footwear does cause injury, it is usually through change: runners who switched to minimalist shoes too quickly in the 2010s had high rates of bone stress injury in the foot. Treat a change of shoe type as a change in load and make it gradually.

• Comfort: the shoe should feel good from the first wear without a break-in period
• Fit: adequate toe box width so the toes aren't compressed
• Drop: most people do well in 8-12mm heel drop; only change this gradually
• Mileage: replace shoes every 600-800km regardless of how they look

FAQs

Is ACWR still useful if I don't run every week?

Yes, but treat your chronic average cautiously if your training has been inconsistent - a low 4-week average from an interrupted month will make even a modest return-to-running week look like a small ACWR jump when your tissue capacity has actually dropped further than the number suggests.

Does cross-training count towards training load?

For injury risk specific to running, what matters most is running-specific load - the impact and eccentric demand cycling or swimming don't replicate. Track running volume separately rather than lumping all training into one number.