Injury Risk in Runners

8 January 2026

Why Single-Session Spikes Matter More Than Weekly Load Ratios.

Julia Caudle

Physiotherapist

Julia Caudle

Physiotherapist

A major 2025 cohort study published in the British Journal of Sports Medicine followed over 5,200 adult runners for 18 months using wearable device data to understand what types of training loads predict overuse injury. The study compared two main concepts:

1. Acute: Chronic Workload Ratio (ACWR)

A common metric comparing a runner’s workload over the past week (acute load) to their average over the prior four weeks (chronic load).

ACWR = Acute Load / Chronic Load

A high ratio (eg. >1.5) suggests a spike in recent workload

A low ratio (eg. <0.8) might suggest undertraining

2. Single-Session Spike

When a runner completes a run that significantly exceeds the longest run in the past 30 days.

Key Findings - Single-Session Spikes Are Strong Predictors Of Injury

A single session run that exceeds your recent longest run by:
- 30–100% increases risk by 52%
- Over 100% (doubling your run distance) more than doubles your risk
These results show a clear, dose-response relationship between session spike magnitude and injury risk.

Environmental Factors Also Matter

While training load is a key driver of injury risk, external factors like running surface, terrain, and environmental conditions also play a significant role. Transitioning abruptly from soft to hard surfaces (eg. trail to pavement), running on uneven or cambered paths, or tackling steep elevation without adequate preparation can all increase mechanical load on tissues.

Similarly, hot, humid, or wet conditions can affect running mechanics, fatigue, and hydration, all indirectly influencing injury risk. These variables can amplify the effects of a session spike, meaning that even a moderate increase in distance could be riskier if combined with more demanding environmental conditions.

Conclusion

While the Acute:Chronic Workload Ratio (ACWR) has been commonly applied in team sports and some endurance settings, this study indicates it is less effective in predicting overuse injuries among individual runners when used alone.

Instead, session-specific spikes in training, where a single run significantly exceeds recent training loads, emerge as a more accurate and actionable predictor of injury risk.

Although ACWR may still offer general insights into training balance, it is the sudden, sharp increases in individual session distance that present the most immediate threat to a runner’s health.

Practical Advice For Runners

Prioritise session-based monitoring: Pay close attention to how today’s run compares to your longest run in the past month.
Avoid spikes >30% above your recent peak running distance in one session.
Use ACWR as a supplementary tool, but don’t rely on it to flag injury risk on its own.
Training decisions should be made based on session context, not just weekly averages.
Prioritise recovery: Adequate sleep, nutrition, and rest days allow tissues to adapt to training stress and reduce the likelihood of overload injuries. Schedule recovery as deliberately as you schedule your runs.
Listen to your body’s psychological and physical cues: If you feel unusually fatigued, unmotivated, or develop early signs of illness (eg a cold, cold sore, or general run-down feeling), take a rest day or reduce intensity as pushing through can elevate injury and illness risk.

Reference

https://bjsm.bmj.com/content/early/2025/07/07/bjsports-2024-109380

Schuster Brandt Frandsen J, Hulme A, Parner ET, et al. How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study. British Journal of Sports Medicine Published Online First: 07 July 2025. doi: 10.1136/bjsports-2024-109380

SUBSCRIBE TO RECEIVE UPDATES AND HELPFUL TIPS FROM OUR PHYSIOTHERAPISTS

Subscribe and recieve 10% off for your first visit!