The Asymmetrical Runner: Debunking the Myth of Perfect Symmetry

In the world of running, symmetry is often hailed as the gold standard – an indicator of biomechanical efficiency, a marker of elite performance, and a safeguard against injury. But emerging evidence challenges this long-standing assumption. Research increasingly reveals that asymmetry is not only natural but may also serve adaptive purposes in the face of fatigue, performance demands, and injury recovery.

In this article, we explore what current science says about running asymmetry – where it matters, where it doesn’t, and how we, as clinicians, can move beyond a one-size-fits-all mindset.

Does Symmetry Improve Performance? It Depends.

A core belief in performance training is that symmetry enhances running economy. Some evidence supports this:

• A 3.5–7.8% increase in metabolic power has been linked to increased asymmetry in step time and GCT (Beck 2018, Stiffler-Joachim 2021).

• NCAA Division I runners with greater GCT imbalance showed higher energy costs – a 1% increase in GCT imbalance led to a ~3.7% rise in metabolic expenditure (Joubert 2020).

Yet, the data isn’t always consistent:

• The same study that linked lower propulsion asymmetry to faster PRs also found that greater asymmetry in ankle dorsiflexion and vertical loading rate was associated with faster runners (Stiffler-Joachim 2021).

• A systematic review revealed no consistent association between asymmetry and running performance, with results varying widely depending on the metric used and athlete population (D’Hondt 2024).

These findings suggest that some asymmetries may be functionally irrelevant or even performance-enhancing, depending on the context.

Asymmetry and Injury Risk: Is There a Causal Link?

The intuitive idea that asymmetry causes injury has led many to treat it as a red flag. But prospective evidence tells a different story.

• Retrospective studies often find links between existing injuries and current asymmetries (Zifchock 2006, Gilgen-Ammann 2017, Zifchock 2008), but these designs cannot determine causality.

• In runners with Achilles tendinopathy, increased GCT on the uninjured side was associated with more pain in the injured leg, likely reflecting a protective compensation strategy rather than a causative factor (Seymore 2024).

Crucially, large prospective studies show that:

• Gait asymmetries in healthy runners are not predictive of future injury risk (Malisoux 2024).

• In some cases, greater asymmetry was associated with lower injury risk, such as increased flight time and peak braking force (Malisoux 2024).

In bone stress injury (BSI) research, subtle asymmetry shifts preceded injury in collegiate runners, including a reversal in ground reaction force patterns between the limbs, hinting at possible early markers rather than causal agents (Archer 2025).

Conclusion? Asymmetry may be a response to pathology, a predictor of injury in specific cases, or a benign individual characteristic. But it's unlikely to be a universal cause of injury.

How Much Asymmetry is “Normal”?

Healthy runners often show some degree of asymmetry, but what counts as “normal” is less clear than commonly thought.

• Kinematic variables (e.g., step frequency, ground contact time) typically show low asymmetry (1–9%), while frontal plane motions (e.g., knee abduction) can vary up to 41% (Vincent 2025).

• Kinetic measures (e.g., joint moments, impact forces) often range between 5% and 38%, with some values exceeding 20% in uninjured runners (Vincent 2025).

Generic thresholds (e.g., 10–15%) have limited clinical utility and don’t account for task specificity, metric variability, or individual biomechanics (Parkinson 2021, Carton-Llorente 2024, Afonso 2022, Malisoux 2024).

Age does not significantly influence asymmetry across most biomechanical variables, suggesting a relatively stable range throughout adulthood (Vincent 2025).

The key takeaway: Individualised assessment is essential. Context matters more than comparison to population norms.

Fatigue and Asymmetry: A Shifting Landscape

Fatigue alters running biomechanics – that much is clear. But whether it leads to increased asymmetry is less straightforward, with studies producing mixed results.

• Some studies report that fatigue increases limb asymmetries. In a recent study, a 30-minute moderate-intensity run led to a divergence in tibial acceleration: an increase on the dominant (right) side and a decrease on the non-dominant side, resulting in increased asymmetry. The authors suggest that fatigue may impair impact attenuation more on the dominant limb, requiring compensation from the contralateral side (Delgado-García 2025).

• Other studies find that certain biomechanical asymmetries remain low and stable, even during fatiguing efforts (Brown 2014, Hanley 2018, Mtibaa 2023). For example, trained runners showed consistent stride symmetry throughout a 30-minute treadmill run, suggesting that not all aspects of gait deteriorate with fatigue.

• A separate investigation observed moderate increases in ground contact time (GCT) asymmetry with fatigue, but these findings contrast with earlier studies showing no change in gait symmetry over time (Mason 2024).

The discrepancies across studies are likely due to differences in:

• Measurement techniques and biomechanical variables assessed

• Running intensity, duration, and surface

• Individual factors such as training status, limb dominance, and injury history

Bottom line: Fatigue-related asymmetry is highly variable, and individual response patterns should guide interpretation.

The Take-Home Message: Embrace Functional Asymmetry?

The pursuit of perfect symmetry in runners may be misguided. Instead, the focus should shift toward functional movement quality, adaptive capacity, and individual variability.

🔑 Key Takeaways for Clinicians:

1. Asymmetry can be normal: Minor differences between limbs are common and often harmless.

2. Not all asymmetries are problematic: Some may reflect natural adaptations or functional compensations that don’t impair performance or increase injury risk.

3. Use your clinical judgement: Evaluate whether an observed asymmetry is relevant for the runner in front of you, considering the broader clinical picture.

4. Remember: running injuries are multifactorial. Rarely is one variable solely to blame. Asymmetry alone is unlikely to be the cause, but that doesn’t mean it should be dismissed.

5. Context is key: Interpret asymmetry in light of the task, the metric being assessed, and the runner’s history, goals, and symptoms.

References

• Afonso, J., et al. (2022). "Why Sports Should Embrace Bilateral Asymmetry: A Narrative Review." Symmetry 14(10): 1993.

• Archer, H. B., et al. (2025). "Can biomechanical variables and asymmetry predict bone stress injuries in collegiate distance runners?" Sports Biomech: 1-16.

• Cartón-Llorente, A., et al. (2024). "Bilateral Asymmetry of Spatiotemporal Running Gait Para

• D'Hondt, J., et al. (2024). "Association Between Inter-Limb Asymmetry and Determinants of Middle- and Long-distance Running Performance in Healthy Populations: A Systematic Review." Sports Med Open 10(1): 127.

• Delgado-García, G., et al. (2025). "Does the Fatigue Induced by a 30-Minute Run Affect the Lower Limb Acceleration Spikes' Asymmetries?" Bioengineering (Basel) 12(3).

• Furlong, L. M. and N. L. Egginton (2018). "Kinetic Asymmetry during Running at Preferred and Nonpreferred Speeds." Med Sci Sports Exerc 50(6): 1241-1248.

• Girard, O. (2025). "Asymmetry in sprinting: The myth of perfection and the reality of performance." J Sport Health Sci 14: 101025.

• Joachim, M., et al. (2021). "Longitudinal Changes in Running Gait Asymmetries and Their Relationship to Personal Record Race Times in Collegiate Cross Country Runners." Symmetry 13: 1729.

• Joachim, M., et al. (2021). "Longitudinal Changes in Running Gait Asymmetries and Their Relationship to Personal Record Race Times in Collegiate Cross Country Runners." Symmetry 13: 1729.

• Joubert, D. P., et al. (2020). "Ground Contact Time Imbalances Strongly Related to Impaired Running Economy." International journal of exercise science 13(4): 427-437.

• Malisoux, L., et al. (2024). "Gait asymmetry in spatiotemporal and kinetic variables does not increase running-related injury risk in lower limbs: a secondary analysis of a randomised trial including 800+ recreational runners." BMJ Open Sport Exerc Med 10(1): e001787.

• Mason, R., et al. (2024). "Instrumenting Parkrun: Usefulness and Validity of Inertial Sensors." Sensors (Basel) 25(1).

• Mtibaa, K., et al. (2023). "Mechanical asymmetries remain low-to-moderate during 30 min of self-paced treadmill running." Front Physiol 14: 1289172.

• Seymore, K. D., et al. (2024). "Asymmetric running is associated with pain during outdoor running in individuals with Achilles tendinopathy in the return-to-sport phase." Phys Ther Sport 67: 25-30.

• Stiffler-Joachim, M. R., et al. (2021). "Lower Extremity Kinematic and Kinetic Asymmetries during Running." Med Sci Sports Exerc 53(5): 945-950.

• Vincent, H. K., et al. (2025). "Reference biomechanical parameters and natural asymmetry among runners across the age spectrum without a history of running-related injuries." Front Sports Act Living 7: 1560756.

• Wayner, R. A., et al. (2023). "Gait asymmetry and running-related injury in female collegiate cross-country runners." Phys Ther Sport 59: 1-6.

• Zifchock, R. A., et al. (2006). "Kinetic asymmetry in female runners with and without retrospective tibial stress fractures." J Biomech 39(15): 2792-2797.