Why Does One Side Always Hurt When Running?

It's always the same side. Left knee, right hip, left Achilles — whatever it is, it's consistently one side and never the other. You've wondered if you're built lopsided, if one leg is just shorter, if something about your gait is fundamentally off-balance. It's a reasonable thing to wonder, and the answer is more interesting than "you're asymmetrical," because — and this might be surprising — some degree of asymmetry is completely normal in every runner's stride.

That's not a typo. Research directly measuring running mechanics has found that essentially every runner shows some side-to-side difference in force, timing, and stride characteristics.[^1] The real question isn't whether you have asymmetry. It's whether one side's tissue can actually handle the specific load it's being asked to handle, relative to the other.

One-sided running pain typically reflects a difference in loading tolerance between your two legs, not simply the existence of asymmetry itself — which research shows is present in nearly all runners and usually isn't harmful on its own. What matters more is whether one side has a genuine strength deficit, a mobility restriction, or is compensating for the other side in a way that concentrates stress repeatedly in the same structures. Identifying which of these is happening determines what treatment actually needs to address.

Here's what the research on this actually says, and why the answer is more nuanced than "you're just uneven."

Why Does One Side Always Hurt When Running?:

Some Asymmetry Is Just Normal Running Mechanics

This is worth sitting with for a second, because it changes how you should think about your own pain pattern. Studies measuring things like ground reaction force, contact time, and leg stiffness between the dominant and non-dominant leg during running consistently find differences — sometimes fairly substantial ones, in the range of 4-13% depending on which specific mechanical variable you're measuring.[^2]

Even more notably, one large study following over 800 recreational runners specifically tested whether gait asymmetry predicted future injury. It didn't. Asymmetry in stride timing and force measures showed no meaningful association with increased injury risk, and in a couple of measures, more asymmetry was actually associated with slightly lower injury risk.[^3]

So if you've been assuming that some fundamental structural imbalance is the reason one side always hurts, that's probably not quite the right framework. Nearly every runner is somewhat asymmetrical. Most of them don't have chronic one-sided pain.

So What's Actually Different About the Side That Hurts?

If asymmetry itself isn't the culprit, the more useful question becomes: what's different about how that specific side is handling load?

Research on limb dominance offers a genuinely interesting clue here. Several studies have found that the non-dominant leg tends to show lower muscle strength and different movement coordination patterns compared to the dominant leg, and researchers have suggested this creates a scenario where the weaker limb ends up compensating with altered movement strategies to make up for what it lacks in raw strength.[^4] Compensation strategies, run over thousands of strides, are exactly the kind of thing that concentrates stress somewhere it shouldn't be concentrated.

There's also a simpler, more common scenario: a previous injury, even a minor one you didn't think much of, can leave one side with a lingering strength deficit or protective movement pattern long after the original issue "healed." The other leg then does slightly more work, indefinitely, without you ever being aware of the shift.

Leg length differences and asymmetric fat distribution do show up as contributing factors in some research, though their actual statistical relationship to injury risk tends to be quite small.[^3] They're worth ruling in or out, but they're rarely the whole story.

Why This Actually Matters for How You Get Treated

This distinction changes what treatment should focus on. If the working assumption is simply "you're asymmetrical, let's make your stride more symmetrical," that's chasing something research suggests usually isn't the actual problem, and isn't even something you can meaningfully or permanently change through effort alone.

A more useful approach asks a different question: does the side that hurts have a genuine, identifiable strength deficit? Is it compensating for a mobility restriction, either on that side or the other? Is there a lingering pattern from a previous injury that never fully resolved? Answering these questions points toward a treatment plan that actually addresses what's driving the one-sided stress, rather than chasing symmetry for its own sake.

What Assessment Actually Looks At

We start by comparing strength side to side — hip abductors, glutes, calf strength, and other key running muscles — since strength deficits are one of the more common and directly treatable contributors to one-sided pain patterns.

We look at mobility on both sides too, since a restriction on the "good" side can sometimes explain why the other side is compensating and taking on extra stress. We talk through injury history in detail, because a previous issue on either leg — even one you've mostly forgotten about — is one of the most common explanations for a lasting one-sided pattern. And we watch you actually run, because visible compensation patterns during a real running stride often make the picture click into place faster than isolated strength testing alone.

What Treatment Actually Involves

Once assessment identifies the actual driver — usually a specific strength deficit, a mobility restriction, or a compensation pattern tied to previous injury — treatment targets that directly.

This typically means progressive strengthening focused specifically on the deficient side rather than generic bilateral exercises that don't address the actual imbalance. Mobility work addresses any restriction that's forcing compensation. And movement retraining helps break old compensation patterns that may have outlasted whatever originally caused them, which matters because these patterns can persist out of habit long after the underlying issue that created them has resolved.

Return to running volume typically progresses gradually as the deficit closes, rather than jumping straight back into pre-injury training loads.

When It's Worth Getting This Looked At

Worth scheduling an evaluation if: pain consistently shows up on the same side, you've had a previous injury on one leg and wonder if it's still affecting you, you want to understand whether there's an actual strength or mobility deficit driving this rather than guessing, or general stretching and rest haven't resolved a persistent one-sided pattern.

Common Questions About One-Sided Running Pain

Does this mean I have a leg length discrepancy?

Possibly, but it's actually a less common explanation than most people assume, and research shows its relationship to injury risk is quite small even when it is present. Assessment can determine whether it's a meaningful factor in your specific case.

Should I try to run more symmetrically on purpose?

Not necessarily — research suggests that natural asymmetry generally isn't the problem to fix. Addressing an underlying strength or mobility deficit tends to be more productive than consciously trying to alter your stride's symmetry.

Could an old injury from years ago really still be affecting me?

Yes, this happens more often than people expect. Strength deficits and compensation patterns can persist quietly for years after the original injury is otherwise fully healed.

Is my dominant or non-dominant leg more likely to be the painful one?

Research has found that the non-dominant leg sometimes shows lower strength and different movement coordination, which can make it more prone to compensation-related stress, though this varies by individual.

Will strengthening the painful side fix this on its own?

Often it helps significantly, but if the real driver is a mobility restriction elsewhere or a lingering compensation pattern, strengthening alone may not fully resolve things. Assessment identifies what specifically needs to be addressed.

How do I know if this is a strength problem versus a mobility problem?

Specific testing differentiates these — strength testing directly measures muscle capacity, while mobility assessment reveals whether restricted range of motion is forcing compensation elsewhere.

Is it possible both legs have deficits, just different ones?

Yes, and this is actually fairly common. Comprehensive assessment looks at both sides rather than assuming only the painful side needs attention.

Do I need to stop running while this gets addressed?

Usually not entirely. Modified volume or intensity is common early in treatment, but most runners continue running in some capacity throughout.

Why Does One Side Always Hurt When Running?: Bottom Line

Nearly every runner has some degree of side-to-side asymmetry, and research indicates that asymmetry itself usually isn't what causes one-sided pain. What matters more is whether one side has a real strength deficit, a mobility restriction, or is compensating for a lingering pattern from a previous injury. Identifying which of those is actually happening — rather than assuming you're simply "uneven" — is what makes treatment effective.

Tired of it always being the same side?

Schedule a Physical Therapy Evaluation at Nashville Physical Therapy & Performance. You'll get completely 1:1 care with your therapist for the entire visit — no aides, no split attention. We'll compare both sides directly, identify what's actually driving the one-sided pattern, and build treatment around the real cause. Available at South Nashville, East Nashville, and The Nations/West Nashville. Call 615-428-9213 or book online at nashvillept.com.

References

[^1]: Injury risk analysis of movement restriction and body asymmetry in sports injury prediction. PMC. 2025.

[^2]: You Y, et al. Biomechanical Effects of Dominant or Nondominant Limb on Asymmetry during Running Stance Phase. Applied Bionics and Biomechanics. 2024;3162919.

[^3]: 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. PMC. 2024.

[^4]: Girard O, et al. The effects of limb dominance and fatigue on running biomechanics. Gait & Posture / Sports Biomechanics. 2013.