Why Do My Knees Hurt When I Run? Causes and Form Fixes

Why do your knees hurt when you run?

Most running-related knee pain traces back to one of two overuse patterns: pain around or under the kneecap (patellofemoral pain, often called "runner's knee") or pain on the outside of the knee where the iliotibial (IT) band crosses the joint. Both are usually driven by a combination of training load, hip and quad strength, and mechanics like overstriding or a low step rate that increase the force each stride puts through the knee — not by one single "wrong" way of running. Patellofemoral pain is among the most commonly reported overuse injuries in runners, so aching knees after a run put you in very familiar company.

Some estimates suggest anywhere from roughly 20% to as high as 50-80% of runners experience an injury in a given year, depending on how "injury" is defined and how long runners are tracked. The knee is consistently one of the most frequently affected areas in these studies, alongside the lower leg and foot.

What is runner's knee (patellofemoral pain)?

Patellofemoral pain syndrome is a dull, aching pain around or behind the kneecap. It's often worse going downhill, on stairs, or after sitting with a bent knee for a while (sometimes called "moviegoer's knee"). It develops when the kneecap doesn't track smoothly in the groove on the thigh bone, usually from some mix of hip and quad weakness, tight surrounding tissue, and mileage or intensity that increased faster than the tissue could adapt to. The American Academy of Orthopaedic Surgeons lists it as one of the most common overuse injuries seen in runners at every level. For a closer look at the underlying mechanisms, see what causes runner's knee.

What is IT band syndrome, and how is it different?

IT band syndrome (ITBS) causes a sharper, more localized ache on the outer side of the knee. The iliotibial band is a thick strip of connective tissue running from the hip down past the outside of the knee, and repeated knee flexion during running can irritate the tissue and structures beneath it where it crosses the joint. Unlike patellofemoral pain, ITBS pain rarely radiates behind the kneecap, and it often shows up at a predictable point in a run — many runners can name the exact mile it starts. It's associated with hip abductor weakness, sudden increases in mileage, and mechanics that let the knee drift inward on landing.

Runner's Knee (PFP) IT Band Syndrome
Pain location Front/around kneecap Outer side of knee
Typical trigger Stairs, downhill, prolonged sitting Repetitive knee flexion, often a fixed distance into a run
Common contributors Hip/quad weakness, rapid mileage increase Hip abductor weakness, sudden training load jump
Onset pattern Gradual, worsens with activity Often sudden and location-specific

How does overstriding or low cadence load the knee?

Cadence is your step rate — steps per minute (spm). Overstriding means landing with your foot well ahead of your hips, typically with a straighter knee at contact. That combination forces more of the landing impact to be absorbed through knee extension rather than shared across the hip and ankle, which raises peak stress on the patellofemoral joint with every stride. A controlled study that had runners increase their step rate by about 5-10% without changing pace found this reduced peak patellofemoral joint loading and hip work compared to their normal cadence. Most recreational runners land somewhere in the 150-170 spm range; shifting toward the higher end of that range, or a touch beyond it, tends to shorten stride length and reduce overstriding without requiring a conscious change in foot strike.

This doesn't mean a single cadence number is "correct" for everyone — cadence naturally scales with height, leg length, and pace, and the evidence linking cadence changes directly to lower injury rates over time is still limited. What the research supports more confidently is the mechanical link: a modest cadence increase measurably lowers knee joint loading in the short term.

What form and cadence adjustments can reduce knee strain?

A few concrete, well-supported adjustments:

Form changes also take longer to become automatic than most runners expect — new movement patterns typically need several weeks of repetition before they hold up under fatigue; see how fast you can actually change running form for a realistic timeline. A phone-video tool like StrideIQ can give you a quick read on your current cadence and general stride pattern from the side, which is useful for tracking whether a cadence adjustment is actually sticking — but it's a screening check, not a substitute for an in-person assessment if pain persists.

When should you see a professional about knee pain?

See a physical therapist or sports medicine doctor if knee pain: lasts more than a few days despite rest, gets worse rather than better, causes swelling or a feeling of instability, is sharp or sudden rather than a dull ache, or shows up during normal walking or stairs — not just running. This article can help you understand common patterns, but it isn't a diagnosis, and it can't replace a hands-on exam, strength testing, or imaging when needed. A clinician can also help build an individualized return-to-run plan, which is not something general guidance online should attempt to prescribe.

What can form analysis and video review NOT tell you?

A single phone video, reviewed side-on, is genuinely useful for one thing above all: estimating cadence, which is measurable frame-by-frame with reasonable confidence. It's much less reliable for ground contact time, vertical oscillation, or true joint angles at the knee and hip — those require high-speed or multi-camera capture to estimate with any precision, and a phone-video estimate should be treated as a rough signal, not a lab-grade number.

More importantly, no video analysis — phone-based or otherwise — can tell you why a specific tissue is irritated, how much load it can currently tolerate, or whether pain is coming from cartilage, tendon, or something else entirely. It can't rule out other causes of knee pain unrelated to running mechanics. If pain is persistent, worsening, or interfering with daily life, an in-person evaluation from a physical therapist, sports medicine physician, or a formal 3D gait lab will tell you far more than any single-angle video ever can.

Frequently Asked Questions

Why do my knees hurt only when I run downhill?

Downhill running increases the force absorbed at landing and often lengthens stride, both of which raise stress on the patellofemoral joint. Pain that shows up specifically downhill or on stairs is a classic pattern for patellofemoral pain (runner's knee) rather than IT band syndrome.

Will increasing my cadence fix knee pain?

A moderate cadence increase (about 5-10%) has been shown to reduce peak knee joint loading in controlled research, so it can help reduce strain going forward. It isn't a guaranteed fix and won't resolve pain caused by tissue that's already irritated — for existing pain, see a physical therapist or sports medicine doctor.

Is it safe to keep running through knee pain?

Mild, dull soreness that fades quickly and doesn't worsen during a run is usually lower-risk, but sharp pain, swelling, or pain that worsens as you run are signs to stop and get it checked. Continuing to run through worsening pain risks turning a minor irritation into a longer-term problem.

Can a phone video diagnose why my knees hurt?

No. A phone video can give a reasonable estimate of your cadence and a general sense of stride pattern, but it can't measure internal joint forces or diagnose the tissue causing your pain. Persistent knee pain warrants an in-person evaluation from a physical therapist or sports medicine doctor.

What's the difference between runner's knee and IT band syndrome pain?

Runner's knee causes a dull ache around or behind the kneecap that worsens with stairs, sitting, or downhill running. IT band syndrome causes a sharper pain on the outer side of the knee that often appears at a consistent point during a run.

Sources