A run that starts comfortably but turns into an ache around or behind the kneecap can quickly change your routine. When people ask, “what causes runners knee?”, they are often looking for one culprit: new shoes, hills, weak muscles, or too many miles. In practice, runner’s knee is usually the result of several factors combining to overload a knee that is not currently tolerating the demands placed on it.
Runner’s knee is a common name for patellofemoral pain, which describes pain around the patella, or kneecap. It can affect regular runners, people returning to exercise, gym-goers, and anyone whose knee is repeatedly bent and loaded. The good news is that it is usually manageable with a clear assessment, sensible activity changes, and progressive rehabilitation.
What Causes Runner’s Knee?
The kneecap sits in a groove at the end of the thigh bone and moves as the knee bends and straightens. Running, stairs, squats, lunges, and sitting with bent knees all increase the force going through the patellofemoral joint. Pain develops when the combined load exceeds what the joint, surrounding muscles, and tissues can tolerate at that point in time.
This does not necessarily mean there is damage inside the knee. Many people are understandably concerned that a clicking sound, a feeling of grinding, or pain on stairs means something has worn away. Those symptoms can be uncomfortable, but patellofemoral pain is more often a problem of sensitivity and load tolerance than a simple structural fault.
The question is therefore less “what is wrong with my kneecap?” and more “why has this knee become overloaded now?” A physiotherapy assessment should consider recent training, daily activity, movement control, strength, recovery, footwear, and the possibility of another knee condition that needs different management.
The Most Common Triggers
A rapid change in running load
A sudden increase in distance, pace, frequency, or hill work is one of the most frequent triggers. This can happen after signing up for a race, returning from a break, changing commuting habits, or adding high-intensity classes alongside running. The knee may cope well with your usual mileage, then become painful after a modest-looking increase made too quickly.
Downhill running is particularly demanding because the quadriceps work hard to control the bend in the knee. Repeated stairs, steep routes, and long descents can produce similar symptoms. The issue is not that hills are inherently bad. They simply require a gradual build-up so your tissues can adapt.
Reduced hip, thigh, and calf capacity
The knee does not work in isolation. The hip muscles help control the position of the pelvis and thigh, while the quadriceps help manage the kneecap as you land, squat, and push off. The calf and foot also contribute to how force is absorbed and transferred through the leg.
If these areas are not strong enough for the current demands of running, the knee may be asked to do more than it can comfortably manage. This is common after time away from exercise, following an injury, or when training volume rises faster than strength training. It is not always about being “weak” in a general sense. A person can be very fit but lack the specific capacity needed for repeated running, hills, or faster sessions.
Changes in running mechanics
Some runners develop pain when fatigue, reduced strength, or a training change affects the way they move. For example, the knee may drift inward more noticeably during a single-leg squat or landing, or a runner may take longer strides that increase braking forces at the knee.
Movement patterns matter, but they should not be treated as a reason to obsess over running form. There is no single perfect running style. Attempts to dramatically alter technique without a clear reason can create problems elsewhere. Small, individualized adjustments may help when they reduce symptoms and work alongside a wider strength and load-management plan.
Inadequate recovery
Training adaptation happens between sessions. Poor sleep, work stress, insufficient food intake, and a busy schedule can all reduce recovery, making a previously tolerable program feel much harder. This is especially relevant for active professionals fitting runs around long days, travel, and limited rest.
Recovery also includes allowing enough time between demanding leg sessions. A hard run, heavy lower-body gym session, and a long walk can be individually manageable but collectively overload the knee when stacked too closely together.
Footwear, surfaces, and equipment changes
Shoes rarely explain runner’s knee on their own, but they can contribute when combined with a sudden training change. Switching to a lower-drop shoe, minimalist style, or a very different pair of trainers changes how loads are distributed through the leg. Replacing worn-out shoes may be sensible, but buying a new pair is not a complete treatment plan.
Likewise, a change from treadmill running to uneven pavement, track sessions, or hilly routes can expose a gap in load tolerance. Rather than avoiding a surface forever, introduce new demands gradually and monitor how the knee responds over the following 24 hours.
Why Pain Is Often Felt on Stairs and After Sitting
Stair climbing, downhill walking, squatting, and getting up from a low chair all involve a bent knee under load. That increases pressure at the patellofemoral joint and can make symptoms more obvious. Many people also notice an ache after sitting through a meeting, a drive, or a movie with the knee bent for a long period.
These symptoms are useful clues, but they do not confirm runner’s knee by themselves. Pain on the inner side of the knee, swelling, catching, a giving-way sensation, or pain after a specific twist may point to a different issue. An accurate diagnosis matters because the right rehabilitation plan depends on the source of symptoms.
What to Do When Runner’s Knee Starts
Completely stopping all activity is not always necessary, and it can make returning to running feel harder. The better approach is usually to reduce the activities that clearly aggravate pain while keeping the knee moving within a tolerable range. For some runners, that means temporarily shortening runs, avoiding hills, reducing speed work, or using a run-walk approach.
Use symptoms as a guide. Mild discomfort that stays manageable during activity and settles back to your usual level by the next day may be acceptable. Pain that progressively worsens, changes your gait, lingers for days, or appears with swelling is a sign that the current load is too high.
Rehabilitation should build capacity, not simply chase pain relief. Targeted exercises often focus on the quadriceps, glutes, calves, and single-leg control. A program may begin with tolerable strengthening movements and progress toward step-downs, squats, hopping, and running-specific drills as the knee becomes more confident. Manual therapy or taping may provide short-term symptom relief for some people, but they are most useful when combined with progressive exercise and practical changes to training.
When to Seek Physiotherapy for Knee Pain
Professional assessment is worthwhile if pain is not settling after a short period of sensible modification, keeps returning whenever you run, or is stopping you from exercising normally. It is also advisable if you are unsure whether the problem is truly runner’s knee.
Seek prompt medical assessment for significant swelling, inability to bear weight, a locked knee, fever, redness or heat around the joint, severe pain after trauma, or a calf that is swollen and painful. These symptoms need a different level of care.
At Movel London Physiotherapy, a one-to-one assessment looks beyond the painful spot. Your clinician can review your training pattern, examine knee and hip strength, assess relevant movement tasks, and create a realistic plan around your goals, whether that is running your first 5K, returning to the gym, or simply taking the stairs without pain.
A painful knee does not mean you have to give up running. With the right balance of reduced irritation, progressive loading, and support when needed, most people can rebuild confidence one comfortable run at a time.