What Is Swimmer’s Knee? Causes, Symptoms, and Treatment

Swimmer’s knee, more precisely called breaststroker’s knee, is a repetitive-stress injury to the inner side of the knee caused by the distinctive whip kick used in breaststroke. It is one of the most common overuse injuries in competitive swimming, with surveys finding that roughly three out of four breaststroke specialists report knee pain at some point in their careers. The condition is far from a minor annoyance: nearly half of affected swimmers experience pain on a weekly basis, and the mechanics behind it involve several different structures inside and around the knee joint.

Why the Breaststroke Kick Is Hard on Knees

The problem originates in the motion itself. Unlike the flutter kick used in freestyle or backstroke, which moves the legs up and down in a relatively straightforward plane, the breaststroke whip kick sweeps the lower legs outward and then snaps them together in a circular path. Underwater photography analysis showed that during the later phase of this kick, the knee transitions from a bent position to a straightened one while the joint is simultaneously being pushed inward (a stress called valgus) and twisted outward (external rotation).¹ That combination of forces is not something the knee is well built to absorb thousands of times per training session. Each kick cycle loads the inner compartment of the knee in a way that land-based sports rarely replicate with such regularity.

The sheer volume of repetitions matters. A competitive swimmer might perform several thousand breaststroke kicks in a single practice. Over weeks and months, the cumulative load on the inner knee outpaces the tissue’s ability to repair itself between sessions, which is the textbook definition of an overuse injury. Age, total years of competitive swimming, and the amount of breaststroke-specific training all correlate with the likelihood of developing pain.²

Which Structures Get Damaged

One reason swimmer’s knee can be tricky to diagnose and treat is that multiple structures can be involved, sometimes at the same time. The pain is most commonly felt along the medial, or inner, portion of the knee, but the precise source varies from swimmer to swimmer.³ Two structures come up most often in research.

The first is the medial collateral ligament (MCL), the band of tissue that stabilizes the inner side of the knee. Each whip kick strains this ligament as the knee absorbs valgus stress. With enough repetition, the MCL becomes irritated and inflamed. Studies have also identified the inferomedial patellar border, the bony ridge along the lower-inside edge of the kneecap, as a frequent pain site.²

The second is the medial synovial plica, a fold of tissue lining the inside of the knee joint. Roughly half of competitive breaststrokers who had weekly knee pain were found to have a tender, thickened medial plica on physical examination.³ This plica can become inflamed from repetitive friction against the femur during the kick and eventually thickens into a painful, palpable band. Because the MCL, the patellar border, and the plica all sit in roughly the same neighborhood of the knee, figuring out which structure is driving the pain often requires a careful clinical exam and sometimes imaging.

How Common It Actually Is

Swimmer’s knee is remarkably prevalent. In a survey of 391 competitive swimmers, knee pain was reported by about 73% of breaststroke specialists and 48% of swimmers who did not specialize in breaststroke.² A smaller, more focused study of 36 competitive breaststrokers found that 86% had experienced at least one episode of breaststroke-related knee pain, and about 47% dealt with it on a weekly basis.³

The fact that nearly half of nonbreaststrokers in the larger survey also reported knee pain is worth flagging. Swimmers in other strokes sometimes use breaststroke kicks during warm-ups, cool-downs, or drill sets, and those lower-volume exposures can still add up. Individual medley (IM) swimmers, who regularly train all four strokes, are an obvious at-risk group even if they do not consider themselves breaststroke specialists.

The Role of Hip Angle and Body Mechanics

Not every swimmer who logs heavy breaststroke yardage develops knee problems. Biomechanical analysis suggests that hip position at the start of the kick is a significant factor. One study using kinematic film found dramatic differences in injury rates depending on how wide the swimmer’s hips were spread when the kick began. Swimmers whose hip abduction angle at kick initiation was either less than about 37 degrees or greater than about 42 degrees had notably higher rates of knee pain compared with those in the middle range.²

In practical terms, this means that both a very narrow kick and a very wide kick increase strain on the inner knee, albeit through slightly different loading patterns. Swimmers who naturally fall into one of these extreme ranges may be mechanically predisposed to trouble. Coaches sometimes talk about finding the “sweet spot” for an individual swimmer’s kick width, and this research gives that advice some biomechanical backing.

Hip internal rotation range of motion may also play a role. Research in other athletic populations has shown that greater hip internal rotation is associated with increased inward collapse of the knee during dynamic movements, and this relationship becomes even more pronounced when athletes are fatigued.⁴ While that particular study looked at soccer players rather than swimmers, the underlying principle is the same: when the hip allows more inward rotation, the knee tends to absorb more valgus stress, and fatigue makes the whole chain wobblier. Swimmers with naturally hypermobile hips, a trait that is common and sometimes even selected for in competitive swimming, may carry extra risk here.

What MRI Reveals, Even Without Symptoms

One of the more striking findings in the research is that competitive swimmers’ knees often look abnormal on imaging even when the swimmer feels fine. A controlled MRI study of adolescent swimmers found that about 69% of the swimmers’ knees showed at least one imaging abnormality, compared with a significantly lower rate in non-swimming controls.⁵

The most common finding was swelling in the infrapatellar fat pad, a cushion of fatty tissue below the kneecap, seen in more than half of the swimmers’ knees. Bone marrow swelling appeared in about a quarter, and fluid inside the joint showed up in roughly 15%.⁵ These are the kinds of changes that in other clinical contexts might prompt concern, but in competitive swimmers they appear to represent a low-grade adaptive response to chronic loading rather than an acute injury. The takeaway for swimmers and parents is twofold: an MRI that shows some edema does not necessarily mean something is broken, and a knee that feels fine is not necessarily free of structural stress.

Treatment and Getting Back in the Water

The good news is that swimmer’s knee usually responds to conservative treatment. The first step, predictably, is reducing or temporarily eliminating the aggravating activity. For many swimmers, that means cutting out breaststroke-specific work while continuing to train other strokes. Complete rest from all swimming is generally not necessary unless the pain is severe or has gone on for months without improvement.

Beyond rest, strengthening the muscles around the hip turns out to be surprisingly important. The knee does not operate in isolation; its alignment and loading during movement depend heavily on what the hip is doing above it. A three-week hip-abductor strengthening program studied in athletes with patellofemoral pain (pain around the kneecap, a close cousin of many breaststroker’s knee presentations) resulted in increased hip strength, reduced pain, and more consistent knee-joint mechanics.⁶ The logic translates well to swimmers: stronger hip abductors help control the inward collapse of the knee during the whip kick, taking some of the repetitive strain off the medial structures.

Common dryland exercises prescribed for this purpose include side-lying leg raises, clamshells, single-leg squats, and resistance-band walks. Many sports medicine practitioners also incorporate quadriceps and hamstring work to improve overall knee stability. Ice and anti-inflammatory medication can help manage acute flare-ups but are not long-term solutions on their own.

Technique Adjustments That Help

Because the injury is driven by the mechanics of the kick itself, technique modification is central to both treatment and prevention. Adjustments typically focus on reducing the amount of valgus stress the knee experiences during each kick cycle.

Kick width: As noted in the biomechanical research, extremes of hip abduction angle at kick initiation correlate with higher injury rates. A coach or physical therapist can use video analysis to assess whether a swimmer’s kick falls outside the moderate range and make adjustments.
Knee tracking: Some swimmers let their knees drift inward relative to their feet during the recovery phase of the kick, which loads the MCL before the propulsive phase even begins. Cueing the swimmer to keep the knees tracking over or slightly outside the toes can reduce this pre-loading.
Kick volume: Periodizing breaststroke yardage so that high-volume breaststroke weeks alternate with lighter ones gives the knee tissue recovery time. This is especially relevant during phases of the season when coaches ramp up yardage rapidly.
Mixed training: Incorporating flutter-kick and dolphin-kick sets into breaststroke specialists’ training provides cardiovascular load without the repetitive medial knee stress. Kick sets using a board with a dolphin kick can maintain leg conditioning while the inner knee recovers.

Structured return-to-swimming protocols that gradually increase yardage exist, though they are more commonly discussed in the context of shoulder injuries. The same principle applies to knee problems: a swimmer coming back from a significant bout of breaststroker’s knee should ramp breaststroke volume incrementally rather than jumping back to full training loads.⁷

When Surgery Becomes Part of the Conversation

Most cases of swimmer’s knee resolve without surgery, but when conservative treatment fails, particularly when a thickened medial plica is the culprit, arthroscopic removal of the plica is an option. A long-term follow-up study of patients who underwent arthroscopic treatment for medial plica syndrome found that knee function scores improved considerably, rising from an average of about 52 out of 100 before surgery to about 80 at the final follow-up years later.⁸

The results were not uniformly rosy, though. Patients who had cartilage damage beyond the mildest grade had significantly worse long-term outcomes, and older patients tended to recover less fully. For a young competitive swimmer whose only real problem is a chronically inflamed plica, the procedure can be effective. For someone with more widespread changes inside the knee, the picture is less clear-cut, and the decision should involve a detailed discussion with an orthopedic surgeon who understands the demands of competitive swimming.

Why Young Swimmers Deserve Extra Attention

Adolescent swimmers face a particular set of risks. The growing skeleton contains growth cartilage at the ends of the long bones, at the joint surfaces, and at the points where tendons attach to bone. These areas are weaker than the surrounding mature bone and are more vulnerable to repetitive loading.⁹ During rapid growth spurts, the risk goes up further because bones grow faster than the muscles and tendons attached to them, creating tightness across joints that amplifies stress with every kick.

The MRI study of asymptomatic adolescent swimmers, which found abnormalities in roughly 69% of their knees, underscores that even pain-free young swimmers are absorbing meaningful structural loads.⁵ This does not mean every young swimmer is on the verge of injury, but it does suggest that coaches and parents should take early complaints of inner knee pain seriously rather than dismissing them as growing pains. Reducing breaststroke volume during growth spurts and making sure young swimmers have adequate recovery time between hard training blocks are reasonable precautions.

The Eggbeater Kick Connection

Breaststroke is not the only swimming-related movement that loads the knee in this way. Water polo players use the eggbeater kick to tread water, and it shares many of the same rotational demands on the knee. A study of eggbeater-kick biomechanics found that as players became fatigued, their hip internal rotation increased and ankle mechanics shifted in ways that predispose the knee to patellofemoral pain.¹⁰ Vertical force output dropped progressively with fatigue, meaning players were kicking less effectively while simultaneously loading their knees in riskier patterns.

The eggbeater kick’s resemblance to the breaststroke whip kick is not coincidental; both involve simultaneous hip abduction and external rotation of the lower leg around a flexed knee. Water polo players who also swim breaststroke in training are effectively doubling their exposure. If you play water polo and notice inner-knee soreness, the same diagnostic and treatment principles that apply to breaststroker’s knee are relevant. Hip strengthening, technique analysis, and smart load management all carry over directly.

Common Misconceptions About Swimmer’s Knee

One persistent myth is that swimmer’s knee is really just a cartilage problem, like the meniscus tears common in contact sports. While cartilage damage can certainly accompany chronic breaststroker’s knee, the primary issue in most cases involves the soft tissues on the inner side of the joint: the MCL, the plica, and the fat pads. Treating it like a meniscus problem, with aggressive rest or premature consideration of surgery, often misses the point.

Another misconception is that the knee itself is the root cause. In many swimmers, the knee is the victim of forces that originate at the hip or even the ankle. A swimmer with stiff ankles may compensate by generating more rotational force through the knee during the kick. One with weak hip stabilizers may let the knee drift inward under load. Addressing only the knee while ignoring the joints above and below it tends to lead to frustrating cycles of recovery and relapse.

Finally, some swimmers and coaches assume that pain during breaststroke is just part of the sport and does not require attention. The prevalence data might seem to support that fatalism, but there is a meaningful difference between occasional mild soreness and the kind of weekly pain that nearly half of breaststroke specialists report. The former may indeed be an acceptable cost of high-volume training; the latter tends to worsen over time if the underlying biomechanical and training factors are not addressed.