Common, usually minor, and tennis is good for youEmerging evidence
A sore knee after tennis is common and usually settles; serious knee injuries are much less common. In a hospital series of 449 recreational players with acute tennis injuries (average age 43.6), "Harmless contusion or strains were the most common injury (49%)", and 88.7% of players aged 60 and over seen in US emergency departments for tennis injuries went home the same day. Nearly half of US high school players with a tennis injury "returned to play within a week of injury".
Tennis is good for you, and for your thighs: in a Copenhagen population study, tennis players' adjusted life expectancy was 9.7 years longer than sedentary people's, racket sport players had about half the risk of dying during a British study (a hazard ratio of 0.53), and senior players aged about 71 had thicker, stronger thigh muscles than their peers. These are associations, not proof.
Sources 1, 2, 3, 4, 5, 6
How often it is the knee: about 1 in 10 to 1 in 5 injuriesEmerging evidence
Estimates vary with who is counted. The knee was 10.4% of injuries in professional players at the French Open and 90 of 467 acute injuries (19%) in recreational players seen at an Austrian hospital, and an older expert review, from 1995, puts knee injuries at "about one fifth of all tennis injuries". The knee was the third commonest site at the US Open on hard courts, and "shoulder, knee and lumbar spine presentations are common" at Wimbledon on grass.
In junior players at one Swedish club, injuries ran at 1.7 per 1,000 hours of tennis in boys and 0.6 in girls, and most new injuries were at the knee. In padel, "The elbow was the most common anatomical site of injury, followed by the knee, shoulder and lower back". Sports Medicine Australia gives "five injuries per 1,000 hours of participation" for tennis overall (a 2008 sheet).
Sources 7, 1, 8, 9, 10, 11, 12, 13
Overuse first, and what tennis asks of the kneeEmerging evidence
In 3,656 recreational Dutch club players (average age 49), "Of these injuries, 3246 (80%) were overuse and 801 (20%) were acute", and the American Orthopaedic Society for Sports Medicine (AOSSM) says "Two-thirds of tennis injuries are due to overuse and the other one-third is due to a traumatic injury or acute event". Tennis Australia, in a summary on elite players: "Tennis injuries are typically attributed to a combination of training and match play loads, court surface, age, equipment, injury history and physical condition of the player".
Tennis is stop-start: a split step (a small hop as your opponent hits), quick first steps, lunges for wide balls, sudden stops and changes of direction, and a serve that bends and drives through the knees. "Knee motion is a significant contributor to serving effectiveness whatever the performance level", and Sports Medicine Australia says "Lower limb (ankle, knee, and thigh) injuries are most common and are caused by the sprinting, stopping, pivoting and pounding nature of tennis". No study has measured the knee's loads in the split step.
Sources 14, 15, 16, 17, 13
The front of the knee: kneecap pain and the patellar tendonEmerging evidence
An expert review: "In younger individuals, patellofemoral pain syndromes are the most frequent and intriguing problems". At the French Open, the patellar tendon was 24 of 207 tendon injuries and the kneecap joint 17 of 163 joint injuries. The International Tennis Federation (ITF) describes jumper's knee as common in tennis players, from the explosive sprinting, jumping and quick changes of direction, often with an ache after play.
No study has counted jumper's knee in tennis players: the two big studies of how common it is didn't include tennis.
Sources 8, 7, 18, 19, 20
The meniscus, from mid-lifeEmerging evidence
The NHS: "A meniscus tear usually happens when you twist your knee while playing sport". In the Austrian series, "Patients who needed surgical treatment for a meniscal tear showed an acute onset of pain after a knee twist on the tennis court", and they were 33 to 66 years old, so the authors suspect "an acute-on-chronic lesion" (a tear in a meniscus already worn with age). The American Academy of Orthopaedic Surgeons (AAOS): "An awkward twist when getting up from a chair may be enough to cause a tear in an aging meniscus".
Sources 21, 1, 22
Arthritis in older players: tennis isn't the causeEmerging evidence
An expert review says meniscus injuries and cartilage wear dominate in middle-aged and older recreational players. A stiff, achy knee from the mid-40s that is worse after a long match is often arthritis; the National Institute for Health and Care Excellence (NICE) says "Diagnose osteoarthritis clinically without imaging" in people 45 or over with activity-related joint pain and no morning stiffness, or stiffness that lasts no longer than 30 minutes.
Playing tennis isn't the cause: in Swedish men, tennis was linked with knee arthritis (odds ratio 2.0) until past knee injuries were counted, and "The sports-related increased risk for knee osteoarthritis was explained by knee injuries". Across studies, sport overall wasn't linked with knee arthritis (7.7% against 7.3%). Former elite women athletes had more X-ray changes than other women but "similar rates of symptom reporting", and only 14 of them were tennis players.
Sources 8, 23, 24, 25, 26
Ligament sprains, the ACL and fallsEmerging evidence
In Danish ACL register patients injured playing tennis, "Lunging, running forward to the net, and movements related to smashing were the most frequent activities leading to injury", and "A majority (72%) of injured players returned to tennis, while a smaller proportion (25%) achieved RTP" (RTP means back to the previous level). Players with an untreated ACL tear found landing after a smash and stopping and changing direction hardest. The inner knee ligament (the MCL) can sprain in a fall or a twist; ligament and meniscus tears were among the acute knee injuries in the Austrian series.
Falls cause most emergency visits by older tennis players, and leg injuries were 22.3% of tennis injuries in players 60 and over seen in US emergency departments. A fall onto the knee can break the kneecap, and a broken hip is occasionally felt mainly in the knee, as the warning signs say.
Sources 27, 28, 1, 2
Who gets it (our reading)
A sore knee from tennis is likeliest after a jump in playing time, such as a club tournament, a holiday or the switch from outdoor clay to indoor hard courts, which the ITF singles out; in players with arthritis or a past knee injury; in singles players who lunge and stop hard; and in worn or wrong shoes. That is our reading of the injury studies and the ITF's advice, not a tested risk model.
Sources 18, 14
When a scan or test mattersStrong evidence
After a fall or a sudden injury, clinicians use the Ottawa knee rule to decide on an X-ray; across 6 studies of 4,249 adults, "the pooled sensitivity was 98.5%" for fractures. For a mid-life knee with joint line pain, a magnetic resonance imaging (MRI) scan often finds a tear whether or not it is the cause: "The prevalence of a meniscal tear or of meniscal destruction in the right knee as detected on MRI ranged from 19% (95% confidence interval [CI], 15 to 24) among women 50 to 59 years of age to 56% (95% CI, 46 to 66) among men 70 to 90 years of age", and a guideline panel says "Meniscus tears are common, usually incidental findings, and unlikely to be the cause of knee pain, aching, or stiffness".
NICE diagnoses osteoarthritis clinically, without imaging. A scan matters for a knee that is locked, after a pop with fast swelling in a younger player, or when surgery is being considered.
Sources 29, 30, 31, 23
How it usually goesEmerging evidence
Bruises and strains settle in days to weeks: about half of acute tennis injuries seen in hospital were of this kind. The NHS says "A meniscus tear can get better on its own without treatment, but it can take a while to heal", and in 140 adults aged 35 to 60 with a worn inner meniscus tear, 12 weeks of supervised exercise did as well as keyhole surgery over 2 years.
Tendon pain is slower: the ITF says "With more serious injuries, training can usually be resumed after six weeks to three months". After an ACL tear, most tennis players returned to tennis, but only a quarter to their old level. No one can promise a date.
Sources 1, 3, 21, 32, 18, 27
When it isn't tennis: the causes the warning signs cover
Some knee pain has causes tennis didn't create, or needs more than a plan, and the warning signs follow the pages behind them. A knee that changed shape, swelled fast or went with a pop needs A&E, in the words of the NHS dislocated kneecap page, and the Ottawa knee rule, tested in emergency departments, picks out who needs an X-ray. A pop at the front of the knee with a knee you can't straighten can be a torn quadriceps or patellar tendon, as the AAOS describes; steroids and some antibiotics can weaken these tendons. A hot, swollen knee can be a joint infection, which the NHS septic arthritis page says is urgent, and gout can look the same, as the NHS gout page says.
A knee that is locked and won't straighten, a tear after a significant twist in a younger person, or a knee that keeps giving way needs to be seen: the NHS meniscus tear page, a BMJ guideline panel that makes an exception for a truly locked knee, two trials in adults up to 45 and a European consensus on repairing tears that can be stitched are behind those lines, and an NHS programme on knee keyhole surgery says when catching or locking that hasn't settled after about three months needs a specialist. The NHS page on why ACL surgery is done, a UK trial of reconstruction against rehab, a US ligament guideline and a review of inner knee ligament injuries say when a loose knee needs a surgeon. Sudden, severe inner knee pain with little or no injury in an older adult can be a small fracture under the joint surface, which the AAOS and a 2025 review describe.
After a fall, a broken hip is occasionally felt mainly in the knee: two case reports describe it, and the NHS hip pain and broken hip pages give the 999 signs. The NHS falls page, NICE's falls guideline and an Otago strength and balance booklet say a fall or near-fall needs a falls assessment, and after a knee replacement the NHS complications page lists the signs of an infection or a clot. A swollen, throbbing calf can be a blood clot or a burst Baker's cyst, and sudden breathlessness a clot in the lungs, as the NHS pages on deep vein thrombosis (DVT), Baker's cyst and pulmonary embolism explain; chest pain on court can be a heart attack or angina, and fainting while exercising needs a 999 call, as the NHS heart attack, angina and fainting pages say.
NHS inform's kneecap pain exercises say when kneecap pain needs a healthcare professional; tender bumps below a junior player's kneecap are usually Osgood-Schlatter or Sinding-Larsen-Johansson, as the AAOS and two NHS trusts describe; the NHS bone cancer page describes bone pain that is worse at night; and pain that travels down from the back is sciatica, in the NHS's words.
Sources 33, 59, 60, 61, 62, 63, 64, 65, 21, 31, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95
