Knee pain on the leg extension: the kneecap, the tendons, range, the pad and what helps

What the leg extension asks of the knee, and what no study has countedEmerging evidence

You sit with a pad on your lower shins and straighten your knees against the weight. Only the thigh muscles (quadriceps) do the work, through the tendon above the kneecap, the kneecap and the tendon below it. The force they need rises sharply over the last 15 degrees of straightening (in cadaver knees, about 350 newtons at straight against 177 between 50 and 15 degrees), and the stress behind the kneecap is highest near straight and lower with the knee well bent. The forward pull on the shin that loads the ACL appears in roughly the last 40 degrees.

Force and stress differ: in 10 men, the squeezing force behind the kneecap on the leg extension peaked in the middle of the range, while the stress (that force spread over the contact area) was highest near straight in the studies that measured it. No study has counted knee pain or injuries from the leg extension, and we found no report of a tendon tear on one; in US emergency data, 90.4% of weight-training injuries involved free weights rather than machines. The lab studies tested healthy adults (10 to 20 per study) in one session, and the treatment trials were small, so this guide's map comes from those studies, condition guidelines and NHS pages.

Sources 1, 2, 3, 4, 5, 6, 7

The kneecap and the tendonsStrong evidence

Pain around or behind the kneecap that is hard to point to, brought on by squatting, stairs or sitting with bent knees, is kneecap pain (patellofemoral pain), diagnosed by examination. Patellar tendon pain sits at one point at the lower tip of the kneecap and rises with load. Quadriceps tendon pain sits at the top edge and is a separate, less studied condition with no exercise trial.

A torn quadriceps or patellar tendon is different: a tearing or popping feeling, then being unable to straighten the knee. The American Academy of Orthopaedic Surgeons (AAOS) says quadriceps tendon tears are not common; tears are linked with kidney failure, diabetes, steroids and fluoroquinolone antibiotics, and people who need surgery do better if it is done soon. The UK medicines regulator says stop a fluoroquinolone at the first sign of tendon pain and contact your doctor. We found no report of a tear on a leg extension machine.

Sources 8, 9, 10, 11, 12, 13

The meniscus and arthritisStrong evidence

Catching, locking, swelling later that day or a knee that won't fully straighten at the inner or outer joint line points to a meniscus tear; from mid-life it is usually a worn tear. A stiff ache in both knees from your mid-40s, worse getting going, is often knee arthritis, which the National Institute for Health and Care Excellence (NICE) diagnoses without imaging from 45 when morning stiffness lasts no longer than 30 minutes.

Sources 14, 15

When a scan or test mattersStrong evidence

Not at first: kneecap and tendon pain are diagnosed by examination, NICE says don't routinely use imaging to diagnose typical arthritis, and a Dutch guideline says imaging adds little for patellar tendon pain. Scans of pain-free knees often show changes: from 40, 43% had a cartilage defect and 19% a meniscal tear on magnetic resonance imaging (MRI) (63 studies), and in 991 adults aged 50 to 90, 61% of those with a meniscal tear had had no knee pain in the past month.

Tests do matter for a knee that locks, a suspected tendon tear (urgently), an injury that meets the Ottawa knee rule (98.5% sensitive for fractures), a hot, swollen knee (blood tests and fluid from the joint the same day), and kneecap pain that hasn't improved after 12 weeks of structured exercise, when the Dutch guideline says to reconsider the diagnosis.

Sources 15, 16, 17, 18, 19, 20

How it usually goesMixed evidence

No study follows knee pain from the leg extension. Kneecap pain is often slow: in 310 people, 55% hadn't recovered well at 3 months and 40% at 12 months, and pain for more than 2 months at the start predicted a worse outcome, so start the exercises early. At 5 to 8 years, 57% of the fifth who replied still hadn't recovered well, but 98% had no arthritis on X-ray. In a trial of 40 young women on a programme with pain monitoring, 85% were doing sport without pain at 12 months, and an NHS trust expects improvement over 3 to 6 months.

Patellar tendon pain usually needs at least 12 weeks of progressive loading, and a review warns it can be slow and sometimes frustrating. The NHS says knee pain can often be treated at home and that you should start to feel better in a few days. No one can promise a date.

Sources 21, 22, 23, 24, 16, 9, 25

Range is the main dialEmerging evidence

In 20 healthy adults, stress behind the kneecap was higher on the leg extension than on the leg press with the knee nearly straight (0 and 30 degrees) and higher on the leg press with the knee well bent (60 and 90 degrees), crossing at about 48 degrees (only 4 angles were measured). In 10 adults, the squat put more stress behind the kneecap at 90, 75 and 60 degrees, and the knee extensions more at 30, 15 and 0; a cam machine (one whose resistance changes through the lift) loaded the kneecap less than ankle weights from 60 to 90 degrees, and the authors advise doing the machine version from 90 to 45 degrees of bend.

That range is also the median of the kneecap pain trials: across 79 studies, 28 used a seated knee extension, at a median of 3 sets of 10, from 90 to 45 degrees, at 70% of the most you could lift once (one-repetition maximum, 1RM). In a computer model, cartilage stresses on the leg extension were no higher than normal, or than in closed chain exercise (with the feet on the ground or a platform), from 20 to 90 degrees. These are lab studies in healthy people; none measured pain.

Sources 2, 3, 26, 27

Leg extensions are used to treat knee painStrong evidence

Clinical guidelines and international consensus put exercise first for kneecap pain, with hip and knee strengthening together, whether or not the exercise is weight-bearing; a Dutch guideline asks for at least 6 to 12 weeks, and to reconsider the diagnosis if 12 weeks haven't helped. Knee extensions did as well as a seated leg press over 6 weeks in 45 men, and an open chain programme (the foot moving freely, as on the leg extension) did as well as a closed chain one in 60 people, with the results holding at 5 years.

For knee arthritis, NICE offers therapeutic exercise to everyone and warns that pain may rise at first; across 139 trials, exercise eased pain slightly and improved function (low to moderate certainty). In 377 adults over 50 with knee arthritis, single-leg machine training including the leg extension, 3 times a week for 18 months, eased pain no more at heavy loads (75% to 90% of 1RM) than at light loads (3 sets of 15 at 30% to 40%), and the heavy group reported more minor complaints (53 against 30).

Sources 28, 29, 8, 16, 30, 31, 32, 15, 33, 34

Patellar tendon pain: holds, then slow heavier repsEmerging evidence

In the largest trial (76 people), daily single-leg holds came first, on the leg extension by preference or the leg press: 5 holds of 45 seconds at about 60 degrees of bend with 2 minutes' rest, building towards 70% effort. Then came slow single-leg reps every second day, 4 sets of 15 between about 10 and 60 degrees, building to heavier sets of 6 and then towards 90 degrees and nearly straight, with two further stages towards sport. Pain was kept at 3 out of 10 or less, and the programme beat painful decline squats by 9 points on a 100-point tendon score at 24 weeks, with a wide margin of uncertainty (1 to 16).

Holds are not magic. They eased pain a lot straight away in 6 volleyball players, but no more than moving reps over 4 weeks in 29 athletes, only briefly in 21 people, and no better across 10 trials; short holds worked as well as long ones (16 men). Moderate loads (55%) did as well as heavy ones (90%) over a year in 44 adults, and a steroid injection's early relief faded by 6 months where slow heavy training lasted (39 men). The Dutch guideline asks for at least 12 weeks, and a review warns rehab can be slow and sometimes frustrating.

Sources 35, 36, 37, 38, 39, 40, 41, 42, 16, 9

After ACL surgery: your team decides, and a limited range looks safeMixed evidence

An international guideline from Aspetar, a sports medicine hospital in Qatar, starts leg extensions at about week 4 between 90 and 45 degrees of bend, on very low certainty evidence with strong panel agreement. Cambridge's NHS programme widens the range by about 10 to 15 degrees a week to full range by week 8, and starts resistance at week 4 after a patellar tendon graft but at week 12 after a hamstring graft; a US guideline from the Multicenter Orthopaedic Outcomes Network (MOON) limits the first 6 weeks to light-load, short-arc work. Across studies, start times ranged from the first day to 270 days.

Across 10 trials, open chain exercise after reconstruction didn't loosen the knee more than closed chain work, and early leg extensions in a limited range didn't loosen hamstring grafts (49 people); open chain work built more thigh strength by 3 to 4 months (9 trials, very low to low certainty), and a 2025 review found it helped most from about week 4, once closed chain work had started (low-quality studies). The caution is going to full range too early: full-range work by week 6 after a hamstring graft added about 1 millimetre of looseness (68 people), and on the leg extension, unlike in the squat, adding weight raised the strain on the ACL. In knees with a torn ACL and no surgery, leg extension work built more thigh strength without loosening the knee (42 people), and a review of 6 small trials found closed chain work no better than open chain. Your team's plan comes first.

Sources 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54

The pad and the pivot: one commentary and small lab studiesEmerging evidence

On the leg extension, the forward pull on the shin that loads the ACL runs from about 40 degrees to straight, peaking at about 14 degrees (10 men lifting a weight they could lift 12 times); in living knees, ACL strain at 10 and 20 degrees rose with a 45-newton weight, while moving between 35 and 90 degrees didn't strain it. One expert commentary puts the peak ACL load at about 158 newtons, below the roughly 303 newtons of level walking, and reports ACL force at 30 degrees of about 400 newtons with the pad near the ankle against about 200 with it near mid-shin.

Small lab studies agree that a pad nearer the knee reduced the shin's forward slide (12 knees without an ACL), and mathematical models say the best pad spot varies with knee angle and that a knee out of line with the machine's pivot changes the loads. No study has tested pad position for pain, so lining the side of your knee up with the pivot and putting the pad on the lower shin is coaching practice, and after an ACL injury or surgery the pad and range are your team's call.

Sources 5, 6, 55, 56, 57, 58, 59

Your leg extension for now (written steps)

Our 3D figure can't show the machine's settings. These steps come from lab studies, trial programmes and coaching sources; where a step is our suggestion, it says so. No trial has tested a change to the leg extension for knee pain.

• 1. Range: with kneecap pain, set the range lever or stop the lift so you work from 90 to about 45 degrees of bend, the range a lab study advises for a cam machine and the median of the kneecap pain trials; add the last part towards straight as it settles. With tendon pain, start with holds at about 60 degrees.
• 2. Pivot and seat: line the side of your knee up with the machine's pivot, with the backs of your knees at the edge of the seat. If the pad slides up and down your shin, move the backrest; in a model, a knee out of line with the pivot changed the loads.
• 3. Pad: on the lower shin just above the ankle for a healthy knee (coaching practice). Higher up the shin lowers the load on the ACL in lab studies; after an ACL injury or surgery, follow your team.
• 4. The top: straighten to just short of a hard lockout, without snapping into it. The last 15 degrees take the most thigh force, and the tendon trial worked to a straight leg without locking.
• 5. Tempo: about 2 seconds up and 3 seconds down, with no swinging (editorial).
• 6. Load: lighter, stopping 2 or 3 reps short of failure, and one leg at a time if one knee hurts (editorial; the tendon and arthritis trials used single-leg work).
• 7. Machine before ankle weights: a cam machine loaded the kneecap less than ankle weights in the bent range, so with a sore kneecap use the machine (our reading).
• 8. Pair it with the leg press's easy range: the leg press loads the kneecap least from nearly straight to about 45 degrees and the leg extension from 90 to about 45, so using each in its own easy range is a lab-based idea (our suggestion); the kneecap pain trials used both.

Sources 3, 26, 35, 59, 55, 56, 1, 34, 2

Back to your usual leg day (written steps)

No trial has tested a return to the leg extension after knee pain, so these steps are our suggestion.

• 1. One change at a time: range, then load, then sets.
• 2. Add about 5% to 10% at a time after a session that passed the pain rule.
• 3. Keep two strength sessions a week with hip and knee work for a few months after the pain has gone.
• 4. If it flares after a big session, go back a stage for a few days, not to nothing.

Arthritis, surgery and injections

With knee arthritis, the leg extension is a reasonable gym choice in a comfortable range: the 18-month arthritis trial used it 3 times a week, light loads did as well as heavy ones with fewer minor complaints, and NICE warns pain may rise a little at first. After ACL reconstruction, a knee replacement or meniscus surgery, your team sets the range, pad and timing; after ACL reconstruction, one NHS programme waits until 12 weeks after a hamstring graft before adding resistance.

For patellar tendon pain, a steroid injection helped at first but did worse than slow heavy training by 6 months, repeated injections into the tendon are linked with ruptures in weightlifters, and the AAOS says doctors typically avoid injections around the patellar tendon. For kneecap pain, consensus doesn't recommend joint mobilisation on its own or electrical and other machine-based treatments.

Sources 34, 15, 43, 44, 42, 61, 12, 29