Knee Flexion: Understanding Range of Motion and Osteoarthritis

Knee flexion refers to the motion of bending your knee, decreasing the angle between your thigh and shin. This movement is essential for nearly every daily task, including walking, sitting, squatting, climbing stairs, and getting in or out of a car. A healthy knee typically bends to about 120 to 140 degrees, allowing for smooth, controlled motion without discomfort.

When flexion becomes limited, it often results in stiff knees, which can make everyday activities more difficult and uncomfortable. Stiffness may develop gradually due to inactivity, swelling, arthritis, or tight muscles, and can significantly reduce mobility and independence if not addressed.

Common causes of limited or painful knee flexion include:

• Muscle tightness or imbalance that restricts the range of motion
• Joint injuries, such as ligament or meniscus tears
• Swelling or inflammation that makes the joint stiff
• Osteoarthritis, where cartilage wears down and bones rub together

When osteoarthritis develops, the cartilage cushioning the knee joint gradually deteriorates. This breakdown leads to stiffness, swelling, and discomfort during flexion, especially after periods of rest or prolonged activity.

Maintaining flexibility and strength through low-impact exercises, stretching, and proper joint care can help preserve your range of motion. Understanding what affects knee flexion and addressing those causes early can keep you moving comfortably and protect your knees as you age.

Understanding Knee Flexion and Its Normal Range

A normal, healthy knee can flex around 120 to 140 degrees when you actively bend it. Passive flexion, when someone else moves your leg, can reach up to 150 degrees. Different activities require different amounts of flexion:

• Walking: About 60 degrees
• Climbing stairs: Around 90 degrees
• Sitting in a chair: Approximately 90 degrees
• Squatting: Up to 115 degrees

Maintaining this range allows you to perform everyday tasks comfortably. Losing even a few degrees can make it hard to rise from a chair, kneel, or squat.

How Muscles and Ligaments Enable Flexion

Flexion involves several structures:

• Hamstrings: These muscles at the back of your thigh are the primary movers. They contract to bend the knee.
• Gastrocnemius: This calf muscle crosses the knee joint and assists in bending.
• Popliteus muscle: A small muscle that rotates the thigh bone to unlock the knee and start flexion.
• Ligaments and cartilage: The anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and collateral ligaments stabilize the joint, while the menisci (cartilage pads) cushion the bones.

Coordination between these structures allows smooth bending. When any component is damaged or tight, flexion may be limited.

Assessing and Measuring Knee Flexion

Clinicians measure knee flexion using a goniometer, a protractor‑like device. You lie on your back with legs straight; the examiner places one arm of the goniometer along the thigh and the other along the shin, then bends your knee to the maximum comfortable angle.

You can perform a simple self‑test at home: lying on your back, slowly slide your heel toward your buttocks, and note when you feel a gentle stretch. If you cannot bend beyond 90 degrees without pain, consider seeking an assessment.

Causes of Limited Knee Flexion

Muscle Tightness

Tight hamstrings, quadriceps, or calf muscles can prevent the knee from bending fully. Stretching improves flexibility and allows greater range.

Swelling or Effusion

Fluid buildup inside the knee (effusion) from injury or inflammation can physically block flexion. Reducing swelling through rest, ice, compression, and elevation (the RICE method) often improves motion.

Injury

Tears of the meniscus or ligaments, fractures around the knee, and post‑surgical scar tissue can restrict bending. Rehabilitation exercises and, in some cases, surgical repair restore movement.

Osteoarthritis

Osteoarthritis wears down cartilage and narrows the joint space, causing the bone ends to rub. Bone spurs may form, and the joint capsule thickens, further limiting flexibility. Stiffness often worsens after prolonged sitting or first thing in the morning and eases with gentle activity.

How Osteoarthritis Affects Knee Flexion

As osteoarthritis progresses, the joint loses its smooth cartilage surface. Inflammation within the joint produces extra synovial fluid, causing swelling. Bone spurs can impinge on soft tissues, and the joint capsule may become fibrotic.

Together, these changes reduce the ability of the knee to bend comfortably. Many people with osteoarthritis also avoid bending due to pain, leading to muscle weakness and further stiffness. Maintaining flexibility through safe exercises and weight management can slow this cycle.

Exercises and Stretches to Improve Flexion

Gentle stretches and strengthening exercises help maintain or regain knee flexion:

• Heel slides: Lie on your back, bend one knee, and slide the heel toward your buttocks. Hold for a second, then slide back. Repeat 10–15 times.
• Seated knee flexion with towel: Sit with your feet on the floor. Loop a towel under one foot. While keeping your foot on the ground, gently pull on the towel to help bend your knee further. Hold for 10 seconds, then release.
• Stationary cycling: Pedaling a stationary bike promotes knee motion without high impact.
• Water exercises: Walking or jogging in a pool reduces load while encouraging a full range of motion.
• Quad and hamstring stretches: Keeping supporting muscles flexible makes bending easier.

Always work within a pain‑free range and increase intensity gradually. If you have had knee surgery or a serious injury, consult a physical therapist to tailor exercises to your needs.

Lifestyle Tips for Healthy Flexion

• Maintain a healthy weight: Excess weight compresses joints and reduces the range of motion.
• Stay active: Regular movement prevents stiffness. Walking, swimming, and cycling are gentle on joints.
• Eat a balanced diet: Foods rich in antioxidants and omega‑3 fatty acids may reduce inflammation and support cartilage health.
• Wear supportive footwear: Proper shoes cushion impact and help align joints.

Medical Treatments for Flexion Loss

Viscosupplementation

If osteoarthritis causes knee pain and stiffness, viscosupplementation, which involves injecting hyaluronic acid into the joint, may improve lubrication and reduce discomfort. While it doesn’t increase flexion directly, decreased pain often makes bending easier.

Platelet‑Rich Plasma Therapy

PRP injections use a concentration of your own platelets to promote healing and reduce inflammation. Some studies show improved pain and function in people with knee osteoarthritis. Learn more on our platelet‑rich plasma therapy page.

Bracing and Supports

A supportive knee brace can provide stability, reduce pain and may help you feel more confident bending your knee. Our back brace for lower back pain and knee brace products are designed to relieve stress on joints while maintaining mobility.

Surgery

When osteoarthritis is advanced or when structural problems block flexion, surgical options may be necessary. Arthroscopic surgery can remove loose fragments or repair torn menisci. In severe cases, partial or total knee replacement restores joint alignment and range of motion.

When to Consult a Specialist

If you have persistent knee stiffness, swelling, pain or limited flexion that interferes with daily life, consult a healthcare provider. Early intervention slows degeneration and helps you maintain function. A specialist can determine whether your limitations stem from soft‑tissue tightness, osteoarthritis, or other causes and recommend appropriate therapy.

Find Relief and Restore Your Knee Movement

Knee flexion plays a vital role in how you move, from walking and sitting to climbing stairs. Even a small loss of range can make daily activities difficult. Understanding what limits knee motion, especially conditions like osteoarthritis, and taking proactive steps through stretching, strengthening, and medical treatment can make a lasting difference.

If knee stiffness or pain is keeping you from doing the things you love, schedule a screening with Arthritis Knee Pain Centers. Our specialists will evaluate your knees, explain non-surgical options, and create a personalized plan to help you move comfortably again.