Range of motion (ROM) describes the distance and direction a joint can move. The knee joint connects the thigh bone (femur) to the main shin bone (tibia) and must move freely for daily actions like walking, sitting, and climbing stairs. Understanding the parameters of normal knee movement helps in recognizing when a limitation is present. A healthy ROM is necessary for mobility.
The Specific Measurements of Normal Knee Movement
Normal knee movement involves two primary actions: flexion (bending the joint) and extension (straightening the leg). Full extension, or a fully straight leg, is the reference point of 0 degrees. Normal knee flexion, which is bending the heel toward the buttocks, ranges from 135 to 150 degrees.
The total range of movement varies based on individual factors like age, body size, and health. For instance, muscle bulk in the thigh and calf may limit the final degrees of flexion. While 0 degrees is the standard for extension, some individuals naturally exhibit hyperextension, where the knee straightens beyond the zero point. This slight backward bend is measured as a negative number, such as -5 degrees.
A functional range of motion is the movement needed for common tasks, which is often less than the maximum possible range. Walking requires about 65 degrees of flexion, and climbing stairs needs around 85 to 90 degrees. Most daily activities can be performed comfortably with a range of 0 to 120 degrees of flexion.
How Range of Motion is Assessed
Healthcare professionals, such as physical therapists and orthopedic doctors, use a goniometer to objectively measure knee range of motion. This device is a large protractor with two movable arms used to measure joint angles. The goniometer provides precise, numerical measurements used for diagnosis and tracking a patient’s progress.
The measurement process requires aligning the goniometer with specific bony landmarks. The device’s central axis is placed over the lateral epicondyle (a prominent point on the outside of the knee). The stationary arm aligns with the hip’s greater trochanter, and the movable arm aligns with the lateral malleolus (the bony prominence on the outside of the ankle).
To measure extension, the patient straightens the leg as far as possible, and the angle is read. For flexion, the patient bends the knee, sliding the heel toward the buttocks, and the resulting angle is recorded. These measurements are recorded in degrees, providing objective data on the knee’s movement.
Common Causes of Reduced Knee Mobility
Reduced knee movement often results from an underlying condition or injury causing mechanical restriction or pain. Osteoarthritis is a frequent cause, as the degenerative disease wears away joint cartilage. This leads to stiffness, pain, and the formation of bone spurs (osteophytes) that physically impede movement, making both bending and straightening the knee difficult.
Acute injuries also severely limit mobility, especially tears to ligaments (like the ACL) or damage to the menisci. A torn meniscus can cause mechanical symptoms, such as catching or locking, preventing full straightening or bending. Swelling (joint effusion) from injury or inflammation also physically restricts movement by increasing pressure within the joint capsule.
Post-surgical stiffness is common due to the formation of excessive scar tissue, known as arthrofibrosis. This condition can occur following any knee surgery, including ACL reconstruction, resulting in a loss of extension and flexion. Additionally, tightness or weakness in surrounding muscle groups, such as the quadriceps and hamstrings, can indirectly restrict the joint’s full range of motion.
When to Consult a Professional About Limited Movement
Consult a healthcare professional if you experience a persistent loss of knee movement or concerning symptoms. A sudden inability to fully bend or straighten the knee, especially after an injury, may indicate a mechanical blockage requiring immediate attention. Professional help is needed if limited motion prevents performing essential daily activities like walking or climbing stairs.
Any movement restriction accompanied by severe or worsening pain, significant swelling, redness, or instability should be evaluated by a specialist. Stiffness that develops gradually and does not improve with rest, or noticing a popping or grinding sound during movement, also requires assessment. Orthopedic surgeons or physical therapists can accurately diagnose the limitation and develop a treatment plan.