The sudden onset of knee stiffness is the feeling of a restricted range of motion, making it difficult to fully bend or straighten the leg. This common experience can range from a minor annoyance to a sign of a serious underlying issue. Stiffness is often caused by inflammation, mechanical changes in the joint, or a thickening of the joint’s natural lubrication. Understanding the nature and duration of this stiffness is the first step in identifying its source.
Temporary Stiffness from Activity and Inactivity
The most frequent reasons for acute knee stiffness relate to how the joint responds to rest or overexertion. This temporary stiffness often resolves quickly with movement. The knee joint contains synovial fluid, a viscous lubricant that cools and thickens when the joint is held still for an extended period, a phenomenon sometimes called “gelling.”
This gelling effect causes stiffness felt upon waking or after prolonged sitting, such as during a long car ride. Movement warms and thins the fluid, allowing joint surfaces to glide smoothly again, typically relieving stiffness within minutes. Muscles surrounding the knee can also become tight from inactivity or mild overexertion, restricting movement.
An increase in physical activity without adequate preparation can lead to minor muscle strains or temporary inflammation that manifests as stiffness. Even mild dehydration can contribute to joint discomfort. These mechanical and fluid-related causes are usually self-limiting and improve rapidly once the joint is active and warmed up.
Localized Inflammation and Acute Injury
When sudden stiffness is accompanied by specific pain, swelling, or warmth, it often points to localized tissue irritation or a minor structural injury. The knee is surrounded by multiple soft tissues that can become acutely inflamed from overuse or direct trauma.
Tendinitis, such as patellar tendinitis, involves inflammation of the fibrous tissues connecting muscle to bone. This irritation causes stiffness and pain, particularly after repetitive activities like running or jumping, restricting kneecap movement. Bursitis occurs when the bursae, the small, fluid-filled sacs that cushion the joint, become inflamed.
Inflammation of the bursae can lead to localized swelling and stiffness in specific areas, such as the front of the kneecap (prepatellar bursitis) or the inner side (pes anserine bursitis). Bursitis often results from repetitive kneeling or direct pressure, making it painful to move the knee through its full range.
A sudden twisting motion while the foot is planted can cause a tear or irritation of the meniscus, the C-shaped cartilage shock absorber. This injury often results in immediate stiffness, sometimes accompanied by a popping sensation or a feeling that the knee is catching or locking. Even a minor sprain of the collateral ligaments (MCL or LCL) can cause swelling and subsequent stiffness.
Systemic and Progressive Medical Factors
If knee stiffness is sudden, severe, involves more than one joint, or is accompanied by other body-wide symptoms, the cause may be a systemic or progressive medical condition. Acute flares of inflammatory arthritis, such as Rheumatoid Arthritis (RA) or Psoriatic Arthritis (PsA), can cause sudden, intense stiffness. This stiffness is characteristically prolonged, often lasting for an hour or more in the morning, differentiating it from temporary gelling stiffness.
Gout is a common cause of sudden, severe joint inflammation and stiffness, resulting from the accumulation of uric acid crystals within the joint space. A gout flare is marked by extreme pain, redness, swelling, and skin that is hot to the touch, with rapid onset. While most commonly associated with the big toe, gout can affect the knee joint, creating a sudden inflammatory crisis.
Osteoarthritis (OA), the degenerative “wear-and-tear” form of arthritis, can experience acute exacerbations causing sudden, noticeable stiffness and pain. Although OA is progressive, symptoms can flare up due to minor trauma, overactivity, or changes in weather or barometric pressure. Unlike inflammatory arthritis, OA stiffness is generally less prolonged in the morning but worsens with activity throughout the day.
Steps for Relief and When to Seek Help
For mild, sudden knee stiffness, initial home care focuses on reducing inflammation and restoring mobility. The R.I.C.E. protocol—Rest, Ice, Compression, and Elevation—is an effective first-line treatment for non-severe, acute stiffness or minor injuries. Apply a cold pack to the affected area for 15 to 20 minutes several times a day to reduce swelling and discomfort.
Gentle movement and low-impact exercises, such as swimming or cycling, help circulate the synovial fluid and prevent further stiffening. Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can manage pain and reduce localized inflammation.
If home care does not provide relief, or if certain red flag symptoms appear, professional medical help is necessary. Seek immediate medical attention if you experience an inability to bear weight, a severe sudden onset of pain that does not subside, or an obvious deformity. Other urgent warning signs include significant, rapidly worsening swelling or systemic symptoms like fever and chills, which could indicate an infection. If stiffness persists beyond a week of home care, a medical evaluation is warranted.