Stiffness refers to a sensation of reduced flexibility, tightness, or resistance to movement in muscles, joints, or connective tissues, making it difficult or uncomfortable to bend, stretch, or move normally. It often feels like the affected area is “locked,” “frozen,” or heavy, and may improve with gentle movement (as in morning stiffness that loosens after activity) or worsen with prolonged immobility. Common localized forms include joint stiffness (e.g., in arthritis—rheumatoid with prolonged morning stiffness >30–60 minutes, or osteoarthritis after inactivity), muscle stiffness (from overuse, strain, poor posture, dehydration, electrolyte imbalance, or delayed-onset muscle soreness after exercise), or neck/back stiffness (from tension, whiplash, or meningitis with severe, board-like rigidity). Generalized stiffness can occur in systemic conditions such as fibromyalgia (widespread achiness and tightness), hypothyroidism (myxedema causing doughy muscle feel), Parkinson’s disease (rigidity with “cogwheel” resistance), infections (e.g., flu or tetanus with muscle spasms), or medication side effects (e.g., statins causing myopathy). It frequently accompanies pain, swelling, fatigue, weakness, or reduced range of motion, and can be acute (short-term, e.g., after sleeping awkwardly) or chronic (persistent, impacting daily function).