Muscle Strain: The most common cause of thigh and calf stiffness is muscle strain, often resulting from overuse, injury, or sudden contraction of the muscles in these areas. This condition is prevalent among athletes and individuals who engage in strenuous physical activities.

Delayed Onset Muscle Soreness (DOMS): This condition occurs after unfamiliar or strenuous exercise, leading to muscle stiffness and pain, typically 24 to 48 hours after the activity.
Overuse Injuries: Conditions like shin splints in the calf or thigh splints can cause stiffness and pain due to repetitive stress on the muscles and bones.
Benign Leg Cramps: Frequent or severe leg cramps can lead to stiffness in the affected muscles, often occurring at night.

Deep Vein Thrombosis (DVT): Although less common, DVT is a potentially life-threatening condition that can cause leg stiffness, swelling, and pain. It requires immediate medical attention to prevent pulmonary embolism.
Compartment Syndrome: A serious condition resulting from increased pressure within a muscle compartment, leading to pain, stiffness, and potentially permanent damage if not promptly treated.
Neurological Conditions: Such as peripheral neuropathy or radiculopathy, which can cause stiffness, pain, or weakness in the legs due to nerve damage or compression.

Dermatomyositis: An inflammatory disease characterized by muscle weakness and skin rash, which can also cause stiffness in the affected areas.
Polymyositis: An inflammatory muscle disease that leads to muscle weakness and stiffness, more commonly affecting the proximal muscles but can involve the lower limbs.
Eosinophilia-Myalgia Syndrome: A rare condition associated with muscle pain, stiffness, and elevated eosinophil counts, often linked to certain medications or toxins.