Weakness (also called asthenia or muscle weakness) is a subjective or objective reduction in strength, energy, or ability to perform physical tasks, ranging from generalized (affecting the whole body) to localized (specific muscles or limbs). It can manifest as profound fatigue making even simple activities like rising from a chair, climbing stairs, lifting objects, or holding arms up difficult; a heavy, limp feeling in limbs; trembling or shakiness; or sudden giving-way of muscles. Causes are diverse and include neurological issues (e.g., stroke, multiple sclerosis, myasthenia gravis, Guillain-Barré syndrome, peripheral neuropathy), muscular disorders (e.g., muscular dystrophy, polymyositis), metabolic/electrolyte imbalances (e.g., low potassium, low blood sugar, dehydration), systemic illnesses (e.g., infections like flu or COVID, anemia, hypothyroidism, cancer, chronic fatigue syndrome), medication side effects (e.g., statins, corticosteroids), deconditioning from prolonged inactivity, poor sleep, depression/anxiety, or acute events like transient ischemic attack. It may be constant, episodic, progressive, or fluctuating, often accompanied by other symptoms like numbness, pain, cramps, shortness of breath, weight loss, or cognitive fog depending on the underlying cause.