Myalgia

Myalgia or muscle pain is a painful sensation evolving from muscle tissue. It is a symptom of many diseases. The most common cause of acute myalgia is the overuse of a muscle or group of muscles; another likely cause is viral infection, especially when there has been no injury.
Long-lasting myalgia can be caused by metabolic myopathy, some nutritional deficiencies, ME/CFS, fibromyalgia, and amplified musculoskeletal pain syndrome.

Causes

The most common causes of myalgia are overuse, injury, and strain. Myalgia might also be caused by allergies, diseases, medications, or as a response to a vaccination. Dehydration at times results in muscle pain as well, especially for people involved in extensive physical activities.
Muscle pain is also a common symptom in a variety of diseases, including infectious diseases, such as influenza, muscle abscesses, Lyme disease, malaria, trichinosis or poliomyelitis; autoimmune diseases, such as celiac disease, systemic lupus erythematosus, Sjögren's syndrome or polymyositis; and gastrointestinal diseases, such as non-celiac gluten sensitivity and inflammatory bowel disease.
The most common causes are:

Overuse of a muscle is using it too much, too soon or too often. One example is repetitive strain injury.

The most common causes of myalgia by injury are sprains and strains.

When the cause of myalgia is unknown, it should be treated symptomatically. Common treatments include heat, rest, paracetamol, NSAIDs, massage, cryotherapy and muscle relaxants.