Which muscle should be biopsied?

Muscle biopsy usually takes muscle samples primarily from the biceps brachii in the upper limbs and the lateral portion of the quadriceps femoris in the lower limbs.

Muscle biopsy is an examination performed under light or electron microscopy, in which a physician removes a small piece of muscle tissue—typically the size of a mung bean to a soybean—from certain parts of the body to diagnose or differentiate neuromuscular diseases. Depending on the nature of the myopathy and the stage of disease progression, muscle biopsy remains an invasive procedure, yet currently no other test—including genetic testing—can fully replace it. Muscle biopsy is particularly useful for diagnosing metabolic myopathies, as it not only provides histological evidence but also reveals underlying biochemical changes, such as in mitochondrial myopathy and lipid storage myopathies. Most myopathies predominantly affect the proximal muscles of the limbs; therefore, clinically, the biceps brachii in the upper limbs and the lateral quadriceps femoris in the lower limbs are commonly selected as primary biopsy sites. For acute myopathies such as polymyositis, areas with obvious pain or muscle weakness should be chosen. For chronic myopathies, sites with moderate damage are preferred, since severely atrophied muscles often have muscle fibers replaced by fatty tissue.

In addition, regions showing significant abnormalities on electromyography (EMG) can serve as reference points for biopsy selection. However, if the EMG site itself is unsuitable for biopsy, the corresponding contralateral muscle may be selected instead.