I only partly agree with previous answer and would like to contrast some of the points suggesting that "muskuloskeletal disorders is a term used in occupational medicine".
According to the International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD-10) of the WHO (which is used by hospitals, health prof. and insurances to "code" a disease), there is a part called "Diseases of the musculoskeletal system and connective tissues" (Chapter 13), diseases are then categorized as follows:
- Arthropathies (eg infectious arthropathies, inflammatory polyarthropathies, arthorsis)
- Systemic connective tissue disorders (eg. systemic lupus erythematosus, dermatomyositis, polyarteritis nodosa)
- Dorsopathies (eg spondylopathies)
- Soft tissue disorders (eg infectious myopathies, mitochondrial myopathies)
- Osteopathies and chondropathies (eg osteoporosis)
- Other disorders of the musculoskeletal system and connective tissue
While some of the listed diseases in each of the above mentioned categories might be work related (or occupational), some may also be congenital (genetic) or acquired (drug associated, infectious, de novo, traumatic, etc...). Following link provides the extensive classification with the corresponding diseases: http://apps.who.int/classifications/icd10/browse/2010/en#/XIII
So MSD is a main category, which entails some subcategories as listed above. Some diseases might be "pure" muscular, other pure "skeletal" (although in most cases, this leads to muscular problems) and some might be both "muskuloskeletal". In general, the medicine speciality dealing with all these disorders is rheumatology and from my current experience, the rheumatology unit in my hospital treats a significant proportion of diseases that are not occupational related.
Some myopathies (e.g mitochondrial) are also listed under Chapter VI, "Diseases of the nervous system" as some of them, while affecting the muscles, are also associated with neurological disorders (stroke, epilepsia).