
The quadriceps muscle group, showing the VMO.
VMO is an abbreviation of 'vastus medialis obliquus', which is a part of the quadriceps (quads) muscle. Page updated June 2023 by Dr Sheila Strover (Clinical Editor)

The quadriceps muscle group, showing the VMO.
Although they come together and unite into a single quadriceps tendon at the patella, the vastus medialis seems to be more independent than the rest of the group.
Peer-reviewed paperQuote from peer-reviewed paper:
Citation: Sawy MME, Mikkawy DMEE, El-Sayed SM, Desouky AM. Morphometric analysis of vastus medialis oblique muscle and its influence on anterior knee pain. Anat Cell Biol. 2021 Mar 31;54(1):1-9. doi: 10.5115/acb.20.258. PMID: 33262319; PMCID: PMC8017455.
Quote from peer-reviewed paper:
Citation: Toumi H, Poumarat G, Benjamin M, Best TM, F'Guyer S, Fairclough J. New insights into the function of the vastus medialis with clinical implications. Med Sci Sports Exerc. 2007 Jul;39(7):1153-9. doi: 10.1249/01.mss.0b013e31804ec08d. Erratum in: Med Sci Sports Exerc. 2008 May;40(5):982. Best, Thomas [corrected to Best, Thomas M]. PMID: 17596784.
The reason for this is that the muscle fibres very readily become inhibited, and struggle to 'fire' when the patient tries to straighten the leg.
Peer-reviewed papersQuote from peer-reviewed paper:
Citation: Pal S, Besier TF, Gold GE, Fredericson M, Delp SL, Beaupre GS. Patellofemoral cartilage stresses are most sensitive to variations in vastus medialis muscle forces. Comput Methods Biomech Biomed Engin. 2019 Feb;22(2):206-216. doi: 10.1080/10255842.2018.1544629. Epub 2018 Dec 31. PMID: 30596523; PMCID: PMC6468979.
Simple practical advice for strengthening the VMO muscle.
Active discussion about VMO exercising.