Hot from the desk of Cool New Things: in a pre-published article submitted to the journal Clinical Anatomy (2016), a group of researchers in Switzerland have determined they have identified a new muscle in the thigh! This muscle becomes part of the quadriceps groups, beginning in the upper-outer part of the thigh and travelling to the knee.
A bit like when you learned Pluto was no longer a member of the planets, anatomy very rarely produces entirely new structures, or redefines known parts of the body. Variations are often noted, but identifying a new muscle is unique. So let’s outline what is known about this new muscle.
The image at right is from Grob et al. (2016). You can see the quadriceps as we previously understood them (left image) vs the quadriceps with the new TVI muscle included (right, highlighted in red). For reference, the inside of your knee is close to 6, and the outside of your hip is close to 4. (1- TVI 2- VL 3- VI 4- TFL 5- RF 6- VM)
WHAT DID WE THINK BEFORE?
> The quadriceps make up the muscles covering the front of your thigh.
- Vastus Lateralis (VL)
- Vastus Intermedius (VI)
- Vastus Medialis (VM)
- Rectus Femoris (RF)
> VL, VI, and VM each begin at the upper part of the femur, and travel down the front of the thigh to the patella — your kneecap — where they attach as one big tendon — the quadriceps tendon (image at right).
> The RF begins higher up, attaching to part of the pelvis before travelling on top of the other three muscles until it also reaches the patella as part of the quadriceps tendon.
> The purpose of these muscles is to extend your knee (eg. a kicking motion), control the patella (kneecap), and the RF also helps with hip flexion (eg. high knees)
WHATS NEW? Tensor of the Vastus Intermedius
> The new muscle is called the Tensor of the Vastus Intermedius (TVI).
> The TVI begins at the upper, outer part of the femur, between the VL and VI origins.
> The muscle itself is actually quite short, but it continues down the leg as a thin aponeurosis (basically a tendon) until it also becomes part of the quadriceps tendon and attaches to the patella (specifically the inside, or medial, aspect of the patella). See #1 in the first image at the top of the page for the TVI path.
HOW COULD EVERYONE MISS THIS?
> The study’s authors suggest a few reasons
- This part of the leg is a rare area for surgical intervention, so it is unlikely many surgeons have needed to study this area in GREAT GREAT detail.
- The muscle bellies of of VL, VI, and TVI are very close to each other, covered in a complex organization of nerves and blood vessels — so unless a detailed study was required, it could be easily missed
- The actual presentation of this muscle changes a bit from person to person — so even if somebody did notice an unusual finding, they are less likely to document and compare it between people
DOES EVERYBODY HAVE THIS MUSCLE?
> Yes — sort of.
> While there are references to similar muscle structures in older anatomical studies, and the TVI muscle was found in every specimen (26 legs) examined by this group, the variation between people noted above means your TVI might not be exactly the same as my TVI. In fact, the authors note that the TVI in my right leg could very well be slightly different than the TVI in my left leg!
> Five identified variations
- Independent type (42%) where the muscle belly and its tendon (aponeurosis) is unique to itself at all times (no blending with other muscles).
- VI-Type (23%) where the TVI tendon shares an aponeurosis with the VI.
- VL-Type (19%) where the TVI tendon shares an aponeurosis with the VL.
- Common type (15%) where the aponeurosis between the VL, VI, and TVI is indistinguishable — they all share a common one.
- Two muscle bellies (19%) where the muscle belly is made up of two or more smaller pieces of muscle instead of one single muscle.
HOW CAN YOU BE SURE IT ISN’T JUST A VARIATION OF THE OTHER QUADRICEPS MUSCLES?
- Consistent Origin: Regardless of how the tendon travels, every specimen studied by this group had an identified TVI muscle belly.
- Consistent Insertion: Every specimen also had a unique presence in the middle layer of the patellar tendon (where it attaches to the kneecap) — so even with the five variations noted earlier as the TVI passes down the leg, it is always clearly distinguished at the beginning (closer to the hip) and the end (closer to the knee).
- Distinct Nerves and Blood Vessels: The TVI is innervated by independent branches of the femoral nerve and is vascularized through unique branches of the lateral circumflex femoral artery. A unique and consistent pattern of nerves and blood vessels should only be present in individual structures.
OK I’M CONVINCED — WHAT PURPOSE WOULD IT SERVE?
> Though probably best left to future research on the TVI, the authors do suggest a couple of likely functions:
- By way of its basic orientation — travelling down the outside of the thigh from the hip, crossing the quadriceps tendon on a diagonal to reach the inside aspect of the kneecap — it is probable the TVI plays a role in patellar control. That is, it works to balance out the much larger quadriceps muscles found at the inside of the knee, so that the kneecap ultimately moves straight up and down. Or,
- As the TVI aponeurosis is often fused or closely related to the VI muscle, it may exert tension on this muscle — contributing to the VI function as well. Hence the name: “tensor of the vastus intermedius”.
Dr. Jim Gilliard is a chiropractor in Burlington, ON at Endorphins Health and Wellness Centre — located in the Burlington Professional Centre at 3155 Harvester Road, Suite 406. If you have questions, comments, or wish to book an appointment, please feel free to contact him at your convenience.
- Grob K, Ackland T, Kuster MS, Manestar M, Filgueira L. A newly discovered muscle: The tensor of the vastus intermedius. Clin Anat. 2016;Pre-Press, available on-line http://www.ncbi.nlm.nih.gov/pubmed/26732825.