Author: Kevin B. Rosenbloom, C.Ped, Sports Biomechanist
Extension at the knee joint is a topic that can be quite extensive. However, the following information will discuss only a limited selection of descriptions and summaries of knee flexion, the muscles contributing to the movement and some research regarding the muscles.
Knee extension is the lowering of the leg at the knee joint from a flexed knee position, thus straightening the knee and entire leg. Occasionally knee extension can overextend anteriorly, past straightening (neutral, weight-bearing stance) into hyperextension. The range of motion is estimated to reach 0° (Soucie et al. 2010, Quinn 2019), but in a study of ROMS in a group of individuals, a mean hyperextension of 1.6° has been observed (Roaas & Andersson 1982). It should also be noted that in a study of more than 600 people, there was a trend of increased flexibility in older aged groups, with 45-69 possessing the closest means to 0° (Soucie et al. 2010). The quadricep group (rectus femoris, vastus lateralis, vastus intermedius, vastus medialis) and the articularis genu are the most significant contributors to knee extension (Visible Body 2019). Some sources have designated the articularis genu of low importance to the motion but will be addressed in this summary.
The Quadricep Muscles of the Thigh
The vastus lateralis muscle is the largest part of the quadriceps. It originates from the greater trochanter of the femur and the proximal linea aspera, while bordering the lateral gluteal tuberosity. As its larger body descends distally down the femur, fibers from the muscle can often be seen encroaching with the biceps femoris short head and inserts into the shared quadriceps femoris tendon that encapsulates the patella. Beginning approximately at the age of 25, men can begin displaying a the process of type II muscle fiber loss (fewer and smaller) and by the age of 50 the vastus lateralis muscle can be reduced by 10% (Lexell et al. 1988). In a separate study, male participants were observed to have larger type IIA muscle fibers, while women displayed larger Type I fibers (Staron et al. 2000).
The vastus intermedius muscle originates at the proximal anterior and lateral femoral shaft, extending to the midshaft. Its body lays just beneath the superficial rectus femoris and between the v. lateralis and v. medialis. The v. intermedius inserts into a deep portion of the shared quadriceps femoris tendon. Current research has suggested that the vastus intermedius has a companion between it and the v. lateralis, named the tensor vastus intermedius. In 22 individuals from one study (Grob et al. 2016) and 36 from a Southern Indian study (Veeramani & Gnanasekaran 2017), the researchers have found several variations of this new muscle. It is described to originate at the anterior greater trochanter, just proximal to the other vasti muscles, and sit beneath the rectus femoris. Its main body is small and occupies the space just proximal to v. intermedius and just medial to the v. lateralis, with a long, thin tensor tendon travelling distally to the patella between them. However, previous case studies have described this new muscle as only a lamella and a mere morphological variation of the vastus lateralis muscle (Willan et al. 1990).
The vastus medialis originates at the intertrochanteric line of the femur and its medial linea aspera. Its origins also borders slightly along the adductor longus and magnus. As its body travels distally down the femur, it finally inserts on the medial border of the patella and the shared quadriceps femoris tendon. Biomechanical researchers have studied portions of the vasti muscles, including the vastus medialis obliquus (Hanten & Schulthies 1990, Mirzabeigi et al. 1999). The v. medialis obliquus (genu) is the most distal segment of the main v. medialis muscle body. However, the validity of some research is a topic of some concern, especially on the v. medialis obliquus and needs to be observed cautiously; because further studies have shown that this portion of the v. medialis may not truly exist (Smith et al. 2009).
Not unlike several different muscles in the body, the rectus femoris have several functions. As a result the muscle has previously been discussed in a different Kevin Orthopedic summary. If interested in reviewing this muscle, please visit the summary for hip flexion.
The articularis genu is a small muscle or muscle group with several variations of structure (Gray 1918). It is found beneath the vastus intermedius and is often distinct enough from the v. intermedius, but can be seen blended into the muscle. It has also been found to be in several muscular bundles. However, it consistently has been found to arise from the anterior surface of the distal femur, just proximal to the epicondyles, and inserting into the synovial membrane of the knee joint. During knee extension, the articularis genu pulls the suprapatellar bursae of the knee (Gray 1918, Blackburn & Craig 1980), and also helps the synovial membrane not becoming caught between the patella and femur within the knee joint (Agur & Dalley 2009).
Muscle Overview - Knee Extensors
Figure 1. Sketch of knee extensors (right), anterior view.
Rectus femoris 
Vastus lateralis 
Vastus intermedius 
Vastus medialis 
Articularis genu 
References & Works Cited
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Gray, H. 1918. “The Muscles and Fasciæ of the Lower Extremity,” Anatomy of the Human Body, 20th Ed. Lead & Febiger. Philadelphia & New York, USA. 470-471.
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Lexell, J., Taylor, C. C., Sjöström, M. 1988. “What is the cause of the ageing atrophy? Total number, size and proportion of different fiber types studied in whole vastus lateralis muscle from 15- to 83-year-old men,” Journal of the Neurological Sciences 84; 275-294. https://doi.org/10.1016/0022-510X(88)90132-3.
Mirzabeigi, E., Jordan, C., Gronley, J. K., Rockowitz, N. L., Perry, J. 1999. “Isolation of the Vastus Medialis Oblique Muscle During Exercise,” American Journal of Sports Medicine 27; 1: 50-53. https://doi.org/10.1177/03635465990270011601.
Platzer, W. 2004. Color Atlas of Human Anatomy, Vol. 1: Locomotor System 5th Ed. Thieme. New York, USA.
Quinn, E. 2019. “Generally Accepted Values for Normal Range of Motion (ROM) in Joints,” verywellhealth.com. Accessed 19 Mar 2019. https://www.verywellhealth.com/what-is-normal-range-of-motion-in-a-joint-3120361.
Roaas, A., Andersson, G. B. J., 1982. “Normal Range of Motion of the Hip, Knee and Ankle Joints in Male Subjects, 30-40 Years of Age,” Acta Orthopaedica Scandinavica, 53:2, 205-208. https://www.tandfonline.com/doi/abs/10.3109/17453678208992202.
Smith, T. O., Nichols, R., Harle, D., Donell, S. T. 2009. “Do the vastus medialis obliquus and vastus medialis longus really exist? A systematic review,” Clinical Anatomy 22; 2: 183-199. https://doi.org/10.1002/ca.20737.
Soucie, M., Wang, C., Forsyth, A., Funk, S., Denny, M., Roach, K. E., Boone, D., Hemophilia Treatment Center Network. 2010. “Range of motion measurements: reference values and a database for comparison studies. Haemophilia. https://doi.org/10.1111/j.1365-2516.2010.02399.x.
Staron, R. S., Hagerman, F. C., Hikida, R. S., Murray, T. F., Hostler, D. P., Crill, M. T., Ragg, K. E., Toma, K. 2000. “Fiber Type Composition of the Vastus Lateralis Muscle of Young Men and Women,” Journal of Histochemistry & Cytochemistry 48(5): 623-629. https://journals.sagepub.com/doi/pdf/10.1177/002215540004800506.
Veeramani, R., Gnanasekaran, D. 2017. “Morphometric study of tensor of vastus intermedius in South Indian population,” Anatomy & Cell Biology 50(1): 7-11. https://doi.org/10.5115/acb.2017.50.1.7.
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Willan, P. L. T., Mahon, M., Golland, J. A. 1990. “Morphological variations of the human vastus lateralis muscle,” Journal of Anatomy 168: 235-239. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1256904/pdf/janat00042-0232.pdf.
Kevin B. Rosenbloom, C.Ped, Sports Biomechanist
Kevin B. Rosenbloom, founder and president of Kevin Orthopedic, is a renowned certified pedorthist and sports biomechanist practicing in Santa Monica, CA. With his continuing research on the historical development of foot and ankle pathologies, comparative evolution of lower extremities and the modern environmental impacts on ambulation, he provides advanced biomechanical solutions for his patients and clients.