A Summary of Knee Flexion Muscles

Author: Kevin B. Rosenbloom, C.Ped, Sports Biomechanist

Obviously, knee flexion is essential in ambulation; which makes it worth a topic for a brief summary. The following information will discuss a limited descriptions and summaries of knee flexion, the muscles contributing to the movement and some "food for thought" research regarding the muscles.

Flexion Essentials

Knee flexion is when the lower leg and foot bends and is raised posteriorly at the knee joint, while the thigh remains fixed in a stationary position. The estimated peak of the range of motion varies from 115-160° (Roaas & Andersson 1982, Gilroy et al. 2006, Pinskerova et al. 2009, Washington State DSHS 2014, Quinn 2019). The sartorius, gracilis, biceps femoris (long, short), semitendinosus, semimembranosus, gastrocnemius, plantaris and popliteus are the most significant contributors to me knee flexion (Visible Body 2019). Higher ranges of knee flexion can be achieved when the hip is flexed (Gray 1918), thus justifying more significant contribution of the sartorius and gracilis than other sources would have expected.

Superficial Posterior Compartment of the Lower Leg

Making up two-thirds of the triceps surae, the gastrocnemius is the most superficial muscle in the posterior compartment of the lower leg or calf. Two strong and flat tendons emerge from both femoral condyles to form two large heads and bodies, a medial and lateral. The slightly larger medial head is longer than the lateral one. Both bodies travel distally downwards and insert into the calcaneal tendon (Achilles), approximately half way down the lower leg before attaching to the posterior calcaneal tuberosity. Absence of the lateral head or entire muscle has been observed (Gray 1918).

In a study where biopsy samples of the lateral gastrocnemius muscles were taken from healthy young and old men and women, it should the amount of Type II muscle fibers were reduced in the old individuals, and that Type I fibers occupied the vacant space (Coogan et al. 1992). Further studies have looked into the biarticularity of the gastrocnemius muscle (Van Soest et al. 1993) and the mechanical properties of its tendons and aponeurosis (Muramatsu et al. 2001).

The plantaris is a small, fusiform muscle that sits between the gastrocnemius and the soleus, and runs the entire length of the lower leg, knee to foot. The plantaris originates on the lower supracondylar femoral ridge, superior to the lateral head of the gastrocnemius. Its insertion is also the posterior calcaneal tuberosity, alongside the Achilles tendon.

Tennis Leg, a common injury to the plantaris muscle and its tendon occurs most frequently during running or jumping with an eccentric load that applies forces across the ankle and an extended knee position. However, the plantaris muscle is susceptible to adjacent ACL injuries (Spina 2007).

The popliteus is a flat, triangular muscle that originates from a small groove on the lateral surface of the lateral femoral condyle. As it travels distally, its flat body obliquely crosses the posterior knee to the medial side where it inserts to the posterior surface of the proximal tibial shaft. The popliteus is connected with seven different structures in the knee joint region (Higgins 1895) and excessive pronation in the foot could cause weakness to the muscle (Fitzgordon 2019).

Previous Muscles

It is quite clear that the mechanisms that form the body, especially the portions of the lower extremities, are complex, with even single organisms capable of displaying multiple functions. Several muscles in the pelvis and thigh region have multiple functions and have been briefly discussed in other Kevin Orthopedic summaries. In order to avoid monotony, it is suggested to review the summaries below for additional information on other knee flexor muscles:

Semitendinosus & Semimembranosus – Hip Medial Rotation
Sartorius – Hip Flexion
Biceps femoris (long and short) – Hip Extension
Gracilis – Hip Adduction


Muscle Overview - Knee Flexors

femur tibia fibula talus calcaneus foot bones biceps femoris long short semitendinosus semimembranosus gracilis sartorius popliteus plantaris gastrocnemius calcaneal tendon sketch

Figure 1. Sketch of knee flexors (right), posterior view.


Sartorius [1]

Origin: Anterior superior iliac spine
Insertion: Medial superior tibial shaft, distal to condyle, via pes anserinus
Additional Actions: Flexion, abduction and lateral rotation at hip joint; medial rotation at knee joint

Gracilis [2]

Origin: Just lateral of the pubic symphysis and along the inferior pubic ramus
Insertion: Medial superior tibial shaft, distal to condyle, beneath sartorius’ insertions, via pes anserinus
Additional Actions: Adduction at the hip joint; medial rotation at knee joint

Biceps femoris long [3]

Origin: Pelvic ischial tuberosity, via shared tendon with semitendinosus and semimembranosus
Insertion: Lateral fibular head
Additional Actions: Extension and lateral rotation at hip joint

Biceps femoris short [4]

Origin: Femoral linea aspera
Insertion: Lateral fibular head
Additional Actions: Extension at hip joint; lateral rotation of tibia

Semitendinosus [5]

Origin: Pelvic ischial tuberosity, via shared tendon with semimembranosus and biceps femoris long head
Insertion: Medial superior tibial shaft, distal to condyle and gracilis, and beneath sartorius, via pes anserinus
Additional Actions: Medial rotation at hip joint

Semimembranosus [6]

Origin: Pelvic ischial tuberosity, via shared tendon with semitendinosus and biceps femoris long head
Insertion: Posterior surface of the medial tibial condyle
Additional Actions: Medial rotation at hip joint

Gastrocnemius [7]

Origin: Two medial and lateral heads from their respective posterior femoral condyles
Insertion: Posterior calcaneal tuberosity via achilles tendon
Additional Actions: Plantar flexion at ankle joint

Plantaris [8]

Origin: Superior to the lateral gastrocnemius head on the lower supracondylar femoral ridge
Insertion: Posterior calcaneal tuberosity, alongside the achilles tendon
Additional Actions: Plantar flexion (assist) at ankle joint

Popliteus [9]

Origin: Lateral surface of the lateral femoral condyle
Insertion: Posterior surface of the proximal tibial shaft
Additional Actions: Unlocks and medial rotation (assist) at knee joint


References & Works Cited

Barclay, T. 2018. “Anatomy Explorer,” innerbody.com. Accessed 19 Mar 2019. https://www.innerbody.com/anatomy/muscular/leg-foot.

Coogan, A. R., Spina, R. J., King, D. S., Rogers, M. A., Brown, M., Nemeth, P. M., Holloszy, J. O. 1992. “Histochemical and Enzymatic Comparison of the Gastrocnemius Muscle of Young and Elderly Men and Women,” Journal of Gerontology 47: 3: B71-76. DOI: 10.1152/jappl.1990.68.5.1896.

Fitzgordon, J. 2019. “Knee Stuff: The Popliteus Muscle,” Corewalking.com. Accessed 12 Apr 2019. https://corewalking.com/knee-stuff-popliteus-muscle/.

Gilroy, A. M., MacPherson, B. R., Ross, L. M. 2006. Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System. Thieme, New York, NY. 398-399.

Gray, H. 1918. “The Muscles and Fasciæ of the Lower Extremity,” Anatomy of the Human Body, 20th Ed. Lead & Febiger. Philadelphia & New York, USA. 482-485.

Higgins, H. 1895. “The Popliteus Muscle,” Journal of Anatomy and Physiology 29;4: 569-573. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1328401/pdf/janatphys00102-0099.pdf.

Mann, R. A., Hagy, J. L. 1977. “The Popliteus Muscle,” Journal of Bone and Joint Surgery 59-A; 7: 924-927. http://citeseerx.ist.psu.edu/viewdoc/download?doi=

Muramatsu, T., Muraoka, T., Takeshita, D., Kawakami, Y., Hirano, Y., Fukunaga, T. 2001. “Mechanical properties of tendon and aponeurosis of human gastrocnemius muscle in vivo,” Journal of Applied Physiology 90: 1671-1678. https://doi.org/10.1152/jappl.2001.90.5.1671.

Pinskerova, V., Samuelson, K. M., Stammers, J., Maruthainar, K., Sosna, A., Freeman, M. A. R. 2009. “The knee in full flexion: An anatomical study,” Journal of Bone & Joint Surgery (Br) 91-B: 830-834. https://doi.org/10.1302/0301-620X.91B6.22319.

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.

Spina, A. A. 2007. “The plantaris muscle: anatomy, injury, imaging and treatment,” Journal of the Canadian Chiropractic Association 51(3): 158-165. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1978447/.

Van Soest, A. J., Schwab, A. L., Bobbert, M. F., Van Ingen Schenau, G. J. 1993. “The Influence of the Biarticularity of the Gastrocnemius Muscle on Vertical-Jumping Achievement,” Journal of Biomechanics 26; 1: 1-8. https://pdfs.semanticscholar.org/cd53/550d29bacaab0e3023d9754a6b6e28ce7390.pdf.

Visible Body. 2019. “Muscle Premium,” VisibleBody.com. Purchasable Application. Accessed 21 Feb 2019.

Washington State DSHS. 2014. “Range of Joint Motion Evaluation Chart,” Washington State Department of Social & Health Services. Accessed 20 Mar 2019. https://www.dshs.wa.gov/sites/default/files/FSA/forms/pdf/13-585a.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.

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