Olympic 800 m champion Keely Hodgkinson has revealed that she has an unusual hamstring anatomy, identified on a magnetic resonance imaging (MRI) scan after a hamstring injury. Speaking to reporters in September 2026, Hodgkinson explained that the scans showed she has two tendons where most people have only one, a variation that may have influenced both how her injuries presented and how she recovered.
While the story has attracted considerable media attention, anatomical variants of muscles and tendons are well recognised in medicine and sports science. They are a fascinating example of how developmental biology, genetics and biomechanics interact to influence athletic performance.
Human anatomy is more variable than most people realise
Medical textbooks are often forced to describe a "standard" human anatomy. In reality, substantial variation exists throughout the body. Muscles, tendons, blood vessels and nerves frequently differ in size, branching pattern, attachment site and overall shape between individuals 1,2.
Many of these variants go undetected for life because they cause no symptoms. Advances in MRI and musculoskeletal ultrasound have increasingly shown that healthy people often have structural differences that would previously have gone unnoticed 3.
The hamstrings are no exception. The group consists of three main muscles:
- Biceps femoris
- Semitendinosus
- Semimembranosus
Together they drive hip extension, knee flexion, sprint acceleration and elastic energy storage during running 4.
Hodgkinson has not publicly said which tendon is affected. However, the semitendinosus tendon is known for a particularly high degree of anatomical variability, including accessory bands, duplicated insertions and split (bifid) tendons 5,6, making it a plausible candidate.
How does a double tendon develop?
To understand how such a variation occurs, we need to go back to embryonic development.
Human limbs begin forming between weeks four and eight of gestation. During this period, cells within the developing limb bud undergo highly coordinated differentiation into bones, muscles, ligaments and tendons 1,7.
Tendon formation is controlled by specialised progenitor cells that express the transcription factor scleraxis (SCX), widely regarded as a master regulator of tendon development 8,9.
During normal development, tendons emerge, lengthen, divide, fuse and remodel before reaching their final configuration. Small alterations in these events may result in:
- Accessory tendons
- Split tendons
- Additional insertion points
- Extra tendon bands
- Variations in tendon thickness and attachment pattern
These differences are generally considered developmental variants rather than disease 1,8,9. Hodgkinson herself has said the variation is simply how she is made, which fits with a congenital variant present since birth.
Could genetics be responsible?
The precise developmental mechanism behind duplicated hamstring tendons is unknown. However, genetics plays a substantial role in how the musculoskeletal system is patterned before birth. Several genes regulate tendon development and organisation, including:
- SCX (scleraxis)
- TNMD (tenomodulin)
- MKX (Mohawk)
- Growth factor signalling pathways, including the TGF-β and FGF systems 8,9
Minor variations in these pathways may influence tendon architecture and branching.
Importantly, there is currently no evidence that a duplicated hamstring tendon is linked to a specific inherited mutation or recognised genetic syndrome. Most cases are considered normal anatomical variants arising during development 8,9.
How common is it?
True duplicated hamstring tendons have not been studied extensively, so prevalence is hard to estimate. However, studies of the semitendinosus tendon consistently report frequent anatomical variation 5,6.
Accessory bands and variant insertions have been described in numerous cadaver studies and in surgical series involving anterior cruciate ligament (ACL) reconstruction. Some studies suggest accessory tendon structures may occur in a sizeable minority of people, although figures vary considerably between populations and classification systems 5,10.
An additional tendon is therefore probably uncommon, but not exceptionally rare. What is rare is identifying one in an elite Olympic athlete, with enough clinical information to consider its effect on performance.
Performance advantage: built-in biological redundancy?
Perhaps the most intriguing question is whether a duplicated tendon could offer a competitive advantage.
In engineering, redundant systems are often more resilient because they provide a backup when one component fails. If an athlete has two functioning tendon bundles connecting a muscle to its attachment point, injury to one may not completely stop force being transmitted.
This fits closely with Hodgkinson's own account. She has described how, when one tendon tore, she still had the other, allowing her to keep training with an adjusted approach.
Potential benefits could include:
- Preserving partial muscle function after injury
- Smaller loss of force production
- Ability to continue modified training
- Faster potential return to competition
- Lower risk of complete mechanical failure
It is important to stress that these remain hypotheses. No controlled studies have looked at elite athletes with duplicated hamstring tendons.
Potential disadvantages
An anatomical variant is not always beneficial. A split tendon could, in theory, change how force is distributed. If the total collagen is divided between two bundles, each may have a smaller cross-sectional area than a conventional single tendon 11.
Potential disadvantages include:
- Lower tensile strength in each individual tendon segment
- Altered loading patterns during sprinting
- Greater susceptibility to partial tears
- Higher risk of delayed diagnosis
Hodgkinson has acknowledged this herself, saying her team is still working out whether the variant is a strength or a weakness. That reflects a basic principle of sports medicine: anatomical differences are rarely purely positive or purely negative.
Other reported cases
The medical literature contains many examples of unusual semitendinosus anatomy. Researchers have reported:
- Accessory tendon insertions into the soleus fascia 5
- Multiple distal tendon branches 6
- Unusual pes anserinus anatomy 10
- Partial tendon duplication 5,6
- Rare semitendinosus injuries involving accessory tendon components in athletes 11
These reports show that hamstring anatomy is considerably more complex and variable than traditionally assumed. Most people never know they have such a variant, but high-performance sport increasingly relies on detailed MRI, which makes these discoveries more common.
The future of personalised sports medicine
Hodgkinson's case highlights an important shift in elite sport. Injury management has traditionally been based on population averages. Increasingly, clinicians recognise that every athlete has unique anatomy, biomechanics and genetics.
Modern sports medicine now draws on:
- High-resolution MRI
- Musculoskeletal ultrasound
- Motion capture analysis
- Genomic testing
- Individualised rehabilitation programmes
This personalised approach lets clinicians tailor treatment to each athlete's biology rather than relying on textbook assumptions. Hodgkinson's anatomy is a powerful reminder that elite performance is shaped not only by training and dedication, but by the remarkable diversity of human biology.
Conclusion
Keely Hodgkinson's "double tendon" is most likely a congenital anatomical variation that arose during embryonic tendon development. Although the exact structure involved has not been formally described, published studies show that accessory and duplicated hamstring tendons are recognised variants of normal human anatomy 5,6.
Whether it has contributed to her extraordinary success remains uncertain. It may, however, have influenced how she responds to injury by preserving function after partial tendon damage. As sports medicine becomes more personalised, cases like hers offer valuable insight into the relationship between anatomy, performance and resilience.
Personalised data for active people
Anatomy is only one part of the picture. For athletes and active people, blood biomarkers such as iron stores, vitamin D, hormones and inflammation markers can add objective data to training and recovery. Our Athletic Performance Panel is designed with this in mind.
This article is informational. Chxhealth is a biomarker and genetic data provider. We do not diagnose, treat or prescribe. If you have concerns about an injury or your health, speak to a qualified healthcare professional.
References
- Tickle C. How the embryo makes a limb: determination, polarity and identity. J Anat. 2015;227(4):418–430.
- UNSW Embryology. Lecture: Limb Development. University of New South Wales.
- Sivasundaram L, Matcuk GR Jr, White EA, Hatch GF III, Patel DB. Partial semitendinosus tendon tear in a young athlete: a case report and review of the distal semitendinosus anatomy. Skeletal Radiol. 2015;44:1051–1056.
- Moźdżeń K, Fibiger G, Chowaniec M, et al. Comprehensive systematic review and meta-analysis of the semitendinosus muscle and tendon. Knee. 2026.
- Olewnik Ł, Podgórski M, Polguj M. An unusual insertion of an accessory band of the semitendinosus tendon: case report and review of the literature. Folia Morphol. 2020;79(3):645–648.
- Morphology of the accessory band of the semitendinosus tendon. Surg Radiol Anat. 2026.
- TeachMeAnatomy. Development of the limbs. 2025.
- UNSW Embryology. Musculoskeletal System: Tendon Development.
- Bobzin L, Roberts RR, Chen HJ, Crump JG, Merrill AE. Development and maintenance of tendons and ligaments. Development. 2021;148(8):dev186916.
- Candal-Couto JJ, Deehan DJ. The accessory bands of gracilis and semitendinosus: an anatomical study. Knee. 2003;10(4):325–328.
- Sivasundaram L, Matcuk GR Jr, White EA, Hatch GF III, Patel DB. Distal semitendinosus injuries in athletes and anatomical implications. Skeletal Radiol. 2015.
