This video is going to explore the fabella, a bone that's found just behind the knee in about 20% of people. The fabella is a type of sesamoid bone, a type of bone which forms within the tendon of a muscle, just like the kneecap. In this case, it forms within the gastrocnemius tendon in response to mechanical stress and may play biomechanical role in bending your knee.

What I found fascinating was that the fabella disappeared in human evolution around the time we transitioned from quadrupedal to bipedal mammals. HOWEVER, a recent study found fabellae to be present in 11.2% of the population in 1918 and 39% of the population in 2018. This is a three and a half fold increase over the past 100 years! Meanwhile the rates of other sesamoid bones have remained constant... So why would the prevalence of the fabella be increasing, if evolution deemed it unnecessary in bipedal mammals?

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Timestamps:
0:00 Intro
0:36 How the fabella forms
2:16 The function of the fabella
3:49 Evolution of the fabella
4:52 Pathologies

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