Tendon Development in Childhood: The Case for Lots and Lots of Movement

This is a summary of Chapter Two in Jake Turra’s Tendon Book.

Tendons are not fully developed until around age 15 to 17. The pulling of bones on one end and muscles on the other trigger their development. Tendons lengthen and thicken as bones do and get bigger and stiffer as muscles grow and get stronger. Interestingly, skeletal muscle is continually replaced over one’s lifetime, but tendon tissue, after being fully developed, remains relatively inactive. This was discovered by comparing Carbon-14 levels in muscles versus tendons following a surge of atmospheric Carbon-14 during a period of nuclear testing in the 1950s. Carbon-14 content in tendons remained high even as atmospheric and muscular levels declined. 

During tendon development, several physiological changes occur. Tendons start out small and short and gradually become larger and longer. The cellular architecture of tendons changes, too: at first, there are many stem cells and immature tendon cells, called tenoblasts. These cells are housed in a gel. The mechanical stress of growing muscles and bones pulling on tendons cause this cell population to shift to less stem cells and tenoblasts and more mature tendon cells, called tenocytes. Gel decreases as collagen production increases. At first, tendons mostly have immature type 3 collagen fibrils in the tendon gel. Over time, more mature type 1 collagen fibrils, which are stiffer and larger, are formed. 

Tendon development is enhanced by movement and harmed by inactivity. An active life during the tendon developmental period fosters bone and muscle growth, which in turn gives rise to healthy tendon maturation.