Tendon: From Body Tissue to Concrete Strength

To most people, a tendon is the cord of fibrous tissue that attaches muscle to bone, transmitting the pull of contraction into movement. You can feel your Achilles tendon above your heel. The word comes from the Latin tendere, to stretch or pull, and the anatomical meaning reflects that: muscle stretches the tendon, which stretches the bone.

In prestressed concrete construction, a tendon is a high-strength steel wire, cable, or bar that runs through or along a concrete beam before the concrete hardens. The tendon is held under tension by an external anchor. Once the concrete reaches full strength, that tension is released, putting the entire concrete member into compression. This compression acts like a clamp, holding the concrete together and preventing it from cracking under its own weight or the loads it will later carry.

The transfer is metaphorical but precise. Just as an anatomical tendon pulls on bone, a construction tendon pulls on concrete. The shared image of tension and transmission made the word natural to borrow. Prestressed concrete became common in the 1930s and 1940s, especially in Europe, where engineers needed to span longer distances with thinner members. The language of the body offered ready vocabulary for this new structural behavior.

The tendon in concrete must be protected from corrosion, kept under exact tension, and released at the right moment in the curing cycle. If it breaks during tensioning, the entire member is ruined. In practice, tendons are often grouped into bundles, called strands, and may be straight, draped (curved along the beam length), or harped (angled in sections). The precision required is far removed from biology but the fundamental action, a controlled pull that strengthens by tension, remains the same.