Without ligaments and tendons our skeletal frame would be just a pile of unconnected bones. Ligaments are elastic structures which connect bone to bone in our joints, whilst tendons are cord-like inelastic structures that connect muscles to bones. Tendons are under extreme stress when muscles pull on them, so they are very strong and are woven into the coverings of both muscles and bones in an intricate matrix. Tendons can be likened to cable-like collagen ropes.

The bodies of humans and animals owe their strength, especially, to a fibrous structural protein called collagen. Collagen is abundant in bones, tendons, ligaments and skin. Like a building, collagen has an hierarchical structure consisting of a complex arrangement of individual molecular components. The basic building block is the collagen molecule itself. Its shape resembles a rope, with three chain-like proteins twisted around each other to form a triple helical framework.

Many of these ‘ropes’ in turn combine to form thicker ‘coils,’ known as collagen fibrils. However, being just 100 to 500 nanometres thick, the fibrils are 100,000 times thinner than actual ropes.

Within the fibrils, adjacent collagen molecules are not simply stacked one adjacent to each other but they are laid to form a staggered arrangement. This results in alternating denser and thinner zones along the length of the fibrils. Many fibrils in turn combine to form collagen fibres.

In evolutionary terms, how long would it take for the body to come up with a muscular substance, position it next to bones and then arrange for the muscles to be attached to the bones? Clearly if the whole arrangement is not working from day one, the whole thing would be useless.

Now how does the tendon bond to the bone? For those who have tried gluing fibrous material to porous material you will know how difficult this is to achieve. In fact, joining of dissimilar materials is a fundamental challenge in engineering.

Researchers have described the bonding as a complex event. The integration of tendon into bone occurs at a specialised interface known as the enthesis. The fibrous tendon to bone enthesis is established through a structurally continuous gradient from uncalcified tendon to calcified bone. The enthesis exhibits gradients in tissue organisation classified into four distinct zones with varying cellular compositions, mechanical properties, and functions in order to facilitate joint movement.

When we read researchers using terms like “specialised interface” and “complex event” we can be sure that this arrangement could not have been easily manufactured. The possibility of getting this complexity right through random mutational chance for one set of bones and muscles, let alone for all joints, would be virtually nil.

Furthermore, tendons come in many shapes and sizes. Some are flattened bands; others rounded cords, and the shape of the enthesis often matches that of the tendon. Inspection of many entheses shows that tendons often fan out at their attachment sites, so as to distribute force over a greater area. Accompanying the fanning out of the tendon fibres, there may be a reorganisation of fibre bundles so that the pull of the tendon is uniform over the entire enthesis, or that different parts of the enthesis bear the strain at different positions of the joint.

All of this reflects a general engineering principle that stress concentrates at interfaces between structures with different mechanical properties. But what is more remarkable is that the body has an appropriate solution. The structure of each tendon enthesis is arranged such that it dissipates stress away from the interface and into the tendon and/or bone itself.

Tendons, like the digital extensors and flexors of the hand, transmit forces in such a way that fingers can be moved with great precision. Other tendons, like the Achilles, function as springs that enable locomotive activities such as running and jumping to be performed efficiently by storing energy during deceleration, and then releasing it to help power acceleration over millions of loading cycles. When tendons get torn due to stretching beyond their capacity the body experiences tendonitis or inflammation of the tendon.

What human engineering could hope to match the tendon’s specifications for decades? Think of every step you take, every object you manipulate with fingers, every bending or stretching exercise.

Most of us accomplish such things easily for many decades. And think of the range of loads these tissues can endure, from the lightest touch on a keyboard to a weightlifter’s 200kg clean-and-jerk. Every basketball player, high jumper or marathoner puts incredible stress on tendons, far beyond what is necessary for mere survival. When man creates machines involving repetitive, stressful activities, they wear out very quickly—but not so with God’s work.

There are seven or so levels of hierarchy in a tendon’s structure (see diagram)—enough to begin thinking about irreducible complexity (IC). Add to that the physical constraints of ability to withstand rupture while storing energy, the crosslinks, and the repair options—and the case for IC grows. It becomes overwhelmingly persuasive when we consider that the fibres are coded by genes and built by ribosomes using two independent codes: the genetic code and the protein code. On top of that, consider that the fibrils must be sent to the proper location in the body and wrapped with others at the proper angle by other molecular machines.

The likelihood of all of this being done by a random association of molecules and proteins is next to impossible. How much easier it is to assign the marvellous work of tendons and muscles to the Master Designer.