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Paper Digest: Ten Biomechanical Animal Joints Enable Extreme Performance

Photo: A sling-jaw wrasse, by Alain Feulvarch, CC BY 2.0 , via Wikimedia Commons.

In 2021, engineer and ID proponent, Stuart Burgess analyzed ten linkage mechanisms in animal joints and published his review of their mechanical functions in Bioinspiration & Biomimetics. He chose animal joints such as fish jaws, knee joints, and bird wings due to their extraordinary performance and the extensive knowledge base regarding how they function. As a veteran mechanical engineer, Burgess is well positioned to assess the mechanics of animal joints. Notice how in the excerpt below, he praises the optimality of animal joint design and notes the potential for bio-inspiration from studying animal joints:

Ten different linkage mechanisms are presented. They are chosen because they cover a wide range of functionality and because they have potential for bioinspired design. Linkage mechanisms enable animal joints to perform highly sophisticated and optimised motions. A key function of animal linkage mechanisms is the optimisation of actuator location and mechanical advantage. This is crucially important for animals where space is highly constrained. Many of the design features used by engineers in linkage mechanisms are seen in nature, such as short coupler links, extended bars, elastic energy storage and latch mechanisms. However, animal joints contain some features rarely seen in engineering such as integrated cam and linkage mechanisms, nonplanar four-bar mechanisms, resonant hinges and highly redundant actuators. The extreme performance of animal joints together with the unusual design features makes them an important area of investigation for bioinspired designs.

As Seen at Home Depot

You may have noticed a four-bar mechanical linkage mechanism if you watched a scissor lift while shopping at Home Depot. That four-bar linkage mechanism allows the lift to extend in order to reach products on high shelves. Collapsing the scissor lift reduces the amount of space the lift takes up. Four bar linkage mechanisms have four bars and four pivot points. The length of the bars may vary as well as how the bars move in relation to each other. By using unique four-bar linkage arrangements, an engineer can optimize mechanical movements. Key points about why engineers use such mechanisms include: 

  • Four-bar linkage mechanisms can increase force by utilizing bars of different lengths.
  • They can improve rotation or optimize the direction of compaction.
  • They move actuators away from the joint providing a mechanical advantage and lowering the energy needed for motion.

In Burgess’s paper, the first four-bar linkage mechanism discussed is the mammalian knee — a joint that has been criticized as poorly designed. As an engineer Burgess is familiar with constraints and design trade-offs. So he first discusses what the mammalian knee requirements are. To summarize, he says the knee must provide a 120o range of motion, be load bearing, and prevent overextension. He explains how through a clever design — an inverted four-bar mechanism — all of these requirements can be accomplished. The four-bar mechanism enables a large extension range, but also has an end stop which locks the knee. This lock decreases the amount of work required by the muscles to stand erect effectively making standing up easier. Because there is a broad area of contact between the femur and tibia, loads can be transferred through the joint and bore. In the knee’s four-bar mechanism, the center of rotation moves, which also provides advantages. When you squat, the center of rotation of the knee joint shifts, which reduces your muscular effort by 35 percent when you rise from the squat position. If you thought squatting was difficult, imagine how difficult it would be without this brilliant design! Burgess points out that one noteworthy constraint for joints of biological systems is that they are restricted from using a shaft inside a hole due to the necessity of a growth and development process. This relevant constraint applies to engineers working to develop self-replicating machines.

The Bird Wing Joint

The second four-bar linkage mechanism discussed is the bird wing joint. Have you ever wondered how birds can fly so long without tiring? Burgess points out that this is due in part to the brilliant engineering in the avian elbow joint, which enables wing tucking and extension. Burgess notes that, according to research done with seagulls, the elbow wing joint decreases 12.3 percent of a bird’s need for force during flapping.

Grasshoppers, dragonflies, and other insects generate lift by flapping and rotating their wings at steep angles. Flapping occurs at a frequency of 20 to 1,000 flapping cycles per second. It’s no surprise that these organisms make such a whirring sound! To accomplish such rapid movement, some incredible hinges are obviously required. Burgess points out that many insect wings have a small bar as part of their four-bar wing mechanism which ends up magnifying the wing rotation. This means that even minor movements on the insect’s body can cause a considerable angle of movement in the wing. Of course, the insect’s body must be correctly built to allow such mobility. Burgess also points out that flapping happens at a resonant frequency, which significantly reduces the inertial energy required to flap. This is only feasible because of the insect’s body architecture.

Another category of four-bar linkage mechanisms Burgess discusses is that of fish jaws. The first example he provides is a sling-jaw wrasse. As it happens, my husband and I owned a wrasse. Why? For the purpose of eliminating flatworms, vermetid snails, and bristle worms from our 75-gallon salt water aquarium. One can’t help but appreciate how incredibly well designed the wrasse’s mouth is. The term “sling-jaw” refers to the fact that these fish can hurl their jaws. Burgess notes that one function of the design is to capture prey with a quick suction approach. The second is that the sling-jaw design minimizes the amount of swimming the fish has to do. Pushing the jaw forward requires significantly less energy than swimming forward when food is nearby. As I was able to observe, the mouth of our wrasse extended so quickly and far that it made the fish an exceptionally agile hunter. Within a month or so of adding the wrasse to our tank, no pests remained — all thanks to the excellent design of the sling-jaw wrasse.

Burgess also describes the four-bar linkage mechanism of the mantis shrimp — a marine creature that punches to eat. The force is produced by a four-bar linkage mechanism connected to a biological battery. When the shrimp is ready to punch, it relaxes a muscle, the latch is released, and the accumulated elastic energy delivers 1000 N of force. That is several orders of magnitude larger than the weight of the organism.

A Gift for Engineers

To conclude, the amazing design structures in organisms provide engineers with inspiring templates for creating better products. Burgess provides three specific examples where direct study could pay impressive dividends:

  1. Improved 3D modeling of avian wing joints has important implications for aircraft wing design.
  2. Jaw mechanisms may result in new and improved designs for robotic clamping.
  3. The punching mechanism of the mantis shrimp could inspire new technology in the field of industrial design.

Burgess’s review has been downloaded over 8,000 times and cited 19 times. The high number of downloads and citations suggests that there is a growing interest among researchers in using nature’s design templates to solve technological challenges. This indicates that biomimetics is becoming an increasingly important field for innovation and advancement in various industries. By studying the intricacies of natural mechanisms like four-bar linkages, scientists can gain valuable knowledge that to enhance human engineering practices. This interdisciplinary approach encourages critical thinking and innovation, ultimately benefiting various industries by inspiring more efficient and sustainable designs.