Walk [Under] Water: The Benefits of Underwater Running

Just because you can’t walk on water doesn’t mean you shouldn’t run under it!

Aqua-jogging. Hydro-running. Water-treadmills. Have you ever heard some combination of these terms and wondered what the hype is?

Running underwater offers benefits for people throughout their fitness journey. Underwater running has proven useful for a variety of focuses, including recovery after injury, cross training, and even improved gait. This article includes a video showing a Runner’s World coach tries out a Hydrotrack and discusses some of the benefits!

So, why does it work?

Three basic water properties: hydrostatic pressure, buoyancy, and viscosity.

Hydrostatic pressure is the force that the water exerts on a submerged point. Hydrostatic pressure acts all around the point. However, since hydrostatic pressure is proportional to the weight of liquid above the point, it increases with increased water depth. This means that your feet would experience greater hydrostatic pressures than your knees. While running, this pressure helps support your body and decrease impact forces. In addition to helping prevent injuries through a decreased risk of falling, it also helps decrease swelling and promote cardiovascular health. This article talks about the specifics of pressure with swelling and the cardiovascular system.

Diagram showing hydrostatic forces. Magnitude of the hydrostatic force is larger as it goes deeper below the surface.
Hydrostatic pressure acts on all sides of a point. The pressure increases with depth. Created in Microsoft PowerPoint.

Buoyancy is the hydrostatic force applied to an object with volume (rather than just a point). Since they are at the same depth, all the horizontal forces cancel out. Since the bottom of the object is deeper than the top, the net buoyant force on the object pushes up. The difference between the buoyant force and the weight of the object submerged determines if the object will rise, sink, or stay in place. Thus, the more submerged a person is, the more of their weight is supported. This research article explains how this support can help make gait analysis more effective to further prevent injury. When water reaches the person’s navel, 50% of their weight is supported. This weight bearing capability of water decreases forces on joints and can even help improve range of motion. This allows physical therapy to begin sooner and, overall, take less time out of the patient’s normal routine. This allows shorter rehabilitation times without sacrificing quality of care or recovery.


Diagrams showing how the hydrostatic force varies around the submerged object due to depth. The side forces cancel out at equal depth leaving a net buoyant force acting upward against the downward force of the object weight.
Buoyant forces cancel out on the sides leading to the second image showing the net buoyant force and the weight of the object. Created in Microsoft PowerPoint.

Viscosity is a fluid property that affects the resistance that an object encounters during motion. In the case of underwater running, viscosity explains why you move significantly slower in water than on land. It also can offer resistance up to 15 times the amount of resistance on land. Forcing your limbs through the water strengthens muscles that are not typically used out of the water and even burns more calories!

As noted above, viscosity can help strengthen muscles as shown in this study on deep water running (DWR) in a community of elderly women shows how viscosity affects overall strength training. It showed that the women who participated in DWR increased their muscle strength (measured through power) and performed better in various tests, including ones that involved sitting down and getting up. The study showed that deep water running helped to mitigate some of the negative muscular effects of aging.

Overall, running underwater offers some great benefits. The basic properties of water (hydrostatic pressure, buoyancy, and viscosity) provide scientific background for why hydro-running provides benefits for all.








Gainz for Dayz: Conventional vs Sumo Deadlift

What’s the best way to pick stuff up and put it back down?

A large tattooed man deadlifting enough weight to bend the bar in the conventional form.
Photo by Alora Griffiths on Unsplash

By deadlifting, of course.  The deadlift is, arguably, one of the most important exercises a weight lifter can perform.  Although primarily thought of as a lower body exercise, the deadlift activates muscles throughout the entire body, and is one of the three all-important lifts for any lifting routine.  Although there are a variety of different deadlift form variations, two of the most prevalent are the conventional deadlift and the sumo deadlift.  The main difference between these two stiles is that in the conventional deadlift, the hands are placed on the outside of the knees, while sumo deadlifting places the knees outside of the hand.  The everlasting debate is therefore which form is the better one?

In attempt to answer this, tracking data was used to employ a biomechanical analysis of these two forms of deadlifting. One of the biggest differences they found was in the distance that the bar had to travel from the beginning of the lift to the completion of the lift.  In the sumo deadlift, the significantly wider foot stance results in a 19% decrease in the distance the bar must travel, decreasing the amount of work that the lifter needs to use.  It therefore seems that the conventional deadlift, if it requires more work, is the better form, right?  Well, maybe.  The researchers also found that the sumo deadlift conveyed a biomechanical advantages compared to the conventional deadlift.  This was mostly due to a more upright trunk at the beginning of the lift, resulting in less trunk extension being required to complete the lift, although it consequently may require more flexibility to perform.  This in turn decreases the moments of the bottom two vertebrae and shear forces on them, and the sumo deadlift therefore seems to have a safety advantage over the conventional deadlift.

The postures of athletes performing the conventional and sumo deadlifts as determined by tracking body points through video footage.
Modified from McGuigan and Wilson, Journal of Strength and Conditioning Research 1996

But what about muscles?  What should you do if you want to gain strength by pushing your muscles to work harder?  As the study showed, the conventional deadlift needs an increased amount of energy to complete, but are all the muscles used in the two forms the same?  Researchers at Duke University Medical Center decided to use electromyography (EMG) to find out.  They found that the wider stance assumed in the sumo deadlift, besides conveying the safety advantages mentioned above, also resulted in an increased recruitment of some of the lower body muscles.  Namely, the vastus lateralis, the vastus medialis, and the tibialis anterior, or the outer (and strongest) and inner thigh muscles as well as the shin muscle.  The conventional deadlift recruited only the medial gastrocnemius (inner calf) significantly more than the sumo deadlift.  The recruitment of the vastus lateralis and vastus medialis make sense, since having your feet placed more directly under you in conventional deadlift would tend to recruit the more central thigh muscles.

With all of the above, it looks like sumo be the better option: it decreases stress on the back, recruits more lower body muscles, and indirectly places a focus on flexibility.  However, if you’re not convinced, Men’s Journal, BarBend, and Starting Strength provide some additional commentary comparing these two methods.

For the following video provides a better explanation of the differences in form between the two styles of deadlift.


Can Plants Talk to Each Other?

When something bad happens, do you tell your friends? Do you warn the people around you?

Similar to human beings telling one another about risks and dangers, plants can communicate with one another about environmental stressors. One stressor for plants is physical contact with other plants, such as their leaves touching. The presence of other plants means competition for resources and the possibility of invasive species. Biologists at the Swedish University of Agricultural Sciences conducted a study on the ability of plants to communicate with neighboring plants through chemical signals excreted through their roots to indicate to neighboring plants that they contacted another plant above ground.

A diagram of a root in a tube with two tubes branching off of it in the shape of an upside-down 'y'. Each branching tube leads to one of the two solutions.
Diagram from Elhakeem et al., PLoS ONE 2018

In their experiment, the Root Choice Test, maize plants were placed in individual chambers where the roots had two possible paths for further growth. One path led to a solution made from the soil of a plant whose leaves had been touched by the leaves of another plant every day for a week. The other solution had soil from a plant that had not been disturbed.

Despite there being an equal chance of growth in either direction, the maize plants chose to grow in the solution of the untouched plant significantly more often. In some cases, a plant would begin to grow roots in the direction of the touched plant’s soil, but then changed growth direction to the other solution. There were no scenarios wherein a plant would switch from the undisturbed plant solution to the touched plant solution.

These results demonstrate a robust system of communication between plants. Plants possess the ability to recognize when their leaves touch another plant’s leaves and can transform this information into a chemical signal released by their roots. Other plants can understand these signals and modify their behavior as a result.

Mushrooms in grass huddled together
Photo by Vince 6800 on Unsplash

Plants can also communicate through physical connection including underground fungal networks. These networks are made up of fungal root hairs called hyphae that insert themselves into the cell membrane of a plant’s roots. Hyphae are made up of long, individual, filamentous branches whose structural characteristics, such as cell wall thickness and branching, are determined by its function. These vast systems of interconnected fungi plants act like telephone wires for communication.

Plants in these networks are able to exchange nutrients, become more resistant to disease, and grow larger. Healthy areas of the fungal network have also been shown to respond defensively when other areas of the network have been under bacterial attack. This shows that the fungal networks and the plants they connect exchange warnings similar to plants communicating though chemical signals in the soil.

Research into plant communication could one day lead to producing desired changes in plants by artificially mimicking their communication. Farmers could create chemical or physical signals that instruct their crops to grow more effectively. As research into this topic continues, perhaps one day, plant communication will be as well understood as any spoken language.

To learn more about plant communication check out these stories: The Scientist, NCBI and PLoS.

Robots Could Soon Replace Human Stunt-Doubles

Imagine an aerial acrobat soaring fifty feet above your head and executing gravity-defying stunts during a live performance. After your initial amazement that a human could be performing acts such as these so fearlessly, you look a bit closer to realize that the performer is actually not human at all. Thanks to a groundbreaking technology recently developed by Disney Research, this could soon become a reality.

Stuntronics robot soaring through air while holding heroic pose
Photo from Walt Disney Imagineering Research and Development, 2018

Over the past year, Disney has been working to produce a robotic stuntman that has the ability to replace its human counterpart in performing dangerous aerial acrobatics. This seamless blend of biomechanics and technology has the potential to ultimately create an immersive and unforgettable entertainment experience.

This project, known as “Stuntronics,” originated from a smaller research project known as Stickman. Stickman was a robot that consisted of a line of three metal rods connected by two flexible joints. Once cast into the air by swinging off a pendulum wire, the robot utilized sensors such as accelerometers and gyroscopes to relay to its microcontroller (or brain) information regarding its position and orientation while flying through the air. Using all of this information from the sensors, the robot then either tucked or untucked its sections to rotate more or less quickly, respectively, in order to land flat and untucked on its back. A diagram explaining the robot’s motion can be found below.

Diagram of Stickman robot's trajectory through the air with labels
Photo from Christensen et al., Disney Research 2018

In order to scale Stickman to a more lifelike and human-sized robot, it was necessary to take a closer look at the science behind how human performers are able to execute their movements. Researchers Spiros Prassas, Young-Hoon Kwon, and William Sands explored these questions in a review focused on the biomechanics of gymnastics.

An important part of acrobatics and gymnastics is the ability to shift angular momentum (the amount a body rotates) between body parts. As a gymnast gets closer to the ground, they can either speed up or slow down their rotation by rotating their arms in order to successfully stick a landing. Performers also are able to speed up or slow down their rotation by manipulating their moment of inertia (the amount a body resists rotating more quickly or slowly) through their body configuration. For example, if the performer needs to speed up their rotation, they could reduce their moment of inertia by tucking into a ball, whereas if they wanted to slow down, they could untuck their body into a full layout.

The Stuntronics robot utilizes these concepts by continually reading the feedback from all of its attached sensors and lasers to tell the entire body which configuration it should be in at any given time. After being launched in an arc from a swinging wire, it is capable of controlling its pose in order to either speed down or speed up its rotation, and thus land perfectly each time (see video below).

This advanced technology could be pushed to the limit to ultimately produce more engaging and immersive entertainment by carrying out stunts that would simply be too dangerous for human performers to attempt. In a world where robots are constantly being implemented to take the place of humans in performing dangerous, dirty, and tedious work, Stuntronics could serve as a foundation for generations of robots, both stuntmen and non-stuntmen, to come.

For further reading, check out these articles from TechCrunch and Popular Science.


Why your Muscles Hurt after a Workout

How often do we cut our overly ambitious workouts short because of exhaustion or muscle soreness? Probably more than we care to admit. But have you ever stopped to ask yourself why your muscles hurt, especially a day or two after your workout? The simple answer is, you’ve put so much strain on your muscles that you aren’t used to, so they tore, and now your body has to repair the tears and build up more muscle, so the same thing doesn’t happen in the future.

Normal muscle tissue in the arm versus strained muscle tissue
Fitness Science, 2015

When your muscles become sore after a strenuous workout, that is called Delayed Onset Muscle Soreness, or DOMS. Matthew Ely, a graduate employee in the Department of Human Physiology at University of Oregon states that DOMS occurs often when you are doing a new workout or using muscles that you typically don’t utilize. What happens then, is that since these muscles are not accustomed to enduring so much stress, they tear. On a microscopic level, what is happening is that after these muscles have torn, local cells begin to work together to repair the muscle fibers. Correspondingly, tissue cells, immune cells, and proteins migrate to the torn ligaments to remove the damaged proteins and repair and replace them with new ones. This process is ultimately the soreness we feel. As the proteins and muscles build up, that exercise that made us sore no longer does as we’ve gotten stronger with visible muscle growth. While this pain ultimately supports the saying “no pain no gain”, this pain shouldn’t last longer than a few days. If it does, the damage could be more serious and require medical attention.

Damaged muscle torn across the fibers
Total Cheer Performance, 2018

Since 2010, a group of Japanese neurologists have been studying DOMS and nerve damage. Ultimately, their findings don’t show us too much more than what is already known.

Inflammation of muscle fibers due to over exertion
Physio4fight, 2014

Their research essentially suggests that the pain we feel is nerve growing pains. However, after examining the effect of DOMS in rats, the Japanese were able to begin working on a “cure” for DOMS, meaning they thought they were able to completely suppress this soreness. So far, though, cryotherapy, stretching, homeopathy, ultrasound, and electrical current modalities have not proven to be entirely fruitful.

There are several reasons that the Japanese and other researchers have proven fruitless in their efforts of understanding this topic, primarily, the early stage in history that DOMS are even being studied. While many remedies have been created to combat diseases, athletic injuries and musculoskeletal research are in their earlier stages of study. The second reason is that DOMS have proven to be more complicated than originally thought. The basic theories tend to prove to be false. But the most accepted theory for DOMS is that when the muscles tear, they swell up and push on nerves which sends a message to our brain that we are in pain, similar to more basic injuries from cuts or broken bones.

In the end, muscle soreness is not a bad sign that you’ve damaged your body like simple injuries. As far as research has shown, muscle soreness is the response your brain receives when too much stress has been placed on an unused muscle so that it tears and swells. When the swelling pushes on nerves, we feel pain. As our bodies repair the damage by replacing proteins, the swelling goes down and the muscle gets larger so it can compensate for the stress it now knows it must overcome when we do that specific exercise. This process would then repeat as we push ourselves further every time we work out.

DOMS – What is it and what to expect


Archerfish: Nature’s Master Marksmen

Picture of an archerfish swimming
Photo by Chrumps on Wikipedia

The name archerfish refers to seven species of freshwater fish that are all members of the Toxotes genus. These fish derive their name from their ability to hunt land-based creatures, ranging from insects to small lizards, using jets of water shot from their mouth with remarkable accuracy. They only grow to a maximum of a foot long, but they’ve been recorded in the wild propelling their water jets distances of up to two meters. A recent study in the Journal of Experimental Biology was conducted by Stephan Schuster to investigate the mechanics behind their unorthodox hunting technique.

Archerfish Learning to Aim

As it turns out, the archerfish has many circumstances working against it. Not only must they account for gravity when hunting, but because their eyes remain underwater during the attack, they must also adjust for the effects of the light refracting through the water. Refraction could cause the fish’s perceived angle of the prey to be off by up to 25˚ from the actual angle. They compensate for these factors by adjusting their spitting angle prior to releasing the jet. The ability to compensate for various environmental factors seems to be a skill developed through experimentation and practice. Archerfish will indiscriminately fire off shots at objects that are new to their habitat. They then judge whether they should continue to attack those objects in the future by remembering if a successful hit rewarded them with food. This was tested in the lab and shown that when the fish wasn’t rewarded, it would ignore the target in the future.

A diagram showing archerfish potential targets. Once the archerfish was only getting consistently rewarded for a single kind of target, insects, that's the only target it would shoot at
Modified from Schuster, Journal of Experimental Biology

In the past, studies were conducted that underestimated the accuracy of archerfish because of the variation in test specimens. Studies now show that a well-practiced archerfish can achieve a 100% hit rate on targets within distance of 65 cm. Accuracy will begin to drop off with increased height and distances beyond 65 cm.

Hunting Mechanics

While most animals who utilize their ability to spit depend on the spit’s glue-like properties or venom, archerfish hunting relies on the force at which the jet contacts the target. Depending on the size of their prey, archerfish can modulate their jets between the range of 40-500 mN. To exert strong enough forces to dislodge targets from leaves and branches, the fish expelling the jet acts like a vessel under pressure. The water starts out slow and increases in velocity as the fish’s mouth stays open, like turning a valve. This means that the jet’s tail catches up to its tip as the water travels along its trajectory, causing a more focused impact. As the water travels, the stream will break into individual blobs of water due to Plateau-Rayleigh instability, creating multiple jets. Since water ejected later is moving faster, these jets will join over time and create a single, larger jet which will impact the target.

Race to the Food

While utilizing their ability to create forceful jets of water can be an effective method of hunting, it does come with one disadvantage. Archerfish have evolved to react to plummeting objects much quicker than other fish, regardless of what caused the object to fall. While this means that an archerfish having its prey stolen by another type of fish is very rare, its prey will often be stolen by other archerfish which happen to be closer to where the prey impacts the water.

Additional information on archerfish hunting can be found at wired.com and in the video from below. There are also some beautiful photos of archerfish in action at stevegettle.com.


Superhero Technology for Super Kids

Researchers have begun using exoskeletons (similar to Iron Man’s suit) to aid children with cerebral palsy in danger of losing their ability to walk.

The problem…

The effect of crouch gait on human posture.
The National Center for Simulation in Rehabilitation Research, 2010

Cerebral palsy is a developmental disorder that affects the ability to move and maintain balance in the body. This neurological condition caused by damage to the brain before birth affects the body and muscles in ways that make it hard for those affected to walk as they get older. There are several different biological symptoms that lead to the difficulty of walking. According to the Mayo Clinic, these issues can include stiff muscles (spasticity), loose muscles, exaggerated reflexes, lack of muscle coordination (ataxia), and the presence of involuntary muscle movements.

These issues compound as children grow older and the normal movements needed for walking can be lost. Specifically, spasticity leads to continuous contractions causing a permanent deformation of the muscles in the legs. For those with cerebral palsy it is seen in the form of crouch gait (pictured above), where patients’ knees bow inward, It is common for those with cerebral palsy to lose their ability to walk when they reach adulthood due to crouch gait.

The solution…

Researchers from the NIH Clinical Center Rehabilitation Medicine Department looked to attack these problems by using exoskeletons to provide a rigid, mechanical, and guided support for the body. The goal of the exoskeleton was to simply assist the participants by alleviating the muscles’ desire to cause a bending of the knees. However, this needed to be done while the participants still had full control of their own walking.

Functional & Applied Biomechanics Section, Rehabilitation Medicine Department, NIH Clinical Center, 2017 

The exoskeleton was tested on seven patients with cerebral palsy, aged 5 through 19, who had been diagnosed with crouch gait, but still had the ability to walk at least 30 feet without crutches or other forms of assistance. The results showed that patients did not lose their ability to use their own muscles and increased their knee angle positively by an average of 13 degrees and a maximum of 37 degrees. This creates a sturdier posture that is conducive to a longer life of walking.

The future…

The use of an exoskeleton as treatment for crouch gait is both promising and needed. Current treatments include physical therapy, surgery, or the use of muscle relaxers. Physical therapy has not proved to be effective in the long term, while surgery and the injections of muscle relaxers are invasive and painful for patients.

The exoskeleton technology is still young. Researchers want to extend the tested time with their current patients, create exoskeletons that can be used outside of a clinical environment, and attempt to use the technology on those who have already started losing movement.

There is hope that this technology can significantly help these children. Exoskeletons have been used to help restore movement in paralyzed adults during rehab for strokes or spinal cord injuries.

At the 2014 World Cup, a paraplegic man kicked a soccer ball while wearing a robotic exoskeleton. Additionally, researchers at Carnegie Mellon have developed walk aiding ankle bracelets that can be worn outside of a lab and adjust their movements to each user’s needs.


The future of hearing might be in your bones


How many times have you walked up to someone and were unable to get their attention because they had headphones on? This is an increasingly important issue as we become more connected to our devices and less connected to the world around us. Recently, several companies, including Aftershokz and Pyle, have tried to solve this issue by creating bone conducting headphones.

How does bone conduction work?

diagrams of the inner ear displaying the differences in bone and air conduction
Modified from Furuichi, GoldenDance 2008


Although these devices may seem futuristic, bone conduction has been used for hundreds of years, especially in applications involving music. In the 18th century, Beethoven, although he had lost much of his hearing, was able to listen to his music by clenching a rod in his mouth that was attached to his piano. In most situations, we hear sounds using air conduction in our ears. Our outer ear channels vibrations that travel through the air into our ear canal where our eardrum transmits these vibrations to our cochlea. Inside the cochlea, each frequency resonates at a different location along the basilar membrane, and these mechanical waves are converted into neural signals that are transmitted to the brain. Bone conduction works by sending these vibrations through our bones directly to the cochlea and bypassing the outer ear and eardrum.

How is bone conduction used?

Szweda, BAE Systems 2015

As time and technology have progressed, bone conduction has become increasingly more common in commercial devices. Currently, the most prevalent use of bone conduction is in hearing aids for those suffering from outer or middle ear damage. Bone conduction is also used in applications where users must still be aware of their environment while listening to music or other sounds. Modern devices are able to transmit frequencies between 20 and 20,000 Hz. This range is perfect for listening to music and voices at reasonable volumes. Bone conduction can also be used in more demanding situations. BAE Systems has utilized bone conducting technology to manufacture helmets that allow soldiers on the battlefield and sailors competing in America’s Cup to communicate with each other while still being able to hear their environment. These grueling environments make perfect use of bone conducting device’s durability in hazardous conditions including water and dust.

What is the future of bone conduction?

image of LG G8 smartphone depicting the cystal sound OLED speaker screen
LG G8 Smartphone, LG Electronics 2019

Although many devices that utilize bone conduction like Google Glass and Zungle Audio sunglasses have not yet become mainstream. This technology still has a bright future. On February 24, 2019, LG unveiled its G8 smartphone which eliminated its top speaker for receiving phone calls. Instead, LG’s design creates sound by vibrating its front glass panel. The user can then press the screen against his or her face conducting the sound through his or her cheek to better hear the person on the other line. As implementations like these become more common, the technology behind bone conduction will only get better. It may seem like the future, but the next headphones or pair of sunglasses you buy might have bone conducting technology inside of it.


For more information on this story, check out The Verge and CNN.


Secret Behind Kangaroos’ Tail

Red kangaroos can reach speed of more than 35 miles an hour, they can also cover an area 25 feet long and get up to 6 feet high in one jump using their tail like a spring to give them more power. When kangaroos want to move slowly, they do kind of lean on their tail, to support their

Schematic representation of the tail involved in accelerating. Photograph: Heather More (theguardian.com)

body. When kangaroos are grazing they move their hind pairs of feet together which makes their movement awkward but the power behind them in their tail is keeping them balanced.There was always a question of why Kangaroos are placing their tail on the ground when they are walking slowly.

Most of the researchers believed that the tail is only used for the purpose of balancing. Professor Max Donelan from Simon Fraser University, collaborated with his colleagues Shawn M. O’Connor, Terence J. Dawson, Rodger Kram trained kangaroos to walk on a measuring device called the force plate, what they found was that the tail was doing a lot more than anyone have realized.  They Found that kangaroos actually used their tail like a fifth leg when they are hopping around or walking. For this study, they documented the movement of five red kangaroos in Sydney Australia which are the largest species of kangaroo and the biggest marsupial on the planet. They observed that kangaroos when walking first put their forelimbs on the ground and when it is the time for their hind limbs to move forward, they use their tail to accelerate and push the whole body forward and then they put their hind limbs on the ground.

They have published a paper in Biology Letters which presents that the tail exerted as much force as four other legs combined. By measuring the commonly work in physics called the mechanical force, the kangaroos tail is as important when it walks as one of our legs as we walk. They found that the kangaroos’ tails are involved on their movement in three ways. First of all, most of the propulsive force which is needed for the movement is provided by the tail. Furthermore, the previous belief that the tail is needed to balance the body weight have been examined and turned out to be that although the tail plays an important role in the balancing, it only provides the 13% of the vertical force needed to balance the body. Besides, investigating on the mechanical work that the tail applies to the whole body for pushing forward, it demonstrates a substantial role of the tail in performing positive mechanical work.

The mechanical representation of the Kangaroo’s movement on the force plate (Shawn M. O’Connor et. al. 2014)
Human’s back leg helps to push the body forward when walking (wikihow.fitness)


In simple words, it can be compared with the role of one of human’s leg when walking. You probably are thinking what exactly makes a leg a leg? The answer could be simple, if a leg exists to play a key role in walking, then kangaroo has five legs.

Kangaroos are the only animals that use their tails as a leg, Max Donelan said.


Continue reading Secret Behind Kangaroos’ Tail

How the Mantis Shrimp Packs its Punch

The mantis shrimp, a six inch long crustacean residing in the warm waters of the Pacific and Indian oceans, may look harmless with its rainbow shell, but it is able punch its prey with the same acceleration as a 0.22 caliber bullet, providing around 1500 newtons of force with each blow. The mantis shrimp can shatter the glass of aquariums, catch and kill their prey with minimal effort, and punches so fast that cavitation bubbles form behind their hammer-like clubs. Cavitation bubbles are pockets of low pressure air that form when a liquid is moved faster than it can react, and collapse with tremendous heat and force—enough to crack the shells of other crustaceans or even a glass bottle.

Rainbow colored mantis shrimp on ocean floor, with two white clubs visible.
Mantis shrimp on the ocean floor, with its two white dactyl clubs visible. (Source: Wikimedia Commons)

The mantis shrimp gets the power for its punches from elastic energy storage—that is, it stores energy in its muscles as they are compressed when cocking its dactyl club back into the locked position. A four bar mechanism within the club and body of the shrimp is used to hold the club back in place until it is ready to punch and a latch is released, transferring the stored energy into rapid motion of the club.

Diagram of how the mantis shrimp locks his club back, storing energy, then releases it and swings it forward to attack prey.
Mechanics of the mantis shrimp’s club swing. (Source: Maryam Tadayon/Nanyang Technological University)

The material composition of the mantis shrimp’s shell enables it to hit so hard without damaging itself. The two layers of the shell on the club allow it to withstand large stresses in both tension and compression, which is uncommon of most shells since they are ceramic materials, which are very brittle. Both layers are made out of hydroxyapatite (HA), a highly crystalline and hard calcium phosphate that is found in human bone, and chitin, a hard biopolymer fiber found in outer shells of most crustaceans and insects. Differing structures and amounts of each material provide different functions in the layers of shell. The outer layer is arranged in a wave-like pattern of HA and chitin that effectively redistributes stresses evenly over the whole surface, greatly decreasing the chance of cracking. The inner layer is less stiff than the outer layer, and is composed of HA and chitin in helical patterns, which allows for small cracks to form in a spiral shape instead of cracking straight through the shell. This greatly increases the lifetime of the club’s shell.

Ceramics are typically not used in robotics due to their brittle nature, but, if modeled after the shell of the mantis shrimp, this could be overcome and provide a stiffer and lighter material to be used in the robotics industry. Additionally, the material composition of their clubs could be used to develop resilient protective body armor for the military and police forces, or helmets for bike riders or football players.

Additional information on the mantis shrimp can be found at Wired.com and theoatmeal.com, and in the following video.