Tag Archives: jumping

Big Air: The mechanics of SKIERS and snowboarders landing after jumps

Snowboarder getting big air off a jump
Photo by Jörg Angeli on Unsplash

Have you ever watched the X-Games or Olympics or any other skiing or snowboarding competition and marveled at the sheer heights that the athletes achieve? Depending on the type of jump the skier goes off, they can reach heights of up to 50 feet off the ground [1]. How exactly do the skiers land what are essentially free falls from such heights? Supposedly “survivable injuries” occur from falling heights above the “critical threshold” of 20-25 feet, so how do these athletes land from heights of up to double this [2]?

First, let’s talk about the limits of the human body and falling. The critical fall height threshold, or the height at which injuries from falling start happening, is defined as “> 20 feet (6 meters)” by the American College of Surgeons Committee on Trauma [2]. This means that “survivable injuries” happen when people fall from the “critical threshold of a falling height of 20-25 feet” [2]. These athletes are falling from heights of up to double this, meaning that they should be sustaining some type of injury, yet they will do multiple runs and emerge completely fine. How is this?

To answer this question, we need to understand the design of the jumps that they are going off. Despite ramps for ski and snowboarding competitions typically being “purpose-built to fit their particular venues,” or built for the specific competition and run they will be used for, the ones that lead to athletes getting big air all share the same general structure, which is displayed below [3].

Diagram of typical ski and snowboarding jump layout

It starts with the inrun, a long, straight drop that allows the athletes to accelerate, and then the “kick,” or the actual jump itself that launches the athletes into the air, and last is the landing ramp, another section that is essentially the same as the inrun where the athletes land [3].

The most important part of the jumps, and the part that allows the athletes to land safely is the landing ramp. The landing ramps downhill slope allows the athletes landing to “convert” their downward momentum from falling into forwards momentum, which spares them the “ruinous impact of a multi-story fall” [3]. The fall impact in situations like this is quantified by physicists as “equivalent fall height,” because “when a snowboarder [or skier] lands at an angle and keeps moving down a slope, the impact is equivalent to falling from a much lower height” [4]. This is the case because gravitational energy gets transformed into “forward-moving energy,” leaving a smaller impact to be absorbed by the knees [4].

Jump designers and builders have become very adept at making the jumps for competitions very safe by using this concept of “equivalent fall height.” It has reached a point that the park and pipe contest director for the International Ski Federation (FIS), Roberto Moresi, stated in an email to Scientific American that “A good jump is when landing, they barely feel the impact,” meaning that through the design of the jump a fall of 50 feet can lead to a nearly imperceptible impact [4].

Sources:

[1] https://www.usskiandsnowboard.org/news/aerial-skiing-101

[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3212924/

[3]https://www.wired.com/story/big-engineering-big-air-olympics/

[4]https://www.scientificamerican.com/article/olympic-big-air-snowboarders-use-physics-to-their-advantage/

Patellar Tendinitis: The Kryptonite of Jumping Athletes

Volleyball is a sport of quick movements. For hitters, one of the most common movements in the game is the jump, whether that be to block or to hit. Although a higher vertical leads to improvement in game performance, it can increase the risk of developing a serious injury that affects many volleyball players: patellar tendinitis. This condition is associated with pain and tenderness directly below the knee cap that is especially apparent during explosive, jumping movements. But what exactly causes this condition? And what can be done to remedy it?

A schematic of the knee and patellar tendon.
Image from Wikipedia “Patellar Tendinitis”

Since volleyball is such a quick game, muscle memory is required to react to different situations that can occur. The main way to build muscle memory is repetition. Therefore, young volleyball players are encouraged to play the sport as much as possible. For many athletes, this means playing for their school during that season and then playing for an independent club for the rest of the year. Although this increases the athlete’s skill level, it also increases the chance of patellar tendinitis, according to a study.

Besides overuse, lack of ankle mobility can also lead to a higher risk of the condition. A study found that players that couldn’t flex their ankle upward past 45 degrees could have 2 times the risk of patellar tendinitis as players with a higher ankle mobility. This is most likely due to the ankle and calf’s role in absorbing impact upon landing. Less absorption by the ankle causes more force to be put on the patellar tendon. This is bad news for volleyball players who often have poor ankle mobility due to a past injury.

There are a few ways to treat patellar tendinitis. For an orthotic approach, players use straps or tape around their patellar tendon. Some think this is simply due to the fact that the strap or tape makes the athlete feel more stable, which allows them to load the tendon more properly. However, a study done in 2011 analyzed the strain in the patellar tendon using a computational model. The researchers found that the patellar tendon strap increased the angle between the tendon and the kneecap, which caused the strain to decrease. Decreased strain means that the tendon stretches less, which would decrease the incidence of patellar tendinitis. Another way to treat the condition is surgically. One of the more simple surgeries is a removal of the dead or torn tissue of the patellar tendon. This allows new, healthy tissue to form.

A strap being put around the patellar tendon that can ease pain.
Image from Sports Injury Clinic “Patella Tendon Taping”

Patellar tendinitis is a serious condition affecting many high-level athletes. Although there isn’t a simple cure, researchers have brought to light different causes and treatments of the condition. These can be used to help athletes remedy the pain they are experiencing and perform at their best.

Sources:

Study on How Vertical Affects Patellar Tendinitis

Study on How Training Volume Affects Patellar Tendinitis

Ankle Flexion Study

Patellar Tendon Strap Proprioception Study

Patellar Tendon Strain After Applying a Strap

Additional Reading:

Clinical Trial on Patellar Tendon Strap

Fish in Flight: The Science Behind Great White Breach Attacks on Cape Fur Seals

Great white shark employs vertical attack on prey decoy
Great white shark employs vertical attack on prey decoy – from Sharkcrew via Wikipedia Commons

If you’ve ever turned on Discovery channel during Shark Week, then you’ve probably seen the iconic footage of a 2.5-ton great white shark leaping out of the water to catch its next meal.  If you’re weird like me and you’ve ever tried to mimic one of these epic breaches in a backyard pool, then you realize just how difficult it is to generate enough momentum to jump even partway out of the water and therefore have a real appreciation for what it takes to pull off this incredible feat.

Great white breaks the ocean surface
Great white breaks the ocean surface – from Alex Steyn via Unsplash

So if a breach attack is so difficult to pull off, how are great white sharks able do it, and why do they do it?  As per usual, some basic physics can help us answer both these questions.

 

Great white shark mid-breach
Great white shark mid-breach – from Alex Steyn via Unsplash

According to a 2011 paper by Martin and Hammerschlag, who spent 13 years studying great white predation in South Africa, breach attacks allow great whites to play to their strengths and maximize stealth.

Millennia of evolution have left great whites with long bodies great for straight-line speed (can reach speeds  >11m/s) but not so great for agility. Additionally, roughly 95% of a great white’s muscle is white muscle, which allows for rapid contraction (e.g. speed bursts) but also results in poor endurance.  Considering these aspects of their physiological makeup, it’s in a great white’s best interest to attack swiftly, avoiding prolonged chases.  Martin and Hammerschlag report that the majority of great white attacks on seals are over within 2 minutes and that the longer an attack drags on, the less likely it is to be successful.

Great white shark chases decoy prey from behind
Great white shark chases decoy prey from behind – from Sharkcrew via Wikipedia Commons

As great whites are less agile than seals, maximizing stealth and minimizing the time seals have to react is imperative.  Having evolved to have a dark grey dorsal (top) surface, great whites are hard to distinguish from the coral on the ocean floor when viewed from above (seal’s perspective).  Additionally, since very little of the light entering the water is reflected back towards the surface, it is estimated that under even the best lighting conditions, a seal could only reliably distinguish a shark a maximum distance of roughly 5m below it, which explains why great whites attack from below rather than behind. Great whites need about 4m to reach top speed, so due to this acceleration distance and seal vision, Martin and Hammerschlag report that great white attacks generally start between 7m and 31m below the ocean surface, with the majority staring closer to 30m.  Looking at data for great white breach attacks ranging from vertical to 45 degree ascents, Martin and Hammerschlag estimate that it typically takes a shark between 2 and 2.5 seconds to go from initial acceleration to surface breach, and that when considering shark speed and average visibility conditions, a seal generally has only about 0.1 seconds to react if it spots the shark before contact is made.  Ultimately, due to the advantages it gives them, great whites are successful in over half their breach attacks when lighting conditions are ideal.

Schematic of geometry and optics of great white shark attacks on cape fur seals from Martin and Hammerschlag - not to scale
Schematic of geometry and optics of great white shark attacks on cape fur seals from Martin and Hammerschlag – not to scale

 

Sources & Further Reading:

Fallows, Chris & Aidan Martin, R & Hammerschlag, Neil. (2012). Predator-Prey Interactions between White Sharks (Carcharodon carcharias) and Cape Fur Seals (Arctocephalus pusillus pusillus) at Seal Island, South Africa and Comparisons with Patterns Observed at Other Sites

Martin, R. Aidan, and Neil Hammerschlag. “Marine Biology Research.” Marine Biology Research, vol. 8, no. 1, 30 Nov. 2011, pp. 90–94., doi:10.1080/17451000.2011.614255.

Egdall, Mark. “New Research Reveals Physics Behind Great White Shark Attacks.” Decoded Science, Decoded Science, 10 Dec. 2011, www.decodedscience.org/new-research-reveals-physics-behind-great-white-shark-attacks/7497.

Sloat, Sarah. “Shark Week: Here Is the Wild Physics of a Great White Leap.” Inverse, Inverse, 25 July 2018, www.inverse.com/article/47437-shark-week-great-white-jumps.

Madrigal, Alexis C. “The Physics of Great White Sharks Leaping Out of the Water to Catch Seals.” The Atlantic, The Atlantic Monthly Group, 9 Dec. 2011, www.theatlantic.com/technology/archive/2011/12/the-physics-of-great-white-sharks-leaping-out-of-the-water-to-catch-seals/249799/.

 

Artificial Turf: Game Changer or Game Ender?

Woman plays soccer on artificial turf field
Soccer player on artificial turf field
From Pixabay

Artificial turf fields were first introduced in the late 1960s and have grown tremendously in popularity since. Today, artificial turf fields can be found at all levels of sport, from youth league to professional, and across many different sporting disciplines. A major reason they are so popular is because they offer a consistent, low-maintenance, year-round green playing field in all weather conditions and climates. However, despite the benefits they provide, artificial turf fields are not without controversy. Even though artificial turf mimics grass in appearance, its properties are much different.

Of these properties, two are especially relevant to injuries sustained while playing sports, especially concussions and injuries to the knee and ankle. The first of these two is the field’s ability to absorb shock. Older artificial turf fields are often much harder than natural grass fields, which leads to greater impacts on athletes, which can then result in higher rates of concussions and other injuries. As artificial turf technology has developed, however, installation procedures and field composition have improved and greatly reduced the risk of injury.

The second of the two properties is the friction of the field, or how well an athlete’s cleats grip the turf. Because of their greater consistency and density, artificial turf fields can generate more friction than natural grass fields, allowing greater force to be generated at the contact point between an athlete’s foot and the ground. This is both a positive and a negative. Positively, the greater friction generated by athletic moves on artificial turf surfaces enable an athlete to perform at a higher level by enabling quicker changes in direction and more explosive movements. However, these greater forces can overload weaker parts of the anatomy, especially the ligaments in the knee, and cause injury. Numerous studies have found that rates of serious knee injuries, such as ACL (anterior cruciate ligament) tears, are found to be increased on artificial turf fields.

A major reason for this is because the knee is a hinge joint, meaning that it only allows straightening and bending motion while resisting rotation. Within the knee, tendons attach muscles to the tibia/fibula and the femur so that the knee can be bent voluntarily, four major ligaments attach the femur to the tibia/fibula to stabilize and restrict the knee’s motion, and a variety of cartilage and fluid sac structures ensure smooth and consistent motion. While robust together, each individual component in the knee is susceptible to injury if it is subjected to a force in an unusual or extreme way, which could happen while changing direction rapidly, incorrectly landing a jump, or during a collision. On an artificial turf field, the risk of damaging ligaments, cartilage, or other structures in the knee is increased because greater forces can be generated from the ground and because the foot may stick in the turf while changing direction and cause inadvertent rotation in the knee.

Nevertheless, even these risks can be somewhat mitigated by taking steps to avoid injury like the one recommended by The Polyclinic.

 

 

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.

 

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