Category: 2022 Spring

Slipping or Tripping? Researchers Find Best Way to Regain Your Balance

Everyone has slipped or tripped at some point in their lives. Whether it is walking on an icy road to get to your car or tripping over the Lego set your kid refused to put away, everyday obstacles can cause us to lose our balance. Often this results in a brief moment of panic followed by the uneasy relief of regaining your footing, but for those who aren’t lucky enough to avoid falling, the results can be devastating. This is especially prevalent in populations more susceptible to falling. Falling in the workplace accounts for 16.8% of all non-fatal injuries leading to days taken off work. It is thought that this is due to the high volume of slipping or tripping obstacles encountered in some occupations. Additionally, 36 million falls resulting in 32,000 deaths were reported for the 65+ year old population of the US (Bruijn et. al 2022). Elderly individuals may lack the strength and reflexes necessary to recover their balance quickly. This is especially worrisome because the elderly are also the most at risk for the major health complications that can be caused by fall related injuries.

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Why Do Your Fingers Make A “Pop” Noise When You Crack Your Knuckles?

When cracking your knuckles, one tends to hear a “pop” noise that is loud, sharp, and irritating to most. This noise can be addicting in the sense that it makes others want to crack their knuckles. The main questions that I focused my research on were “Does cracking your knuckles or joints cause potential health issues for your future?” and “ Why does cracking a joint such as your knuckles make a “pop” noise?” 

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Ski Racing: Where Champions are Made on the Course and in the Lab

If you have ever watched the winter Olympics, you have probably watched in awe as the alpine ski racers flew down the course. Years of training to perfect technique and build strength are essential for any athlete trying to compete with the best, but in a sport where hundredths of a second can separate first and second place, racers are always looking for ways to shave time. Understanding the forces that slow them down and their relationship to body positioning gives these athletes a competitive advantage.

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How to Optimize Biomechanics Forces of Olympic Giant Slalom Skiers

Giant Slalom skiers skiing down slope in tandem.
Photo GEPA pictures

How to Optimize Biomechanics Forces of Olympic Giant Slalom Skiers

            Do you ever wonder what differentiates a casual skier from an Olympic level skier? The distinction lies in the immense forces these Olympic skiers’ output as they naturally transition from incredibly high speeds to sharp turns on the icy slopes. The four famous alpine skiing events held at the Winter Olympics are the slalom, giant slalom, downhill, and super-G events. In these events the human body is pushed to its limit with skiers experiencing forces of up to 2000N during turns through closely spaced poles and gates [1]. Which is the equivalent of a 440-pound weight laying on top of you. These forces are integral in achieving faster times, better technique, and winning Olympic gold. How can these forces and techniques be optimized for the best possible ski run?

            Let’s explore the turn mechanics and forces experienced by giant slalom skiers. Giant slalom was chosen because it combines the quick turns of slalom and the high speeds of the super-G and downhill events [2]. Therefore, it is an adequate choice for investigation. First, we need to understand the forces experienced by a skier during a turn around a gate, which are displayed.

Skier bending down while turning
Forces on alpine skiers (J. Smith representation)

            Skiers experience four basic forces during a turn. They undergo friction forces between the skis and snow and the force of air drag as they blister down the slopes. The force of their body weight due to the constant force of gravity. As well as ground reaction forces (GRFs), which represent the force of the skiers on the ground. These GRFs are highly important as the skier pushes hard against the ground to carve through turns and around the gate. 

            The most important part of the ski turn is the change of direction. Skiers change their direction by digging into the snow while changing the angle of their skies. This causes variability in their outputted forces. Prior to changing direction, skiers output 500N of GRFs, but as they begin to change angle around each gate they skyrocket to 2000N [1]. This is due to the adjustment of ski angle against the direction of motion. The understanding of these forces can be the difference between Olympic gold and last place. 

            These forces are important for the success of the skier as well as their safety. Every skier has different approaches towards their turn around a gate, so it is important to gather data to optimize the success of each ski run. Ski equipment also plays a vital role and could influence ski times, as each skier has difference in preferences for their equipment [3]. 

Understanding these forces are important for the success of the skier as well as their safety. Every skier has different approaches towards their turn around a gate, so it is important to gather data to optimize the success of each ski run. Ski equipment also plays a vital role and could influence ski times. As each skier has difference in preferences for their equipment. Comprehension of these forces play a huge role in enhancing skier safety. Slipping due to high speeds and improper technique can cause crashes and devastating injuries. The goal of this study is to produce a point of fast ski times where safety, equipment, and technique are optimized. There are many unknowns in the optimization of the perfect ski run. With future study of the alpine skiing, skiers and their equipment will continue to develop and their times will become faster. 

References 

[1] Gilgien, Matthias. et al. “Determination of external forces in alpine skiing using a differ- ential global navigation satellite system.” Sensors (Basel, Switzerland) vol. 13,8 9821-35. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3812581/

[2]  Supej, M and Holmberg, H. “Recent Kinematic and Kinetic Advances in Olympic Alpine Skiing: Pyeongchang and Beyond.” Frontiers in Physiology. https://doi.org/10.3389/fphys.2019.00111

[3]  Cross, M. et al. (2021) Force output in giant-slalom skiing: A practical model of force application effectiveness. PLOS ONE 16(1): e0244698 https://doi.org/10.1371/journal.pone.0244698

Secrets of the Rapid Snapping Mechanism of a Venus Fly Trap

Dionea Muscipula, also known as the Venus Fly Trap, is universally considered an interesting and eye-catching plant. Most people are fascinated by its ability to snap its lobes closed around prey, allowing it to then chemically dissolve the trapped animal and subsequently absorb nutrients from its body. What most people fail to realize is the incredible amount of biomechanics required for this plant to survive. Not only is this information useful and interesting to know, but it is crucial for scientists to better understand how plants can respond to physical stimuli.

Labeled diagram of the parts of a Venus Fly Trap

The Venus Fly Trap is native to southeastern United States and is popular due to its unique method of absorbing nutrients. Unlike most plants, they do not obtain most of their nutrients from the soil, instead they absorb them from dissolved insects that they trap within their lobes. Despite the name Venus Fly Trap, their primary prey are spiders, ants, and other insects, especially those that do not fly. These animals are attracted to the plant via the scent of its nectar that is used to lure them to their fate. A detailed process of how they do this can be seen below.

Process of the Trap Closure

  • Attracts insect with a sweet nectar
  • Trigger hairs are touched
  • Sends chemical signals to the midrib
  • Outer lobe expands while inner lobe simultaneously contracts
  • Trap shuts and teeth interlock, sealing off the prey
  • Digestive enzymes break down the insect
  • 7-10 days later the trap reopens
  • Repeat
GIF of a slug being eaten by a Venus Fly Trap

Another interesting aspect of the closure of a Venus Fly trap that most people may not expect is that the hydration of the plant has a massive effect on its closure. Turgid plants, which means they are fully hydrated and cannot absorb any more water, are able to close much quicker than dehydrated plants. It was even found that if a Venus Fly Trap becomes dehydrated in its lifetime before being rehydrated, it still shuts slower than turgid plants. Another side effect of the plants being dehydrated was that the opening angle of their lobe was 10-15 degrees wider than when they were fully hydrated. While the exact cause of this is unknown, it is speculated that as the plant becomes more dehydrated, it is more desperate to absorb nutrients and by widening the opening of the trap, there is a larger surface area for an insect to crawl across and subsequently become trapped. Below is a video of a dehydrated plant that closes much slower than the turgid one shown above.

All in all, the Venus Fly Trap is an incredibly exciting plant that takes advantage of some very unique biomechanics to carve out its niche. While there is much we know about this special plant, there is still much more to for us to discover in the near future regarding its rapid closure and overall biology.

Image of many Venus Fly Traps in various states of trap closure
Venus flytrap” by gwen is marked with CC BY-NC-ND 2.0.

Which Body Mechanics Help You Jump Higher?

Vertical jumping is an essential aspect of many sports. In volleyball and basketball, for example, jumping higher than your opponent gives you a significant competitive advantage. Volleyball players need to be able to block and spike, while basketball players need to be able to rebound well and finish tough shots over opponents. Most athletes know the basics of jumping, but few know what specific body mechanisms contribute to jump height. This article will discuss four key elements to vertical jump height:

  • Squat depth
  • Non-extension movements
  • Arm swing
  • Toe flexor strength

Understanding the mechanics behind each of these elements can help guide athletes in training regimens to better increase jump height.

Squat Depth

Three illustrations displayed, showing the squat depth at each studied angle.
The different squat depths studied. Taken from Gheller et al. (2014).

It seems obvious that squat depth is a part of jumping, but does the average athlete really consider how deep they squat during a jump? A study was done by Gheller et al. (2014) to determine the optimal squat depth to increase jump height. The depth was measured by the angle of the inside of the knee at the bottom of the squat. Participants were instructed to squat to three different depths, < 90◦, > 90◦, and their own preferred, natural squat depth, before jumping as high as they could. Surprisingly, the squats at < 90◦ produced higher jumps than squats at preferred depth. This is primarily due to these jumps producing the greatest takeoff velocity.

Non-Extension Movements

Non-extension movements are movements not related to any lower leg extension. In other words, these are movements seemingly unrelated to the core mechanics of the jump. However, some of these movements were found to have a significant impact on jump height. A study was done by Sado et al. (2020) regarding a running jump off one leg, where the amount of mechanical energy generated by various non-extension movements was calculated. This mechanical energy is converted to energy needed to produce higher jumps (Evert). Again, the velocity of the participants’ center of masses was recorded, from which these energies were calculated. During the takeoff phase, 59% of the increase in Evert was found to be due to the rotations of the stance-leg (jumping leg) calf, free-leg thigh, and the pelvis. The free-length thigh was the largest contributor, followed by the stance-leg calf, then the pelvis.

Arm Swing

The arm swing in a jump is intuitive for most people, but it is important to still understand its mechanics to better utilize the mechanism. During a jump, people swing their arms back as they squat down, then swing them back up as the propel vertically. How does this impact jump height? A study was performed by Hara et al. (2006) where participants stood on a force platform and jumped with and without swinging their arms. Every participant’s jump was higher with an arm swing. This is because the ground reaction force from the force platform increased with the swing, meaning the participant had pushed off the ground with greater force. This created a higher takeoff velocity, resulting in a greater height.

Two step-by-step illustrations comparing what jumps with and without an arm swing look like.
Top: a jump without an arm swing. Bottom: a jump with an arm swing. Modified from Hara et al. (2006).

Toe Flexor Strength

Test subject standing on toe grip dynamometer and squeezing the grip bar with his toes.
Toe flexor strength experimental setup. Taken from Yamauchi and Koyama (2020).

Toe flexor strength is rarely considered when jumping, which is why it is important to understand. In this study by Yamauchi and Koyama (2020), participants stood on a toe grip dynamometer and squeezed the grip as hard as possible. The maximum force was recorded, then separately, participants jumped as high as possible. Participants with greater toe flexor strength also had greater jump heights. This is a lesser-known correlation that can help athletes gain a slight advantage over competitors by training their toe flexors.

This is not an all-intensive list of what goes into jumping but knowing how these mechanisms work can still guide an athlete’s training program. In today’s world, sports are so competitive; everyone is always looking for a leg up (pun intentional). Knowing the biomechanics behind jumping can truly lead to better sports performance.

Falling for You: How to Reduce Fall Risks?

The majority of people know what a fall is and, in fact, many people have unfortunately experienced one or a few. But what would be a good definition for what a fall is? Simply put, a fall is something that happens when you lose your balance and cannot recover. Falls have the potential to ruin anyone’s day. For some, however, the risk is far more severe than that as falls are one of the leading factors in injury and death among the elderly population. This will continue to be a problem as the number of elderly people in the United States is expected to increase dramatically over the next fifty years.

Changes in bone mass for women and men, decreasing as a function of their age.
Graph of bone mass for men and women as they age (Wikimedia Commons)

There are a few reasons that make the elderly a more at-risk population. A major reason is the reduced reaction time that occurs as people age. Another reason is due to the reduction in bone density as people age. Additionally, some elderly people may suffer from disorders such as Parkinson’s Disease or other neurocognitive issues that cause diminished motor control, leading to more instability and a higher chance of falling. Parkinson’s Disease or (PD) is a nervous system disorder that affects both cognitive and motor functions. In PD patients, increased falling is often linked with cognitive decline according to this paper by Fasano, et al. While there is not much research on PD and falls, one benefit is that fall rate could be used as an early predictor of PD in individuals as injuries stemming from falls have the potential to be higher than normal for people even ten years before the onset of Parkinson’s Disease.

Chart with effects of Parkinson's Disease, split between motor and non-motor impairments
Effects of Parkinson’s Disease on the Body (Wikimedia Commons)

The best way to reduce fall risks is to prevent falls. A study by Hahn and Chou attempted to predict fall risk in individuals using a system similar to machine learning called a neural network to evaluate a group of subjects and place them into different groups based on level of fall risk. By categorizing the fall risk of individuals, one can know if they are likely to suffer from falls and can take appropriate measures to prevent them. The subjects of this study were split into two groups classified as healthy, with no preexisting conditions, and fallers, with self-reported issues. Each subject participated in three walking trials and different measurements were recorded including gait measurements, center of mass movement, and muscle response. The first distinction made was to classify each participant as either healthy or as a faller. After finding a baseline for healthy subjects, the participants were further classified by level of fall risk from very low risk to very high risk. This neural network had about ninety percent accuracy in separating healthy patients from ‘fallers’ and was successful in sorting by risk level.

Experimental setup of a trip recovery study
Illustration of Testing Setup for Recovering from Trips. Taken from Bieryla et al. (2007).

Another way to reduce the risk of falls is to reduce the severity of them. This study by Bieryla, et al. contained twelve healthy subjects and was designed to test the application of motor learning to reduce fall risk. It was conducted as a pretest/posttest study where the participants were divided into two groups: a control group and an experimental group. A first trial was performed where a trip was simulated using obstacles placed on a treadmill as shown in the figure. The participants were in a harness and were told to attempt to recover from the trip incident. The dummy obstacles were images that appeared to be obstacles and were used to ensure the validity of the results. After this first trial, the control group spent time walking on the treadmill while the experimental group underwent trip recovery training. The recovery training consisted of practicing stepping over an obstacle that resembled the motion of regaining balance after tripping. Then, a second trial was performed to see the effect of the recovery training. Overall, the results were positive and they showed beneficial effects of the trip recovery training.

An Exploration Into The Growth Of The Heart As A Result Of Certain Exercise

Have you ever wondered what happens to your heart when you begin to consistently exercise? How does the heart change and why? Well, the answer may not be very complicated.

During intense exercise, our heart is put under stress as it has to rapidly pump blood throughout the body. The heart often responds to this by increasing its size, but it does not do this like our other muscles. The heart has to add mass to its existing cells instead of adding new cells as we only have a limited amount of cardiac muscles; the amount we are born with is all we have. The health of our hearts is important. In the US, heart disease and injury are the number 1 cause of death. So, it is in our best interests to learn more about our health so as to minimize our risks of heart-related ailments.

Results of Marathon training

Studies have been performed to analyze the changes in the heart of people as they were training to run a marathon. The main study that I looked at tracked the heart taking measurements every three months. As a result of marathon training the test subjects’ hearts had increased wall thickness, left ventricle end-diastolic volume, and right ventricle end-diastolic volume. The end-diastolic volume is the amount of blood in the heart while it is full. This is all just to say the heart could pump more blood per heartbeat than before.

This graphic shows that the Left and Right Ventricles of the heart increase in volume as a result of running with the left growing more towards the end of the first year while the right grows more initially. Wall thickness is also shown to increase after the first three months then leaving off.
The result of marathon training on the left and right end-diastolic volume as well as the heart’s wall thickness. (Arbab-Zadeh et al., 2014)

The Result of Constant Swimming… on mice

I could not find a study on how the human heart adapts to constant months of swimming. However, I found a study that does it for mice. Each day, mice were forced to swim 5 hours a day for 6 days a week for 9 weeks. At the end of the experiment, the heart of the mouse that endured swimming was 73% larger in weight than the control group.

I could not find a study on how the human heart adapts to constant months of swimming. However, I found a study that does it for mice. Each day, mice were forced to swim 5 hours a day for 6 days a week for 9 weeks. At the end of the experiment, the heart of the mouse that endured swimming was 73% larger than the control group.

Which Exercise is better for the Heart?

While both exercises can promote heart health. For a person who wants to get serious about their heart, running is better for the heart. A study, by Currie, Katharine D et al. in 2018, was done comparing the heart of Olympic-level runners and swimmers. While the sizes of the hearts of the athletes were almost identical. The runners had a lower resting heart rate as a result of larger pumping volume. This may have to do with the fact while someone is swimming their body is horizontal so the heart does not have to pump blood up against gravity. More studies should be done to see the direct impact of swimming on the heart. In the end, both exercises are beneficial to our heart’s health.

Hip Hip Hooray: Joint Functionality Can Be Restored After Hip Labral Tear

Do you experience deep, sharp pain in your groin? Or a feeling of “catching” or “popping” in your hip joint as you go about your daily activities? Is your range of motion you once had now severely limited? If so, you could be experiencing symptoms of a hip acetabular labrum tear, an ever-increasing problem in society that fortunately, has effective treatments.

What is the labrum?

Anatomy of the hip. (Taken from injurymap)

The acetabular labrum is a portion of tough cartilage that lines the rim of the acetabulum, or the “socket” of the “socket-and-ball” structure of the hip that allows us to move and rotate our leg in various directions. The main role of the labrum is to decrease friction between the bones, as well as protect the joint from strong impacts. Cartilage in the hip also stores synovial fluid, releasing it with motion to lubricate the joint similar to how a sponge releases water when squeezed.

Labrums can be either traumatic or degenerative tears. Traumatic tears are typically a result of an accident, fall, or sports injury. Degenerative tears are chronic, and typically the result of femoroacetabular impingement (FAI). FAI is a condition in which there is abnormal bone growth around the joint, causing more friction and leading to eventual wear and tear of the labrum. There are two types of FAI: CAM and pincer, illustrated below. Surgeons can shave down the bony abnormality to recreate the normal shape.

CAM impingement. <a href="http://(Taken from injurymap)
Pincer impingement. (Taken from injurymap)

The most common surgical techniques to repair labral tears in the hip include:

Labral reconstruction with iliotibial band tissue (by Jenny Yates from personal procedure images)
  • Labral resection – smoothing or trimming down of frayed labral tissue is done to remove tissue causing pain
  • Labral repair – the labrum is reattached to the acetabulum with suture anchors that hold it in place 
  • Labral reconstruction – a piece of tissue is used to replace a damaged labrum when it is not repairable, the tissue is held in place with suture anchors along the acetabular rim

For a technique to be valid, the procedure must return the mechanical functionality of the hip joint to its intact state.

The Hip Fluid “Suction” Seal

During compressive loading experienced in walking, the suction seal of the hip pressurizes the synovial fluid stored within the cartilage. The suction seal is critical to the stability of the joint as well, as a negative pressure is created that restricts the femoral head’s motion due to unpredictable external forces.

One study investigated how various labral conditions affected the pressurization of the hip. Cadaver hip specimens with an age and gender distribution were secured in testing equipment in a position that modeled normal walking gait. Pressure sensors were placed in the capsule to measure fluid pressures as specimens were subjected to compressive forces. The results showed that a labral tear does significantly lower the intra-articular pressure of the hip, showing pressures that were 75% relative to the intact state. Labral resections reduce the pressurization even further below the values of a tear, while labral repairs and reconstructions both restore, and even improve, the fluid pressurization to the intact state.

Adhesion breaking the fluid seal by pulling labrum (LA) away from femoral head (FH). (by Jenny Yates from personal procedure images)

Another prominent issue related to the hip fluid seal is the increase in the number of revision hip arthroscopies due to capsulolabral adhesions. Adhesions are a build up of scar tissue that forms as the joint heals. They join to the surface of the labrum and the hip capsule, sticking to both and causing damage to their surfaces. In severe cases, these adhesions are so sticky that they can tether to the labrum and pull it away from the femoral head as pictured on the right, breaking the fluid seal and producing feelings of “giving way.” Dr. Marc Philippon, a leading hip preservationist, has developed both surgical and rehabilitative solutions to address this concern. In the surgical solution, the surgeon creates a “spacer” offset between the labrum and capsule to prevent adhesions from binding the two surfaces together and allow the labrum to maintain contact with the femoral head. The rehabilitation techniques utilize continuous passive motion to actively prevent the formation of adhesions, most notably circumduction. Low impact active range of motion exercises such as cycling or aquatic therapy is also encouraged to return blood flow to the joint.

Contact Area and Pressure

Since distributing load pressure and increasing the contact area to improve range of motion are two of the main functions of the labrum in protecting and stabilizing the femoral head, it was also important for researchers to investigate how contact area and pressure of labral conditions compared to the intact labral state. The measurements were taken in various degrees of extension and flexion to mimic stair climbing. It is optimal to have a greater contact area to distribute loads and increase range of motions, and a lower contact pressure to protect the bone against impact, so the labral resection was determined to be an insufficient technique once again since it did the opposite. Under the same criteria, it was determined that a reconstruction can return the labrum to more ideal conditions than the intact state.

Jumping into Better Bone Health: Impact Exercise and Your Bones

When exercising for overall health, the general public tends to disregard the importance of bone health. Often the focus is on consuming milk or calcium rich foods, but are there certain exercises that can increase bone health? Studies show that the presence of impact in exercise plays a major factor.

As we age, everyone loses bone mineral density, which is a determining factor in bone strength and stiffness [1]. Decreasing bone mineral density can lead to bones that easily break and fracture, and will, in extreme cases, result in the disease osteoporosis. Women are at a higher risk of osteoporosis because they lose more bone mineral density as they age due to the process of menopause [2]. Increasing bone mineral density at younger ages can ensure that even with the inevitable bone loss, peoples’ bones are still strong.

Bone mass as a function of age (Wikipedia Commons).
Simone Biles Back Band Handspring on Beam. (KSN: Tokoyo Olympics).

Adding high impact activities into one’s exercise routine can increase bone mineral density and prevent weak bones from developing. When athletes engage in high impact activities, the forces from the impact induce small strains in their bones. Strain in bones causes bone growth which increases the bone mineral density of the impacted bones, therefore making them stronger [3]. When gymnasts perform back handsprings or flips, or when volleyball players jump high into the air to spike the ball, they load their bones with high forces. A study done on college athletes showed that gymnasts and volleyball players had significantly higher bone mineral density than swimmers, which is considered a low-impact sport. Additionally, the swimmers did not vary much from the non-athlete group signifying that rigorous exercise is not enough to increase bone health without the presence of impact. The gymnasts also had higher levels of bone mineral density in their arms than all groups, because they were the only sport that induced significant strain in their arms [3].

There are many types of impact sports, from cross country to gymnastics. All impact sports increase bone mineral density in their participants, but the sports that involved the highest loads such as jumping increased bone mineral density more than medium impact sports [4][5]. Running is a great pastime for overall health but if bone health is a priority more focus should be given to even higher impact activities. Like any other type of workout plan, consistency is key. College athletes show significant increases in bone mineral density from the pre-season to the post-season, meaning that they often lose bone during their time off working out.

Long Jumps and Plyo Jacks (Verywell Fit).

It is still unclear what precise frequency of bone impact or amounts of strain optimize bone growth. Higher levels of load generally led to stronger bones, but obviously there is a limit to the amount of load bones can take without breaking. What is the optimal amount of strain to put bones under to produce the strongest possible bones? This is still up for debate, but it is well established that only activities with impact play a role in increasing bone strength. If you’re an avid swimmer or cyclist, consider adding running or some jump squats into your cross training to keep bones strong and healthy.

Works Cited

16 Mar. 2022

[2] Nazarian, Ara, et al. “Tensile Properties of Rat Femoral Bone as Functions of Bone Volume Fraction, Apparent Density and Volumetric Bone Mineral Density.” Journal of Biomechanics, vol. 44, no. 13, Sept. 2011, pp. 2482–88. DOI.org (Crossref), https://doi.org/10.1016/j.jbiomech.2011.06.016.

[3] Fehling, P. C., et al. “A Comparison of Bone Mineral Densities among Female Athletes in Impact Loading and Active Loading Sports.” Bone, vol. 17, no. 3, Sept. 1995, pp. 205–10. DOI.org (Crossref), https://doi.org/10.1016/8756-3282(95)00171-9.

[4] Carbuhn, Aaron F., et al. “Sport and Training Influence Bone and Body Composition in Women Collegiate Athletes.” Journal of Strength and Conditioning Research, vol. 24, no. 7, July 2010, pp. 1710–17. DOI.org (Crossref), https://doi.org/10.1519/JSC.0b013e3181d09eb3.

[5] Dook, Jan E., et al. “Exercise and Bone Mineral Density in Mature Female Athletes:” Medicine amp Science in Sports amp Exercise, vol. 29, no. 3, Mar. 1997, pp. 291–96. DOI.org (Crossref), https://doi.org/10.1097/00005768-199703000-00002.