Category: 2019 Spring

Female Athletes Compete Against Higher Risk of ACL Injuries Than Males

Female athletes face a greater rate of anterior cruciate ligament (ACL) rupture than males. According to Dr. Karen Sutton and Dr. James Bullock from the Department of Orthopaedics and Rehabilitation at Yale University, female athletes are 2 to 8 times more likely to tear their ACL than male athletes. The majority of these injuries (more than two-thirds) are from non-contact situations. A variety of anatomical, biomechanical, and hormonal factors attempt to explain this difference.

Female soccer player stretching her leg
Photo by rawpixel on Unsplash
Differences between female and male lower-body anatomy show the disparity in Q-Angle that results
Taken from Desrosiers, Soccer Nation 2018

Some anatomical factors that help stabilize the knee joint and may be linked to ACL injuries include: the quadriceps angle (Q angle), tibial slope, and intercondylar notch. The Q angle is the angle formed between the upper leg at the hip joint and the lower leg at the knee joint. This angle tends to be 3.4-4.9 degrees greater in females than males when measured in a standing position. The figure at right shows the Q angle difference between men and women that is caused by anatomical differences including a wider pelvis in females. A greater Q angle causes more strain on the quadriceps muscle away from the centerline of the body, which can affect the position of the ACL to be more prone to rupture.

Tibial slope is a quantity used to describe the position of the tibia relative to the femur. When the tibia is positioned more forward than the femur there is a greater posterior tibial slope and therefore increased ACL strain. On average, females have shown to have a greater tibial slope, which may contribute to the higher incidence of ACL injuries. The figure below illustrates the biomechanics of posterior tibial slope: the effect of the knee joint compressive load (down arrow) and the force of the quadriceps (up arrow) result in an anterior shear force, causing anterior translation of the tibia relative to the femur (right-directional arrow) .

Biomechanical force diagram describing posterior tibial slope
Modified from Sutton and Bullock, JAAOS 2013

In terms of biomechanical differences between men and women, women have greater natural muscle contractions for movement away from the centerline of the body. This translates to a difference in landing positions for women compared to men – females tend to land more straight, creating more force on the knee joint, while males absorb the impact better by naturally flexing their knees upon landing. The hamstring to quadriceps ratio (H:Q ratio) is the functional strength of the hamstring muscles (peak torque) relative to the strength of the quadriceps in motion. Poor muscle strength has been linked to higher risk of lower extremity injury. Males have the ability to increase their H:Q ratio during sport motion, but females fail to do so. Women have also shown greater internal rotation laxity – slackness or lack of tension in a ligament – than men. Generalized laxity was also significantly greater among individuals who suffered a noncontact ACL injury compared to an uninjured control group.

Hormonal factors are an additional consideration that researchers have explored, but the results have been inconclusive in making a direct link between hormone levels and the rate of ACL injury.

Additional reading on this topic can be found at VeryWellHealth and SoccerNation. The following video shows some advice for female ACL injury prevention.


Striking Out the Myths behind the Curveball

Anybody who has played baseball growing up was probably told “Don’t start throwing a curveball until you are ‘X’ years old.” That “X” in there for the age was normally around fifteen or sixteen years old depending on who you asked. When an eager, young ball player responded with “Why,” it was normally answered by “Because you will hurt your elbow and shoulder.” No sixth or seventh grade kid is really going to question that statement beyond asking another adult, and subsequently getting the same answer. Likewise, no youth baseball coach has really put in the effort to research whether or not learning to throw a curveball is detrimental health of young athletes.

A study was recently conducted by professionals at Elite Sports Medicine at Connecticut Children’s Medical Center to find out the answer. The study was aimed to analyze the shoulder and elbow joints of several teenage pitchers as they threw multiple fastballs and curveballs. They were specifically looking at the moments put on the elbow and shoulder and comparing those between pitches. A moment is a measure of a force on an object and the distance away from the object the force is being applied, mostly resulting in rotation. A moment can also be thought of as torque.

This image shows the grip and wrist position for a curveball
From McGraw, How to Play Baseball, a Manual for Boys

After warming up, the athletes selected for the study had reflective markers placed on their body. These markers assisted in gathering information for “3-Dimensional motion analysis”. This analysis allows the researchers to record “kinematic and kinetic data for the upper extremities, lower extremities, thorax, and pelvis” for both the fastball and the curveball. The researchers found that the moments in the shoulder and in the elbow are lower when throwing a curveball compared to a fastball. This means that the rotational force put on the joints is actually less severe in a curveball than a fastball. The only thing found that is more intense in a curveball than a fastball is the force on the wrist ulnar, which is used when making the motion trying to touch the wrist to the pinky finger. The wrist and forearm motion and forces were the only significant differences between the two pitches.

From this data it is easy to see that the reason for not learning curveballs at a young age has nothing to do with shoulder and elbow injury. There may be a reason related to wrist injury, but that is yet to be explored. A fastball is actually harder on the joints than a curveball. For whatever reason, youth coaches have always preached not to throw curveballs until you absolutely need to. They may have their reasons, but science has shown that it is not realistic to blame injuries.

For further reading on this topic, please see these articles from Driveline Baseball, The New York Times, and Sports Illustrated.

Are Muscle Loads During Irish Dance Unsafe?

Do you think your ankle can bear loads more than 14 times your body weight? Can you look graceful while doing it?

This question is one of great importance for many athletes participating in high-impact sports. Dancers of all kinds have strived, for years now, to perfect the balance of athleticism and grace that their competitive markets demand. Achieving this balance is no simple feat, and many dancers fall victim to injuries during their countless repetitions of high impact leaps and landings, and Irish dancers are no exception. Researchers have recognized the need for biomechanical analysis of muscles and joints of Irish dancers, and created a model to do just that.

The method for one such study by James M. Shippen and Barbara May assumed an Irish dancer’s body to be a composition of discrete rigid bodies, each having a unique mass and set of inertial properties that dictate its motion. Joints connect these segments, and muscles that cross numerous sections exert forces between those segments.  The study analyzed 196 muscles, which in turn generated 53 torques on various joints. A least-squares regression minimized muscle activation, which is a physiologically sensical model, as lower muscle activation decreases fatigue. The solution for the model was subject to three logical conditions: muscle torques are equal to the sum of the external and inertial torques at each joint, the muscle loads must be positive (as muscle contraction only allows for pull), and the load must be smaller than that which the muscle can maximally generate. The finalized model represented 35 joints, each constrained in accordance with its anatomical analog (i.e. the human hip joint is spherical, so the hip joint in the model has equivalent degrees of freedom.)

Researchers attached optical tracking markers to the test subject dancers (similar to the image on

Optical trackers attached to a walking test subject
Photo from Advanced Real Time Tracking

the right) to analyze motion at 250 frames per second; force plates were mounted into the ground to measure reaction forces at 24,000 samples per second. 5 Irish dancers performed the rock step (video), among other common Irish dance moves, during which the dancer uses one ankle to push the other to the ground (see video below for example of rocks).

Muscle activation in the gastrocnemius and soleus during the rock step
Shippen & May, Journal of Dance Medicine & Science 2010

The relevant components analyzed during the rock step were the Achilles tendon and attached soleus and gastrocnemius muscles. Shippen and May’s calculations yielded forces of 2700N on these ankle muscles and tendon.  Normal muscle forces due to walking are in the range of 500-1430N for worthwhile analogy. The figure on the left shows the activation of the two relevant muscles during the rock step, and it is evident that the gastrocnemius reaches an activation of 1.6, far above the maximal isometric force activation of 1.0.  This is demonstrative of the risk associated with this specific move, and why the loads can be unsafe.

Componentized contact forces in the right ankle during the rock step
Shippen & May, Journal of Dance Medicine & Science 2010

The figure on the right shows the contact forces in the right ankle componentized into external loads (reactions forces with the floor) and forces generated internally (from muscles acting at the joints). The maximum load for this dancer was about 14 times her body weight, with the great majority of that coming from muscle forces.

This study demonstrated the risk associated with the rock step in Irish dance. It also is helpful for future research, as it suggests that emphasis should be placed on reducing the internal muscle loads that work to stabilize the ankle, as opposed to modifying ground reaction forces, which have a smaller margin for success.

The mathematical model developed in this research can be applied to many other athletes and examples of human motion in the future to understand internal forces and reduce risk of injury in athletes.

For more research on biomechanical models of the human body and their applications, read about this humanoid robot and human sitting posture for driving.



Do Running Injuries Depend on the Running Surface?

Photo by MabelAmber on Pixabay

Imagine that you are on your typical route for a morning run when you decide to change things up. Instead of following the path of the sidewalk like usual, you take a shortcut across the soft grass and run alongside the concrete for awhile. No big deal, right?

Although this change in running surface may not seem to be a big deal, there is a lot more going on behind the scenes than runners are consciously aware of. As you step from the firm concrete to the soft soil, your body automatically adjusts its stride to accommodate the change and you hardly notice the difference. Specifically, your legs calibrate their angle relative to the ground and tension in the quadriceps before taking a step onto a new surface to maintain balance and speed as you run.

Image of a person running with one leg modeled as a spring
Modified image by Kulmala et al. in Scientific Reports from

In a study performed by Daniel Ferris et al., researchers examined the mechanics of surface-dependent running. Participants ran at a constant speed across a track with both a rigid and a compliant surface, and the force of the leg, time of foot-to-ground contact, and angle of the leg at initial ground contact  were measured for each trial. The resulting data were then analyzed by modeling the legs of the runners as simple springs (left). The research concluded that runners quickly and instinctively adapt the stiffness of their legs in response to changes in surface elasticity. This allows runners to keep a constant running speed without wasting energy due to excessive vertical motion of the torso as the body adjusts to the new surface.

Leg modeled as springs moving from one running surface to another
Image by Ferris, et al. in Journal of Biomechanics

Without this intrinsic adaptation of leg stiffness, runners would have to drastically change their strides to remain upright as they ran from surface to surface (right, with stiffness denoted as kleg). In fact, leg stiffness decreases when you run on rigid surfaces and increases when you run on springy surfaces. In short, the stiffness of your legs compensates for the firmness of the ground you are running on.

So how does leg stiffness translate to injuries and injury prevention?

First of all, it is important to note that injuries to muscles and ligaments have different causes and symptoms than injuries to the bone. Knowing this, experts suggest preventative measures specific to the type of injury. For example, one article claims that high leg stiffness while running corresponds to a higher risk of bone injuries. As a result, we can use the information above to deduce that the best way to prevent stress fractures is to run on hard surfaces such as concrete or asphalt in shoes with little cushioning. Conversely, low leg stiffness corresponds to a higher risk of muscle and ligament injuries and the best way to prevent these injuries is to run on dirt, grass, or a track in padded shoes. The video below explains this correlation in further detail.

See also padded shoes paradox and leg stiffness and running performance for further reading.


Do Wrist Guards Prevent Snowboarding Injuries?

Snowboarder grabbing board while in the air after going off a jump.
Photo from Markos Mant on Unsplash

Snowboarding is a breathtaking sport yet carries with it an inherent risk of injury. Wrist protectors provide potential protection against snowboarding wrist injuries. However, some studies have argued that wrist protection transfers the injury to other parts of the forearm.

A 2001 joint study by the Lillehammer Central Hospital (Now part of Innlandet Hospital Trust) and University of Oslo Department of Orthopedic Surgery explored the efficacy of wrist protectors in preventing snowboarding injuries.

Studies like this are very important in growing winter sports, as more athletes will pick up snowboarding or alpine skiing if the risk of serious injury can be further decreased.

A total of 5029 snowboarders were included in the study, with 2515 snowboarders wearing a brace and 2514 snowboarders not wearing a brace. The brace used was a D-ring wrist brace. A physician examined the participants at the end of each day snowboarding and was not aware if the subject had worn a wrist protector or not. The physician defined a wrist injury as an evident fracture, sprain, or pain in the wrist that lasted for at least 3 days.

Front and Side view of a participant wearing a D-ring wrist brace.
Front (A) and side (B) views of D-ring Wrist Protector. Modified from Rønning, Rønning, Gerner, and Engebretsen, The American Journal of Sports Medicine 2001

A limitation of this study comes with setting an endpoint for what qualifies as a wrist injury. Both fractures and sprains qualify as wrist injuries. Wrist pain must be accompanied by decreased range of motion for 3 days to qualify as a meaningful wrist injury.

The study results showed that the braced group experienced 8 wrist injuries, while the control group recorded 29 wrist injuries. This is a statistically significant difference in the number of wrist injuries experienced by each group.

Of the subgroups explored in this study, beginner snowboarders with less than 5 days of snowboarding experience were found to have significantly more wrist injuries than the snowboarders with more than 5 or more days of experience.

A snowboard constrains both legs and feet in strapped bindings. When a snowboarder begins to lose their balance, a snowboarder will commonly extend their arms to brace the fall. When the wrist is flexed upwards during a fall, the wrist absorbs the energy of the fall and causes a fracture or sprain.

An effective wrist protector absorbs as much energy as possible without providing additional stress areas to the forearm. A wrist protector that is designed with too much rigidity will generate a high stress force above or below the wrist. The study confirms the benefits of wearing a protective wrist guard while snowboarding, and the physician found no injuries in the arm due to the use of a brace.

However, most wrist guards still available are uncomfortable to wear with winter gloves, so the study recommended future gloves be designed with built-in wrist guards. By improving the safety of alpine sports, snowboarders will feel comfortable pushing the boundaries of the sport and attempting more unforgettable tricks!

For more on injury prevention in snowboarding, check out this article by the Daily Herald or click here.

What Happened to Markelle Fultz’s Shot?

What happened to Markelle Fultz? This is the question on the minds of many basketball fans who have watched a promising player slip into a sharp decline in his first two seasons in the NBA. The former 1st pick in the 2017 NBA draft was known in college for his ability to score; however, so far in his career, his shooting statistics have fallen dramatically as he seemingly forgot how to shoot the ball. A couple of painfully awkward shots can be seen below as Fultz tried new methods of shooting the basketball:

A few months ago, his difficulties were diagnosed as neurogenic thoracic outlet syndrome (TOS). But what is neurogenic TOS and how does it impact Fultz’s shot?

Male figure shown with location of thoracic outlet between the base of the neck, the clavicle and the arms.
White shaded area shows the position of the thoracic outlet on the body. From University of Washington School of Medicine in St. Louis.

A paper by neurosurgeons Jason Huang and Eric Zager of the University of Pennsylvania on TOS gives insight into Fultz’s diagnosed condition. The thoracic outlet is an intersection of nerves and blood vessels that run through the gaps between the base of the neck, the clavicle, and the arm. Neurogenic TOS occurs when there is compression of the brachial plexus, a bundle of nerves that run between the scalene muscles, the clavicle (or collarbone), and the subclavian arteries. When certain arm motions are performed, the space in the thoracic outlet can become smaller, leading to increased compression.

A picture shows the muscle, nerves, arteries, and bone that make up the thoracic outlet.
Representation of the thoracic outlet including the scalene muscles, the brachial plexus nerve bundle, the subclavian arteries, and the clavicle bone. From Huang and Zager, in Oxford Academic.

Particularly in men, it is common for the scalene muscles to cause TOS, and research has shown that it can happen through repetitive use or sports. There have been reports of baseball pitchers diagnosed with TOS because of the awkward arm motions from throwing the ball.  Often TOS is accompanied by a dull pain in the neck, shoulders and arm where affected, but is not sharp and is often characterized by discomfort, especially with overhead motions. This would explain why Fultz’s shooting motion could be uncomfortable and cause his brain to focus on the pain caused by the nerve compression.


So what is the treatment and what is Fultz’s timetable for return?

Sometimes for patients with TOS, surgery is an option, but not often for the type Fultz is likely experiencing, since they are tricky and carry high risk due to the presence of major nerves and arteries. Often a more conservative treatment is prescribed, and it seems as though Fultz is doing physical therapy. His initial timetable for return was listed at 3-6 weeks, but there is no indication of an immediate return, and there is little data to predict the length of recovery with physical therapy.

Because of the unpredictability of the treatment, the uncertainty surrounding Fultz seems to be just as thick with the diagnosis of TOS as it was before. However, the ability for Fultz to recover and relearn how to shoot will be imperative in determining whether he will return to his original form as an elite scorer or become one of the biggest busts in the history of the NBA.


Further reading on this topic can be found from The Washington University School of Medicine and In Street Clothes.

How do Hummingbirds and Nectar Bats Hover?

What do hummingbirds and nectar bats have in common?

Bat feeding. Photo from Pixabay.
Hummingbird feeding. Photograph from Shutterfly.


Due to their dietary needs, evolution played an important role in the flight mechanisms of these species. In order for them to collect nectar, they developed the ability to hover over flowers.

Understanding hovering capabilities of these animals has been unclear for a long time. Hence, researchers, Ingersoll, Haizmann, and Lentink, set on discovering how exactly these species do it in this research paper.



The researchers headed to the tropics, Costa Rica, for 10 weeks to conduct the study. This destination was chosen, because it is home to 10% and 15% of the worlds’ respective bat and hummingbird populations. In the neotropical environment, they studied 17 hummingbird species and 3 bat species. They chose popular species that were representative of the environment.

The researchers captured living birds and bats to measure forces exerted from their wings. They also digitized their wing kinematics to see similarities and differences between them. They placed the species into a 0.125 cubic meter box as seen in figure 1. On the walls of the box, they installed force sensors and plates to measure the forces exerted by their wings. They recorded the species with a high resolution camera. After they collected the data, they released the animals back into the wild.

Figure 1: The experimental setup. Modified from Ingersoll, Haizmann, and Lentink, Science Advances 2018. 

After running a convergence study, they found that hummingbirds and nectar bats have different wingbeats. Hummingbirds create a quarter of vertical aerodynamic forcing during the upstroke of their wingbeat—meaning that when their wings go up, they create a force that is 1/4 of their body weight. Hummingbirds’ wingbeats are more horizontal than generalist birds and bats, which helps generate this lift. On the other hand, nectar bats generate elevated weight supporting during the downstroke, by inverting their wings more than hummingbirds with a greater angle of attack. Theoretically, this takes up more power than hummingbirds’ wingstroke. However, due to the fact that bats have a large wingspan, energy costs are made up and power used becomes similar to the hummingbird per unit body mass.

The researchers also decided to look into interspecies differences to see if different hover poses, due to different diets, produced different upstroke support. In both hummingbirds and bats, there was no remarkable difference. 

Therefore, the study concluded that hummingbirds are more efficient, due to symmetry in beating back and forth, which creates a lift force upward to reduce drag and power required. However, bats are able to compensate for the lack of vertical force during upstroke, with large wingspan and a higher angle of attack to maximize aerodynamic force to combat gravity, by combining lift and drag forces on the downstroke. 

These findings will largely help engineers understand design tradeoffs, like the ones discussed, with aerodynamic power to help aerial robots, like the Nano Hummingbird and Bat Bot seen in these videos:

For more information check out this!


Find out more about Bat Bot here.