Tag Archives: impact

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.



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.








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.