Kinesiology and the 2016 Olympics: Part VI Race walking

Today we have a guest blogger,  my doctoral student, Jeffrey Simpson.  Jeff is a second year student in our department working towards his PhD in biomechanics, and he is a big fan of race walking. Check out his thoughts below, and check you can check out Jeff's race walking form by clicking on this link.

This morning while I was eating breakfast and drinking my daily cup (or two) of coffee, I received a text message from Dr. Knight telling me that Olympic race walking was on NBC Sports. Since I arrived at Mississippi State in the Fall 2015 to work on my PhD in biomechanics under Dr. Knight, my lab mate and colleague, Brandon Miller (other PhD student in biomechanics), and I are always talking about race walking and discussing how competitive the sport has become. If you’ve never heard of race walking, I promise that you’re missing out.

In the 2016 Rio Olympics there are both men’s and women’s 20 and 50 km races. Believe it or not, these race walking athletes are able to “walk” 20/50 km faster than the majority individuals can run a 20/50 km. So how do we classify someone who is walking or running? The two main phases during the gait cycle we examine are the stance phase (when the foot is in contact with the ground) and the swing phase (when the leg moves forward). The phase we focus on when determining if someone is walking is the stance phase, which has two periods. The double limb stance period (when both feet are in contact with the ground) and the single limb stance period (only one foot in contact with the ground). However, if neither feet are in contact with the ground at any given moment this would be considered the flight phase (period of no support), which indicates running. This is how the IAAF enforces the rules and regulations of race walking, which states that the athlete must have no visible loss of contact with the ground (one foot must always be in contact with the ground) and the knee must be extended from the moment of initial contact (when the foot strikes the ground) until the athlete reaches the upright position. Throughout the race (20 or 50 km), officials constantly monitor athletes and issue red cards if the previously mentioned rules are broken. If an athlete receives 3 red cards, they are then disqualified from the race.

From a biomechanics perspective, we are able to analyze the gait kinematics and determine the individuals walking speed. As with running, the two main factors that determine speed are stride length and stride frequency (also referred to as cadence). In order for a race walker to optimize their stride length/frequency, they must exaggerate their gait by excessive pelvic rotations (shifting their hips left and right) and shoulder movements to oppose the pelvic rotations. Additionally, athletes will decrease the time their foot is in contact with the ground (ground contact time).  They also position their support leg close to their midline so that their center of mass passes directly over the support foot in order to maintain walking speed by decreasing breaking forces, which helps maintain movement efficiency. Although we might view this event as a joke, or even think it is funny, it is quite amazing what these athletes are able to accomplish. Race walking is actually a difficult task, if you try to “race walk” you’ll quickly find out that it’s hard to maximize your stride length/frequency without running. Provided in the link is a YouTube video of elite race walkers from the 2012 Olympic Games in London. Watch how quickly they are able to move while still being able to maintain walking gait.

https://www.youtube.com/watch?v=rdXD2Fe6Hx4

Kinesiology and the 2016 Olympics Part V: Appreciating Usain Bolt

Usain Bolt made history on Sunday night, becoming the first athlete to win the men's 100 m race at three different Olympics (2008, 2012, 2016).  There have been very few to win the 100 m race at back to back Olympics, so for Bolt to do it at back to back to back is very impressive.  He is definitely the best sprinter of all time and this accomplishment may never be matched.

From a biomechanics perspective, there are several factors that enable Bolt to run at such a high velocity.  At six feet, five inches tall, he has very long legs, which enables him to cover more ground with each step and stride than his competitors.  However, being tall and having long legs does not automatically mean you will be a fast runner. Take Shaquille O'Neal for example.  What Bolt is able to do that most people with long legs cannot do is to have a very high step and stride frequency, meaning he can apply a force very quickly to the ground, pick his foot off the ground, and swing his leg through and get it back on the ground in a very short amount of time. We can measure how hard it is to rotate something by an object's moment of inertia, which is the product of the mass of the object and how the mass is distributed about the axis of rotation (radius of gyration, which is a squared value).  For Bolt, he has very muscular and long legs, which increases their mass. The length of his legs also moves the mass further away from the axis of rotation (the hip).  So, Bolt has a greater moment of inertia when swinging his legs while running than his opponents.  Imagine swinging a baseball or softball bat.  As the bat gets more massive (heavier) and longer, it becomes more difficult to swing the bat. The fact the Bolt can overcome this larger moment of inertia and still achieve a very high step and stride frequency really sets him apart from all the other sprinters.

Bolt is also a very powerful runner.  He is able to apply a larger amount of force to the ground in a shorter amount of time than his opponents, thus giving him a greater amount of impulse and a greater change in momentum.  This interactive piece from the New York Times offers a really good analysis of Sunday's race.   Although Bolt has a poor reaction time (0.155 s on Sunday, which was the second worst in the field), he is able to make up for that with his stride length, stride frequency, and his superior ability to apply more force to the ground in less time than his opponents.

Here is another piece from the New York Times that shows how Bolt stacks up against all previous medalists in the 100 m race.  It's a very interesting look at the history of the race.  Bolt still has the 200 m race to run, it will be very interesting to see if he can also win that one again.

Kinesiology and the 2016 Olympics Part IV: False Starts and Reaction Time

With the Olympics shifting away from the pool and towards the track, you might notice that a race starts but they stop it immediately, call all the runners back, and make them start again.  Also, the runner that started early is disqualified from the race. Sometimes, it is obvious that a runner starts before all the others and is disqualified.  Other times, it may be tough to tell that a runner started too soon.  So, how do they know that a runner "anticipated" the starting gun and started too soon? 

The starting blocks the athletes use have sensors in them that detect the amount and timing of the force applied to them by the runners' feet.  If the force applied to the blocks exceeds a certain threshold (I haven't been able to locate what this threshold is) in less than 0.1 (100 ms) seconds after the gun is fired, the runner is called for a false start and disqualified from the race.  This happened here to Usain Bolt in at the 2011 World Championships.  Why is this rule in place and why was 0.1 seconds chosen?

The purpose of the rule is to prevent the athlete from anticipating the firing of the starting gun and gaining an advantage on their opponents.  Most humans can react voluntarily to a stimulus (auditory, visual, etc) in about 120-150 ms.  Now, well trained athletes can react quicker than this because of their training, and there have been calls to lower the threshold to 90 ms (0.09 seconds).  Why does it take these athletes anywhere from 100-150 ms to start the race after the hear the firing of the gun? The answer lies in the nervous system and muscular system.

When responding to an auditory stimulus, the signal has to travel from the ears through sensory neurons to the brain.  Since this is a simple reaction time scenario (only one stimulus and one response), the brain can quickly process this signal and activate the alpha motoneurons that control the muscles in the legs, but it will still take some time for these signals to get from the brain down the motoneurons and to the muscles.  The time it takes from when the gun is fired to when the signal reaches the appropriate muscles is called the pre-motor time.  Once the signal reaches the muscle, it still takes time for chemical and mechanical process to occur within the muscle, force to be developed, and movement to occur.  This is known as the motor time.  Although it takes time for both of these processes to occur, it is remarkable to me the human body can respond so quickly.  However, if it occurs too quickly at the start of the race, the athlete will be disqualified.

Kinesiology and the 2016 Olympics Part III: Propulsive Impulse

The last post discussed impulse (the product of the average amount of force and the time over which it is applied) when landing from a jump, and how it was important to increase the amount of time over which the force is applied when landing with a "soft" landing to decrease the injury risk.  When applying force, especially in a sport like volleyball, the force has to be applied to the ground when jumping or to the ball when attempting a spike over a short time period in order to be successful.

First, let's examine the vertical jump.  Volleyball players have to jump repeatedly over the course of a match.  If they are jumping to spike a ball or block their opponent's spike, they want to apply a large amount of force to the ground in a very short time period so they can reach the peak of their vertical jump as quickly as possible.  If two volleyball players, one on offense, and one on defense, both have the same vertical jump height, but one player can reach their peak height faster (in less amount of time) than the other, then that player will gain an advantage.  One way to reach the peak of their jump faster is by applying a large amount of force over a shorter time period than their opponent.  The more powerful the person is (power is the product of force and velocity, or how quickly someone can develop force), the more successful they will likely be.

Second, let's examine the volleyball spike.  When a player spikes a volleyball, their hand is in contact with the ball for a very short period of time.  They have to apply as much force as possible over a very small time period.  The same principle is true for other striking activities like hitting a baseball/softball, tennis ball, etc.   The more velocity the player can generate (the faster the can swing their arm), the greater the momentum they can transfer to the ball, which will result in a greater amount of impulse, because change in momentum is equal to the change in impulse (a post for another day).  The first graph on the bottom figure demonstrates how in the volleyball spike the propulsive impulse is generated by applying a large amount of force over a very short period of time.  The second part of the first graph relates to the last blog post regarding landing, which was applying a smaller amount of force over a longer period of time.  The second graph would relate more to a person jogging/running at a constant velocity.

Flannagan, S.P. (2014) Biomechanics: A case based approach (1st ed.), Jones & Bartlett.

Kinesiology and the 2016 Olympics Part II: Impulse when landing

During the first post on the Olympics I discussed how many different injuries can occur when an athlete lands from a jump.  Not only is the amount of force they have to absorb when landing a key factor, but the time over which that force is applied is also a very important factor.  Impulse is the product of the average amount of force and the time over which that force is applied.  In the graph below (Flannagan, S., 2014), the impulse for both the "stiff" landing and the "soft" landing are the same, but the amount of force that has to be absorbed by the jumper with a "stiff" landing is twice that of the force in the "soft" landing.  So, what is a "stiff" landing and what is a "soft" landing?  In a "stiff" landing, the joints of the lower extremity, primarily the hips, knees, and ankles, stay in a more extended position and do not give, or flex, as the person lands.  This means that the ground reaction force is applied over a very short time period.  In a "soft" landing, the person flexes their hips, knees, and ankles (technically dorsiflexion of the ankles) as they land to increase the amount of time over which the ground reaction force is applied.  This leads to a lower amount of stress placed on the lower extremity. That's why it is important to teach young athletes to perform a soft landing by flexing the joints of the lower extremity when they land (second figure below).

Flannagan, S.P. (2014) Biomechanics: A case based approach (1st ed.), Jones & Bartlett.

Kinesiology and the 2016 Olympics: Part I Tibia/Fibula Fracture

Yesterday, French gymnast Samir Ait Said suffered a horrible fracture (warning, graphic picture at the bottom of this post) of his tibia/fibula during the vaulting event.  Gymnastics as a sport carries a high degree of risk, and to me, vaulting might be the most dangerous event.  The gymnast runs as fast as possible, uses the vault to launch themselves as high as possible into the air, and then performs a series of flips and twists before landing.  Here is the video of the injury if you want to watch it. Gymnasts, and athletes in other sports, such as basketball and volleyball, land repeatedly during practice and games/competitions, and these types of fractures are very rare.  So, what went wrong to cause this injury and others similar to it (think about Kevin Ware in the NCAA basketball tournament a few years ago).

First, because Said projected himself so high in the air, when he landed he applied a large amount of force to the ground, and due to Newton's 3rd law of motion (for every action there is an equal and opposite reaction), the ground applied a large amount of force back to his foot.  Also, because gymnasts are taught to "stick the landing" in order to maximize their score, a large amount of force is applied in a very short time period (the product of the average amount of force and the time over which it is applied is called impulse, which applies to almost every Olympic sport/event).  Now, the mat helps dissipate some of this force, but it was still likely hundreds of pounds of force, probably 4-5 times greater than his own body weight.  However, he has probably landed from this same height with around the same amount of force hundreds of times during his gymnastics career, and never suffered an injury like this.  So, what really went wrong this time?

If you watch the video, you will notice that he did not complete his last rotation, and therefore was not able to get his feet directly underneath his body, or his base off support was not directly under his center of mass.  Ideally, when landing from a jump, it's best if the feet are directly underneath the person' center of mass.  Going back to Newton's third law, this will ensure that the ground reaction force is applied in the vertical direction and will be transferred directly up the person's lower extremity.  When Said landed, his feet were not directly under his body, so instead of the force from the ground being applied mainly in the vertical direction, it is applied at about a 45 degree angle from the ground.  The created a bending moment around the midpoint of his tibia and fibula, and because of the large amount of force, both the tibia and fibula broke.  I'll do another post discussing the relationship between the amount of loading placed on a bone and how much it can deform before breaking.  While this was a devastating injury for Said, hopefully with surgery and many months of rehabilitation he can make a full recovery.  It also demonstrates that in many sports, the most dangerous part is landing from a jump.  While fractures like this are pretty rare, other injuries such as ankle sprains, achilles strains/tears, and ACL tears often times occur when landing from a jump.

Kinesiology and the Olympics and Brett Favre

 Four years ago during the 2012 London Olympics, I did a series of blog posts about "Biomechanics and the Olympics" which highlighted some events and the biomechanical aspects of those events.  I've been encouraged to do that again (by my wife), so this year I'm going to call it "Kinesiology and the Olympics" and broaden the scope some to examine some other aspects of Kinesiology and how it relates to the Olympics.  I'm going to try and analyze some non-traditional events like race walking and hope to do 3-4 posts a week.  I might even have a guest blogger or two.

On another sports related note, Brett Favre is being inducted into the Pro football Hall of Fame this weekend.  Favre was my favorite football player growing up.  I got to watch him play at Southern Miss when I was a kid, watching several great performances including the upset over Alabama in 1990.  I followed his professional career through Green Bay, New York, and Minnesota, and eventually got to see him play in the NFL for the Jets against the Titans.  Last year my uncle, Steve Knight, was inducted into the Mississippi Sports Hall of Fame (he has the most wins of any college basketball coach in Mississippi history) and Favre was also inducted the same night.  It was a real pleasure to watch him play although I do feel older now that he is in the Hall of Fame.  Thank you for the memories, Brett.