Biomechanics and the Olympics: Part VII

I was asking my wife what today's topic should be when I came across this video of weightlifter Matthis Stenier dropping 432 pounds directly onto his neck (video courtesy of Deadspin.com).  Before you read any further, watch the video (disclaimer: it looks worse than it really is, but if you have a weak stomach, you may not want to watch it).  This video itself contains numerous possibilities for discussion, but let's talk about Newton's second law of motion, since we have previously discussed the first law.

Newton's second law of motion is the law of acceleration, represented by the equation F=ma, where F is force, m is mass, and a is acceleration.  This equation tells us that an object's acceleration is directly proportional to the force applied to it and inversely proportional to the object's mass.  Steiner is attempting to perform a lift called the snatch, where he must lift the weight over his head, and then stand up.  During his failed attempt, he was attempting to lift 432 pounds.  This weight represents a force, since weight is the result of gravity acting on the mass of an object.  To calculate the weight of an object, you multiply the object's mass by the acceleration due to gravity (-9.81 m/s^2).  Pounds is the US unit of measuring force, in the metric system, a Newton is the unit used to measure force.  One pound is equal to -4.45 N, so in this example, the barbell had a weight of -1,922.4 N.  We can also calculate this weight by multiplying the mass of the barbell (195.95 kg) by the acceleration due to gravity (-9.81m/s^2), which also gives us approximately -1,922.4 N.  In this example, the negative sign indicates that the force is acting in the downward direction, which is the case with weight, which always pulls us or objects towards the ground.

For Steiner to lift the barbell off the ground, he must exert a force in the upward direction to the barbell greater than 1, 922.4 N, or 432 pounds.  If the force he exerts is less than or equal to the weight of the barbell, no movement will occur.  Steiner is clearly able to exert a force greater than this because he is able to move the barbell over his head.  The muscular force he generates is greater than the weight of the barbell, and the muscles shorten, allowing him to begin the movement.  This causes an acceleration of the barbell in the upward direction.  When Steiner gets the weight over his head, he pauses for a second, and the barbell does not move.  At this point, the force he is exerting is equal to the weight of the barbell, and the muscles develop tension while remaining at a constant length.  Since the forces are balanced, there is no acceleration.  After this short pause, Steiner begins to stand up, meaning he is exerting a force greater than the weight of the bar.  However, he is not able to complete the lift, and because he is now exerting less force than the weight of the bar, the bar is now moving in the downward direction right onto his neck.  Now, the barbell has an acceleration in the downward direction.  Normally, after we have lifted something and are attempting to lower it back down, we do so in a controlled manner, in order to avoid injury and slamming it on the ground.  The muscles gradually develop tension as they lengthen to control the movement.  Steiner was not able to do this (which is very difficult since he was lifting 432 pounds).  It is difficult to tell from this angle exactly what happened, but I have two theories.  1) He was lifting too heavy of a weight and simply could not continue to produce enough force to lift it over his head or 2) the barbell moved too far back behind his head, causing a tremendous amount of tension on his shoulders and elbows, to the point where if he did not drop the weight, he likely would have dislocated one of those joints.  It was probably a combination of these factors as well as fatigue.

I am happy to report that he was not seriously injured, and actually attempted another lift.  Weightlifting can be a very dangerous sport, especially when attempting to lift something this heavy.  My advice is to leave these kinds of lifts to the professionals, and if you are attempting to lift something over your head, be very careful.

Biomechanics and the Olympics: Part VI

Sometime yesterday afternoon (although NBC didn't show the race until later last night), Usain Bolt cemented his place as the world's fastest man by again winning the 100 meter race, with a time of 9.63 seconds.  As you can see in the picture above, Bolt is clearly a few meters ahead of his closest competitors.  What makes Bolt so fast?  Well, there are a number of factors, but today I am going to talk about his stride length, step length, and stride frequency.

Stride length is the amount of distance covered from the touchdown of one foot (let's see left foot strike) until the left foot touches the ground again.  Step length is the distance covered from the touchdown of one foot (left foot) until the touchdown of the other foot (right foot).  Bolt clearly has an advantage here because he is taller than the other sprinters.  I went back and watched the race in slow motion and it took Bolt 41 steps, or 20.5 strides, to run 100 meters.  The third place finisher, American Justin Gatlin, took 44 steps or 22 strides to complete the 100 meters.  Simply put, Bolt covers a much greater distance with each step and stride than any other competitor, giving him a distinct advantage.  On average, Bolt covers 2.44 meters with each step, which is roughly equal to 8 feet, and 4.88 meters per stride, or nearly 16 feet.  Justin Gatlin covers 2.27 meters per step, which is equal to 7.45 feet per step.  These numbers are just the average stride and step lengths over the entire race.  Since each runners takes a shorter stride at the start of the race, the actual stride and step lengths are going to be greater towards the middle and end of the race.

Another critical factor in determining running velocity or speed is stride frequency, which can also be broken down into step frequency.  Stride frequency is the number of strides taken during a given time frame, typically strides per second.  In this race, Bolt took 20.5 strides over 9.63 seconds.  This averages to 2.13 strides per second, or 4.26 steps per second.  For Justin Gatlin, he took 22 strides over 9.79 seconds, which averages to 2.25 strides per second, or 4.5 steps per second.  This means that Gatlin is able to swing his legs through the running gait cycle (put his foot on the ground, and swing it back and then forward to the ground again) at a faster rate than Bolt.  But, because Bolt is able to take such a longer stride and step than any of the other sprinters, he still finishes the race faster than them.  Based on Bolt's stride length and stride frequency, and Gatlin's stride length, Gatlin would have to increase his stride frequency to 2.28 strides (4.56 steps) per second to equal Bolt's time of 9.63 seconds.  Simply put, Bolt is very difficult to beat when he is dedicated to his training and focused on the race.

Biomechanics and the Olympics: Part V

Now that the focus of the Olympics has shifted to track and field, and more specifically the men's and women's 100 meter race, one of the critical factors in winning this race is the reaction time of the sprinters.  Often times, just like in swimming, these races are decided by a few hundredths of a second, and a poor start can be the difference between first place and last place.

Reaction time is the amount of time it takes the body to prepare and initiate a response to a stimulus.  There are three different types of reaction time (RT).

1. Simple RT: there is one stimulus/signal, and only one response to be made to the stimulus.  This is what happens at the start of a race in track and swimming.
2. Choice RT: there are several stimuli/signals, and each one requires a different response.  A traffic light is a good example.  Each color signal requires a different and timely response to avoid an accident, although one could debate what response is to be made to a yellow light.
3. Discrimination RT: there are several stimuli/signals, but the person is only going to respond to one specific signal.  A quarterback calling out the snap count is a good example of this; he is going through several different signals, but the offensive players are only responding to one specific signal and should ignore the rest.

The start of a track race involves simple RT, because the only signal the runners are paying attention to and responding to is the starting gun.  The faster the runners can respond to this signal and begin to move, the greater the chance they have of winning the race.  Reaction time can be broken down into two components, pre-motor time and motor time.

1. Pre-motor time is the amount of time from the onset of the stimulus (the sound of the gun) until electrical activity is detected in the muscle groups used to perform the movement.  This time can be measured using electromyography (EMG), which typically involves placing electrodes over the muscle in order to record the electrical signal associated with the muscle contraction.  What happens during this pre-motor time?  Well, the auditory signal has to be detected by the sensory receptors in the ear, this signal has to travel to the brain for processing, and the brain has to send signals down to the appropriate muscle groups.
2. Motor time is the amount of time from the onset of electrical activity in the muscle until the first movement is detected.  It takes time to develop tension in the muscle and transmit this tension from the muscle to the tendon to the bone for movement to occur.  This time is also known as the electromechanical delay of the muscle.

The take away message is that it takes time for the body to prepare and initiate a response, even to simple signals.  As movement complexity and the number of signals and possible responses increase, reaction time also increases. In all sports, a shorter (faster) reaction time typically leads to a greater chance of success. 

For the elite level sprinters in the Olympics, this entire process takes between 100-200 milliseconds (a millisecond is a thousandth of a second).  This does not seem like a large amount of time, but in a short sprint of 100 or 200 meters (or even 400 and 800 meters), a few milliseconds can make a large difference.  I am not an expert in track, but I know these sprinters spend a lot of practice time working on their starts.

Biomechanics and the Olympics: Part IV

Many different Olympic sports involve rotational motion of the entire body, with the most popular probably being diving and gymnastics (trampoline too, that is crazy!).  When we examine motion, one of the first things we have to look at is the inertia of the object or the person.  Inertia is resistance to change in motion, and is measured by an object or a person's mass.  The greater the inertia of an object or person, the greater the force that is required to start, stop, or change the object or person's motion (this is Newton's first law of motion; objects in motion stay in motion and objects at rest stay at rest unless acted upon by an outside force).  For example, if you were going to lift a 25 kg block off the floor and a 50 kg block off the floor, it would take more force to lift the 50 kg block, because it is more massive and has more inertia.  The force applied to an object or person has to be greater than its inertia in order to cause a change in motion.  When we examine rotational motion, such as the flips, tucks, twists, and spins that occur in diving and gymnastics, we have to consider the mass moment of inertia of the person.  The mass moment of inertia is resistance to change in rotational motion, and is the mass of the person and the way the mass is distributed about the axis of rotation.  In these rotational motions, we can consider the axis of rotation to be the center of gravity.  The further away from the axis the mass is located, the greater the mass moment of inertia, and the closer to the axis the mass is located, the smaller the mass moment of inertia.  When the divers and gymnasts want to increase their rotational velocity, they get into a tucked position, such as in the picture above.  This brings the athlete's mass towards the axis of rotation, and reduces the mass moment of inertia.  When the athlete wants to slow down and decrease their rotational velocity (when they are about to enter the water or land), they come out of a tucked position into a more extended position (in the picture below), which will increase their mass moment of inertia and reduce their rotational velocity.  The same principle is demonstrated with figure skaters; when they want to rotate at a high angular velocity, they get into a crouched position and bring the arms in towards the body, and when they want to slow down, they get into a more upright position.  Since track and field is now in full swing, the next few posts will examine some of those events.

Biomechanics and the Olympics: Part III

 In my opinion, some of the best athletes in the world are gymnasts.  The combination of strength, power, flexibility, balance, and stamina are a rare combination.  All of the events are extremely difficult, but the one I find the most fascinating is the balance beam.  The balance beam has a width of 10 cm (3.9 inches).  There are many factors when it comes to maintaining balance, but two of the most critical are the base of support and the center of gravity.  The majority of the time during human locomotion, our base of support is our feet.  In the second picture below, the gymnast's base of support is her hands and chin.  We can change our base of support in order to be in a more stable position or a less stable position.  A wider base of support is better for stability and balance, but does not allow for a great deal of mobility (try walking with your feet spread far apart, you can't move very quickly, but you are less likely to fall over), while a narrower base of support is better for mobility but does place us in a less stable position.  The second factor is the center of gravity.  This is the balance point for the body, or the point where the weight of the body acts.  The center of gravity is slightly lower in females versus males in a standing position.  When we move, the location of the center of gravity will change.  In order to remain in a stable position, the line from the center of gravity (directed towards the ground) must remain within the base of support.  Consider what happens when you are standing in an upright position and start to lean forward.  As you lean forward, your center of gravity moves forward.  If you lean forward far enough, your center of gravity will move outside of your base of support, and if you don't take a step forward to change your base of support, you will fall.  Now, consider how a gymnast moves when she is performing a routine on the balance beam.  The shape of the balance beam automatically reduces the size of the base of support, making it difficult not to fall.  Many people, including myself, would have difficulty just walking across the beam.  When you factor in all the different moves, jumps, and landings the gymnasts have during their routine, which constantly changes the location of their center of gravity, it is truly remarkable what they are able to do on the balance beam.  Many of the gymnasts use their hands as their base of support during the routine, which increases the difficulty.

Biomechanics and the Olympics: Part II

 There is little doubt about the greatness of Michael Phelps and it is truly remarkable all the Olympic medals he has won.  However, it is the race that he barely lost that caused him much frustration.  During the Men's 200 meter butterfly two days ago, South Africa's Chad le Clos beat Phelps by five hundredths of a second.  The race was very close and Phelps had a slight lead on le Clos coming into the wall at the end.  However, instead of taking another half stroke before touching the wall, Phelps decided to go ahead and reach for the wall, while le Clos did take a half stroke.  The announcer commented that Phelps lost the race because he did not have any momentum at the end.  This was only partially correct.  Momentum is the product of a person's (or objects) mass and velocity (think speed with a direction).  Any time a swimmer is moving, they have momentum.  When they are not moving, either before or after the race, they do not have any momentum.  The faster a person is moving, the more momentum he or she has.  In order to increase momentum, it requires a reactive force in the same direction you are moving in.  In order to change momentum to the opposite direction, it requires a reactive force in that direction.  When swimmers complete a turn, they push into the wall, and the wall pushes back in the opposite direction, thus giving them momentum in that direction.  Back to the finish.  When Phelps decided not to take another stroke or half stroke into the wall, he was not applying any force to the water to increase his momentum.  Due to the drag force from the water, which acts in the opposite direction that the swimmer is moving in, Phelps was losing momentum.  le Clos, by taking another half stroke, was able to produce more force and increase his momentum, thus allowing him to touch the wall right before Phelps.  If you remember the 2008 Beijing Olympics, Phelps won this same event by one hundredth of a second because he did take an additional half stroke.  These are incredible athletes, and often times the difference between winning a losing can be just a few hundredths of a second.  Phelps did not lose because he did not have any momentum, he lost because he was losing momentum while le Clos was gaining momentum.  The video of the race can be found at this link at around the 30 minute mark.

Biomechanics and the Olympics: Part I

With the 2012 London Olympic games underway, I thought it would be a good time to do a series of blog posts and discuss biomechanics and the role it plays in analyzing and improving the performance of Olympic athletes.  My former doctoral adviser and mentor, Dr. Wendi Weimar, an associate professor at Auburn University, got me interested in analyzing movement several years ago when I was a Master's student at Auburn.  Dr. Weimar is highly skilled at watching an athlete perform a motor skill, or just watching a person walking down the street, and then analyzing their performance or walking gait.  She has worked with several former and current Olympians that attended Auburn, and as a graduate student, I had the unique opportunity to assist her.  We had an underwater video camera that was used to capture the swimmer's motion from a unique position, and then we were able to use a motion analysis program called Dartfish to analyze variables such as the swimmer's body position, joint angles, and the mechanical efficiency of the movement.  In many of these Olympic events, the difference between winning a medal or coming in last place is only a few hundredths or tenths of a second, so even the smallest biomechanical details are critical.  Over the next few days, I am going to take some of the sports in the Summer Olympics and discuss biomechanical factors related to the skill.  If you would like to read more about Dr. Weimar and her work, please click on the link below.

Weimar specializes in the science behind Olympic sport