Biomechanics and the Olympics: Part IX

Another scary moment at the Olympics the other day occurred when pole vaulter Lazaro Borges had his pole snap during an attempt.  As you can see in the picture above, he was very fortunate not to be injured by this unfortunate accident.  Pole vaulting can be a potentially dangerous sport, as often times the athletes are attempting to use a pole to project themselves over a 15-17 foot high bar.  Why did Borges' pole break during this attempt?

When a structure is loaded, it will deform to a certain extent before it starts to break, and if the a load is still applied to the structure, it will eventually fail and rupture.  Think about holding a tree limb and applying a force to it.  Eventually, the limb will start to break, and if you continue loading it, it will rupture completely and become two separate parts.  We could plot a stress-strain curve for different materials (metal, glass, bone, muscle, tendon, etc.) to examine how they will behave when they are loaded.  Stress is similar to the load applied to the material, and strain is the amount of deformation.  The stress-strain curve is comprised of three main parts: 1) the elastic region 2) the plastic region 3) and the ultimate failure point.  If a material is loaded within the elastic region, it will undergo deformation, but it will return to its original shape once the load is removed.  If it is loaded past the elastic region into the plastic region, the material will start to show some small tears (microtrauma), and will not return completely to it's original shape after the load is removed.  If a material is loaded to or past the ultimate failure point, it will completely rupture and tear (break into two or more pieces). 

Now, the pole used in pole vaulting is highly elastic, which is beneficial because it will deform extensively and store potential energy which will be used later to project the vaulter over the bar.  Most of the time in pole vaulting, the pole is not loaded past its elastic limit, and no permanent deformation occurs to the bar.  Think about using a rubber band, if you pull the band back (but not too far), and then let it go, it releases a lot of energy and returns to its original shape.  If you pull the band back past its elastic limit, then it starts to show some little tears, and if you keep pulling the rubber band back, which increases the stress on the band, it will eventually break.  This is what happened to Borges' pole.  It was loaded past the elastic and plastic limits, to the ultimate failure point, and it broke.  Why did it happen on this attempt?  There are a few possible explanations.  Perhaps his pole had been loaded previously past its elastic limit to its plastic limit, and had already sustained some small deformation, and was weaker for this attempt.  It might have been due to pole placement and how the load was applied to the pole (unfortunately the video has been removed from the internet).  The fortunate thing is the he was not seriously injured.  In many of the Olympic sports,  the athletes are moving at very high velocities, producing a large amount of force, and sometimes have implements that they use or obstacles that are in their way.