06 August 2012

Tuesday Preview #2: Men's High Jump

I got enamored by the men's high jump in this year's Olympics because I was asked to work on a story that now appears in the current issue of Outside magazine (click here for a link to the article).  The article concerns Jesse Williams of the US, who is my pick to win Tuesday's high-jump final.

What intrigues me about Williams is that at 1.84 m (6' 1/2") tall, he is a bit shorter than most of his competition.  All jumpers, regardless of height, have to clear a bar that is some distance above the ground.  Taller jumpers have the advantage of possessing centers of gravity that are higher at the start than for a jumper like Williams.  It takes energy to elevate mass vertically upward.  Kinetic energy obtained in the approach and muscle energy released upon jumping are partially converted at the apex of the jump to gravitational potential energy and a little kinetic energy needed to pass over the bar.  All jumpers make use of the Fosbury Flop, which allows their centers of gravity to pass under the bar as they are contorted in a Ç shape over the bar.

How can Williams make up for starting with his center of gravity lower than most of his competition?  Note that as athletes approach the bar and prepare to jump, they do not run in a straight line to achieve sprint speeds.  Instead, they circle into the jump.  Williams, because of shorter limbs, is able to accelerate better than his longer-limbed competitors.  I discussed scaling and notions of top acceleration favoring short people and top speed favoring tall people when I discussed the 100-m sprint (click here for that post).  Because jumpers don't reach top speeds before they leap from the ground, someone like Williams may use his acceleration advantage to help his speed at takeoff.  It turns out that Williams is 3% -- 12% faster on his approach than most of his competition.

Williams also has slightly less lean at the jump compared to others.  His powerful leg muscles and strong core enable him to initiate just the right amount of twist upon leaping.  Why does he need to twist?  He must cross over the bar with his back closest to the bar.  In a sense, he "rolls" over the bar.  That he and others can execute such a move is due to angular momentum conservation, something I've mentioned in other posts (click here for gymnastics and diving, here for long jump, and here for shooting).  Once athletes leap from the ground, their angular momenta are fixed.

The men's world record is 2.45 m (8' 0"), set by Javier Sotomayor of Cuba in 1993.  At 1.95 m (6' 5") tall, Sotomayor combined great height and great technique to set a record that is now 19 years old.  Williams has been clocked at 7.59 m/s (27.3 km/hr or 17.0 mph) on his approach.  To elevate over the bar, Williams needs about 5.2 m/s (19 km/hr or 12 mph) vertical launch speed to match his personal best jump of 2.37 m (7.78 feet).  To match Sotomayor, Williams will need to increase his vertical launch speed by about 3%.

The women's high-jump final is not until Saturday.  My pick there is Blanka Vlašić of Croatia.  At 1.93 m (6' 4") tall and possessor of great technique, she is quite the sight to behold.  As for tomorrow, I'll root for Williams and fellow Americans Erik Kynard and Jamie Nieto.

1 comment:

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That he and others can execute such a move is due to angular momentum conservation, something I've mentioned in other posts