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Sport Science Puts Athletic Performance in Engineering Terms
In Sport Science, Fox Sports used wireless accelerometers, high-speed cameras, tactile sensors, load cells, gyroscopes and more to learn about athletic performance
Charles J. Murray, Senior Technical Editor -- Design News, November 18, 2007
With the debut of the television show “Sport Science,” the intangibles of sports are suddenly falling. The force of a major league fastball, the impact of two colliding Sumo wrestlers and the catching technique of Jerry Rice have all left the realm of folklore and are now measurable in engineering terms.
“We said, ‘Why not take the ultimate biomechanical technology and apply it to the ultimate human athletes?’” says Mickey Stern, co-creator of the new program, which airs on the Fox Sports Network on Sunday nights. “We wanted to do this in a way that’s as big and outstanding and outrageous as possible.”
Indeed, Fox Sports’ new show is big and outstanding for engineers, as well as for sports enthusiasts. By analyzing sports phenomena that occur in a fraction of a second — and which have therefore been previously unmeasured — “Sport Science” has broken new ground. The show’s engineers and scientists, for example, used wireless accelerometers to measure the impact of two colliding Sumo wrestlers and the take-off force of basketball’s best straight-up jumper. They employed capacitive touch sensors on the hands of former football wide receiver Jerry Rice to explore how he catches a football. They used inertial measurement units (IMUs) to measure how fast a soccer player’s leg travels during a so-called “bicycle kick.” They also employed infrared (IR) cameras, high definition (HD) cameras, instrumented crash dummies, load cells and a variety of other technologies to analyze everything from “hang time” to punching power.
“There’s a definite advancement of technology and it’s enabling us to make measurements that we couldn’t have made 10 years ago,” says Cynthia Bir, an assistant professor of biomedical engineering at Wayne State University, who serves as the lead scientist on the show. “We can instrument these athletes and see what’s going on and determine what forces they generate and how much they can endure.”
Fox executives say they launched the show with the idea that it would be a myth-buster, disproving countless glamorized ideas held by sports fans. And while they did burst the bubble on a few myths, such as the concept of so-called “hang time,” they ended up being drawn into the testing in a way they hadn’t anticipated. After spending a few weeks testing athletes in an instrumented airplane hanger at Hawthorne Airport in California, the show’s creators were in awe.
“When we set up our experiments, we didn’t know exactly how they would turn out,” says John Brenkus, co-creator and one of the stars of the series. “The results really opened our eyes to how amazing these athletes are. It makes you realize that we’re only scratching the surface of sports science.”
In the next five sections of this article, we’ve broken the experiments out by technology, the better for engineers to understand the measurement techniques employed by Fox Sports’ engineers.
Wireless Accelerometers and Gyroscopes
Bir and other engineers on the show used Inertia-Link, a wireless inertial measurement unit (IMU) to test a combination of angular rotations and accelerations on athletes. Built by Microstrain, Inc., the IMUs contain three accelerometers, three gyroscopes, six A/D converters and a radio transceiver, all in a package measuring 41 x 63 x 24 mm.
In one significant application of the IMU, Bir placed it on the ankle of pro soccer player Jason Hernandez and measured the velocity of Hernandez’s leg during a “bicycle kick.” Microstrain engineers say the unit was the only possible means to measure the speed of Hernandez’s kick, which involves linear acceleration and angular rotation.
“We blend the gyro and the accelerometer output, so we can tell the difference between linear movement and angular movement,” says Mike Robinson, vice president of sales and marketing for Microstrain. “Our processor does the calculation and the module spits out a linear acceleration and an angular rate.”
Bir of Fox Sports says the biggest advantage of the system is that it’s wireless.
“In past research, we’ve always used wired accelerometers, which have a trailing cable,” she says. “When you’re measuring events that are very kinematic — such as running and jumping — it’s difficult to have someone be tethered by a wire.”
In “Sport Science,” Bir used the wireless IMU, as well as wireless accelerometers known as V-Link and G-Link, to measure forces on Sumo wrestlers, basketball players, ultimate fighters, martial artists and pro football players.
Load Cells and Crash Test Dummies
To determine simple compressive forces, engineers applied load cells to the backs of flat surfaces, then measured the outputs when the surfaces were struck. They used the technology to measure the speed of softball pitcher Jennie Finch and the force delivered by martial artists breaking concrete blocks.
Made by Robert A. Denton, Inc. , the Model 2881 load cells employ piezoresistive strain gages that change their electrical resistance, and therefore current output, when a load is applied. Since the difference in current output is proportional to a change in loading, the units can measure forces. The load cells, measuring 3 x 3 x 1.5 inches, were applied to the back of a piece of Plexiglas to measure the force of Finch’s fastball and to the bottom of a concrete block to measure a 2,000 lb-f load generated by a martial arts expert.
Fox Sports also used the technology to compare the force generated by Finch’s fastball to that of a 95-mph major league fastball. They found that although the baseball was smaller than the softball, it transmitted more force.
“Although the velocities were the same and although the softball had greater mass, it generated smaller loads,” Bir says. “Because of the softer nature of the material, it was able to dissipate some of the force.”
Bir also used the technology to measure the force of a body slam by ultimate fighter Rampage Jackson. Employing a load cell inside the “head” of a Hybrid 3 50th percentile crash test dummy from Denton, Bir calculated the head injury criteria (HICs) to be between 3,000 and 4,000 HICs (HICs are unit-less), meaning the trauma to the head during the body slam was about three to four times that of a 35-mph head-on auto collision.
Tactile Pressure Sensors
Football coaches teach receivers to catch footballs with their fingers, not with their hands or body. To find out if receivers really do it that way, “Sport Science” used tactile pressure sensors on the fingers and palms of former NFL football player Jerry Rice. The sensors, built by Pressure Profile Systems, use a capacitive sensor technology weaved into a stretchable, Lycra-type fabric. Known as FingerTPS, the sensors change their capacitance level as pressure is applied. Capacitance levels on the sensors are extraordinarily small — on the order of 10-15 Farads — but the sensors are able to read those values and subsequently infer a change in pressure.
“By measuring the capacitance, we can tell how the gap is changing between the sensor’s electrodes and therefore we know how much pressure is applied,” says David Ables, chief technology officer of Pressure Profile Systems.
Using the system, the Fox Sports crew was able to determine that the football never touched Rice’s palms.
The crew also used the technology to prove that a National Basketball Assn. (NBA) rule was based on a faulty assumption. The rule, which says a player cannot catch and shoot a ball with 0.3 sec or less left on the shot clock, was disproven by an NBA three-point shooter outfitted with a sensor on his middle finger. The shooter, Jason Kapono of the Toronto Raptors, consistently beat the clock.
“If we threw the ball to the right spot, we found he could catch and shoot the ball in 0.22 seconds,” Ables says. “So the rule is wrong.”
Motion Capture Kinematics
To fully understand the movements and power techniques of athletes, Fox Sports employed motion capture cameras. Commonly used to create lifelike cartoon motion in software games made by Atari, Sega and Nintendo, the motion capture cameras use retro-reflective targets, LED-based strobes, custom-designed high-resolution cameras and software. Athletes wear the retro-reflective targets on their joints, atop a “motion capture” suit, while multiple cameras monitor their movements in three-dimensional space. Using the technology, engineers are able to analyze a subject’s biomechanics.
“You can get a 3-D position reading of the athlete’s joints in space at any given time,” Bir says. “You can see where an athlete’s foot is and what he’s doing with it. You can see where his head is compared to his center of gravity.”
Engineers from Vicon, creators of the motion capture system, say they’ve developed a distributed processing architecture that enables users to employ multiple self-synchronizing cameras, which can work together to provide a kinematic analysis of the most complex motion applications. The technique has
been used by NASA for analysis of control surfaces of prototype model aircraft in wind tunnels, the company says.
Fox Sports used the technology to analyze the jumping technique of “Skywalker,” the falling methods of so-called “free runners” and the pitching mechanics of Jennie Finch.
“We use these systems in biomechanics quite commonly,” Bir says. “But to apply it to sports has rarely been done in the past, especially as we’ve applied it here.”
High-Speed Video
Researchers say the best way to understand athletic performance is to slow it down. On “Sport Science,” they did that by employing two types of high-speed video cameras.
Developed by Vision Research, the two cameras are the Phantom v7.3 and the Phantom HD. The v7.3, an 800 x 600 resolution camera that can operate at almost 7,000 frames per sec, offered higher speed, while the Phantom HD enabled Fox Sports to create 1920 x 1080 images at 1,000 frames per sec. Both cameras are able to operate at high speeds because they employ CMOS sensors and have pipelined architectures designed to process huge amounts of data. The systems capture photons as analog data, transfer it to A/D converters and then store it as raw digital data in on-board memories ranging from 8 to 32 Gbytes.
Once the data is stored, software can perform slow-motion video analysis, enabling researchers to pinpoint when a physical movement starts and stops.
“You can take a point and ask the software to analyze how far that point moved in a certain period of time,” says Rick Robinson, director of marketing for Vision Research. “From that, it can determine velocity and acceleration.”
The technology was particularly important in disproving the myth of so-called “hang time” in basketball. By looking at high-speed video of basketball players, engineers proved jumpers are always moving up or down, never defying gravitational forces by hanging in the air. Such myths would be impossible to prove or disprove without slowing down those physical movements, engineers say.
“There’s a whole alternate universe taking place out there — phenomena that we’ve never really seen,” Robinson says, “because we can only perceive things at a certain frequency.”
| What They Learned |
| A few of the important — and surprising — lessons Fox Sports learned along the way. —Street basketball player “Skywalker” has a 50-inch vertical jump. In jumping over a convertible and dunking a basketball, he generated 1,200 lb-f on liftoff. —Professional soccer player Jason Hernandez’s “bicycle kick” was measured at 1,800 degrees/sec, or 300 revolutions/min. His kick is comparable in rotational speed to that of an Apache helicopter. —Two Sumo wrestlers generated 1,000 lb-f on impact. —Ultimate fighter Rampage Jackson registered a head injury criteria (HIC) number about four times as high as that of a 35-mph head-on crash when he body-slammed a 180-lb crash test dummy. —There’s no such thing as “hang time.” High-speed cameras revealed that leaping basketball players are never in the air for more than 1 sec. They’re always moving up or down, never defying gravitational forces by hanging in the air. —Normal human anticipatory reaction time is 0.19 sec. —A blind-side hit by NFL linebacker Joey Porter generated 1,600 lb-f, about the same as that generated by a bull, kicking with its hind legs. —The human skull can absorb about 1,400 psi before fracture, roughly the same as a coconut. —A 95-mph major league fastball hitting a human head could induce mechanical stresses exceeding 1,400 psi. However, batting helmets reduce that stress by at least 50 percent. —The swing, and subsequent contact, of a baseball bat can generate 25 times as much force as the strike of a hockey stick. “A baseball bat is the most potentially dangerous piece of equipment in all sports,” says “Sport Science” co-creator John Brenkus. —A 95-mph fastball thrown by a baseball pitcher reaches home plate in about 0.38 sec, about the same time as it takes to blink an eye. —Major league baseball hitters never see the bat contacting the ball. A 95-mph fastball becomes invisible 25 ft from home plate. “The closing velocity of a pitch is so fast that you literally cannot see it when it reaches you,” Brenkus says. “When you think you see it, it’s really just your brain projecting where it believes the ball to be.” |
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Sport Science Puts Athletic Performance in Engineering Terms
In Sport Science, Fox Sports used wireless accelerometers, high-speed cameras, tactile sensors, load cells, gyroscopes and more to learn about athletic performance
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> Capacitance levels on the sensors are extraordinarily small — on the order of 10-15...
- 2007-1-11 12:27:38 EDT -
We thoroughly enjoyed reading the latest technology in these fields. It puts a new prospective...
- 2007-1-11 11:02:06 EDT
