Expert Advice: Keep up the Pace
March 1, 2009 By: M. Elizabeth Cannon, Gérard Lachapelle, Sidney P. Kwakkel GPS World
Kwakkel, Lachapelle, and Cannon.
Watching the Olympics in Beijing this year inspired awe at the performances. One sport in particular, running, offered some of the greatest entertainment, as sprinters and marathoners alike showed why putting one foot in front of the other is easier for some than for the rest of us.
Non-athletes may find it hard to envision the day-to-day work that goes into training for those events, and the technology used to fine-tune such performances. While most runners boast needing only a pair of shoes and an open road to practice their discipline, many are not immune to the gadgetry that promises to provide what Mother Nature did not.
In recent years, GPS has made a strong impact on the running community. Wrist-top GPS-equipped running computers tell runners not only their current pace, but also provide trajectory, speed, distance traveled, and a host of other scrutable statistics. The combination of position and speed anywhere at anytime brings obvious advantages to the throng of runners. Moreover, the ability to download the information to a home computer for analysis has made runners a burgeoning market for end-user GPS applications.
How fast did I run downhill? Uphill? Where did I slow down during the race? The competitive runner and the technology-minded enthusiast now power a running revolution.
What lies around the next turn in the world of running? In 2007, we endeavoured to push this envelope. Armed with low-cost inertial measurement units (IMUs) and a GPS antenna/receiver,
we set out to see what was possible. Our goal: collect position, velocity, and rotation data for each segment of a runner’s lower body while running the 42.2-kilometer Las Vegas Marathon. The results were promising.
With an IMU attached to each segment of both legs and a GPS antenna/receiver, the first author set out on the course. Each IMU was comprised of triaxial accelerometers and gyroscopes — one attached to his belt as well as each of his thighs, shins, heels, and forefeet. Atop his head, he wore a GPS antenna attached to a receiver that also provided the IMUs with a pulse-per-second (PPS) to facilitate data synchronization. In order to capture the 20-Hz GPS pseudoranges as well as the 100-Hz accelerations and rotations rates, he wore a custom data recorder called the NavBox, developed by the Position, Location, And Navigation (PLAN) Group at the University of Calgary. In all, the setup weighed slightly more than 600 grams — about the weight of a small water bottle. Figure 1 shows the placement of each of the sensors as well as the hardware.
FIGURE 1. Hardware and sensor placement of GPS
and inertial units during the Las Vegas Marathon.
(Inset) Photos of the first author during the race
In effect, each IMU in combination with the GPS antenna formed a separate GPS/INS system. Using the fusion of a tightly coupled Kalman filter, the data from both sensors gave position, velocity, and attitude parameters. A step-detection algorithm separated one step from the next. The ability to detect the gait phases within a step also made the application of zero-velocity updates (ZUPTs) possible during periods when the foot was stationary and in contact with the ground. Figure 2 shows the algorithm for this system.By combining the attitude information between adjacent body segments (such as thigh and shank, and shank and heel) the joint rotations were observable in three dimensions. Then, by combining this information with the step-detection algorithm, the knee and ankle rotations could be compared on a step-by-step basis.
FIGURE 2. Algorithm for determining step and rotation kinematics using GPS/INS
Now, not only is it possible to see pace and speed information along the trajectory (see Figure 3), but also joint and step kinematics (see Figure 4). Whether or not this constitutes the next generation of runner-friendly gadgetry remains to be seen. However, the possibility of combining GPS and inertial information brings vast potential to consumer markets. Until new products arrive, we’ll all have to be content with our Sunday morning runs.
FIGURE 3. Trajectory of runner during the race:
(top) planimetric view and (bottom) elevation view
FIGURE 4. Kinematics at 1.5 km (•) and 41.1 km (—):
(top) position of heel in sagittal plane; (bottom left) knee flexion;
(bottom right) ankle flexion.
SIDNEY KWAKKEL just completed his M.Sc. in the Schulich School of Engineering, University of Calgary, where GERARD LACHAPELLE holds a research chair and ELIZABETH CANNON is dean.
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