In this post, we will go over the contents of the Running Performance report page automatically generated by the MotionMetrix system. The purpose of this post is to explain the output variables of the system, as well as to demonstrate how the system can be used to determine the effects a runner's stride has on his or her mechanics at different running speeds.
After completing a data capture, the user is taken to the first of 3 pages generated by MotionMetrix: the Running Performance summary. In this page, there are 11 key outputs relating to a runner's stride, including: Strike Type, Pelvic Tilt, Stride Frequency, Stride Length, Contact Time, Forward Lean, Footstrike Position, Vertical Displacement, Braking Impulse, Vertical Force (max), and Lateral Force (Max). Additionally, the system includes 3 ratings: Running Economy, Running Stride Rating, and Joint Loading. One key question relating to the actual ratings is their relevance: the team at MotionMetrix developed their rating system based on real test results from multiple reference groups - from elite runners down to more average runners.
In terms of long distance running, Running Economy is often thought of as the most important factor into overall performance. The value for Running Economy in MotionMetrix is derived from the total mechanical energy of a runner's motion, and is presented in Joules/Kg/m - i.e. how much energy it takes to move one Kg of body mass one meter forward at a set running speed. A simple way to think about the calculated value: the lower your score the better - think about fuel in a car: the less fuel you consume based on distance traveled results in higher fuel economy. In MotionMetrix's grading system, the scale moves in increments of .15, with a score less than 2.7 considered "A++" or very good, and a score greater than 3.45 as an "E," or very bad (F). Furthermore, the system provides value of where the runner scores within a reference database of 600 runners (in this example, we can see that the runner is in the top 30% by this scale).
One of the most interesting considerations that must be made when taking Running Economy into account is that it can change depending on a runner's speed. Runners should strive for similar stride performance at both distance and race paces. If these values vary too much, distrance training mileage may not be as beneficial to the runner.
Running Stride Rating
The displayed running stride rating takes into account specific stride parameters that have shown to have an impact on running economy. The parameters are rated on a scale from 1 to 5 stars, with 5 stars corresponding to "elite" status. The different parameters are weighted based on their effect on running economy (get in touch with us if you are interested in learning more about how the system weighs each variable).
Key Stride Parameters
Stride frequency is described as the number of stride cycles per minute. This can be an important factor, because a stride frequency that is too low can result in "overstriding," increased vertical displacement, and prolonged ground contact times - all of which can have a negative effect on running economy. On the contrary, an excessive stride frequency can cause a runner to exert too much effort at a given speed, which reduces overall efficiency.
While running, the majority of mechanical work is undertaken while the foot is in contact with the ground - i.e. to support the body's weight and to recover speed lost during the braking phase. Contact time is a very important factor when it comes to running economy, as it largely controls the amount of elastic energy that can be recycled during stance. The most economic runners tend to exhibit short contact times - reduced contact time can be achieved in the short-term by manipulating the stride, e.g. by reducing the footstrik position (see below) or long-term through running drills.
Footstrike position is the horizontal (antero-posterior) distance between the ankle joint and the ground projection of the center of mass at the moment the foot contacts the ground. If the footstrike position is too far in front of the center of mass, overstriding can occur, which leads to increased braking action, increased contact time, and consequently a degraded running economy.
Vertical Displacement is the range of motion of the center of mass vertically. More simply put, it is how much you (your center of mass, more specifically) move up and down while running. Insufficient vertical displacement results in a low center of mass trajectory and premature touchdown of the swing leg - leading to a footstrike far in front of the center of mass (see above). Excessive vertical displacement increases the work done against gravity and subsequently causes runners to expend more energy. Changing one's stride frequency is an efficient way to control vertical displacement.
The Max Vertical Force measure is a breakdown of the vertical force exerted on the ground during the stance phase (i.e. when one's leg is supporting body weight). This variable is measured in relation to body weight (in the example above, the runner's vertical force is around 2.5 times her body weight), so it is essential to enter an accurate body weight value at the beginning of an analysis. The measure focuses in on the stance phase because this is when maximum force is typically generated - as the runner transitions from absorbing the shock from touchdown to propulsion. Why is vertical force important? Not enough vertical force reduces energy recycling via elastic elements of the leg such as the Achilles tendon. Too much vertical force may cause a runner to work more against gravity, which in the end reduces efficiency and expends more energy. Vertical force is most often affected by both vertical oscillation and footstrike position.
If you have any questions about how the analysis is conducted or about the system itself, please get in touch with us below.