Hit the ground running – lightly! Metric Series #2.

By Milestone Sports athlete, Jackie Merritt.

From an injury risk perspective, rate of impact (ROI) is one of the most valuable metrics the Pod gives you as a runner. It’s also the metric that keeps me honest as I continuously work to improve my running mechanics in training. Maintaining a low ROI doesn’t always come naturally to me, especially during tempo runs, speedwork, and downhill running. However, it’s critical for me to monitor, particularly as I increase my overall mileage and thus the amount of stress my body sees in ultramarathon training.

It is pretty intuitive to most runners that a high ROI = bad. But what exactly is ROI, and why should we pay attention to it?

What is ROI?

Simply put, rate of impact is the rapid load your body experiences when your foot first strikes the ground. Each time your foot hits the ground, it transmits a certain amount of force to the ground. The ground, in turn, transmits an equal and opposite force to your foot. This is called a ground reaction force. When the vertical component of this ground reaction force is transmitted through the foot too quickly, your body has less time to spread out the force, resulting in a high rate of impact (see MilestonePod Metrics). In the image below, ROI is depicted as the slope of the red line. The steeper the slope, the higher the ROI.

vGRF (BW): vertical ground reaction force normalized to body weight. 0% stance: foot strike. Notice you have greater than 2.5 times your body weight when your foot is on the ground, but this is not what has been linked to injury.  The speed at which that force has been transferred has been linked to injury.

Why should runners care about ROI?  

High ROI increases stress experienced by the foot, the lower leg and up the kinetic chain. In particular, the tibia (shin bone) and soft tissue structures such as Achilles tendon and plantarfascia are subject to much of the ROI stress at foot strike. Consistent with this increased biomechanical stress, research has linked high rate of impact in runners to a high risk for developing bony stress fractures1. Stress fracture. Now those are two words that will instill terror in the hearts of runners everywhere!

If your Pod metrics do indicate that your ROI is on the high end, never fear! Here are some tips for lowering your ROI:

  • Increase your running cadence. By far, the easiest and safest way to lower your impact forces is to increase your stride frequency. Research shows that increasing running cadence as little as 5-10% above baseline can decrease overall impact forces and loads through the knee joint by about 20%2. That is a pretty profound change! I recommend trying this first.

  • Lean your trunk forward. In the presence of weak core and glute (butt) muscles, runners can sometimes lean their trunk back to compensate. This will shift the center of mass posteriorly (backwards) and make it more likely that your foot will land too far in front of you relative to the center of mass, increasing impact forces. This can also cause increased loads through the knee3. Incorporating a slight forward lean of the trunk can help with this. Try engaging (lightly flexing) your front abdominal muscles and your butt muscles while you run. This is a very subtle adjustment. If you try this, make sure that you are not rounding your back and adopting a flexed position of the spine. If your abs and your glutes are sore the next day, you’re doing something right!

  • Strengthen the hips and core. This will not necessarily lower your ROI directly but may help promote better mechanics associated with impact forces. As mentioned above, increased core and glute muscle utilization may help promote a slight forward trunk lean and better position of impact forces relative to the trunk. Developing strength and endurance in these muscles is important for incorporating long-lasting changes in these mechanics. I like doing planks in variations and using a theraband for glute strengthening and stability work.

  • Strengthen the calves.  Additionally, calf muscle strengthening plays an important role in force distribution of the impact through the lower leg. Previous research found that weak calf muscles were associated with tibial stress fractures, an injury that can result, in part, from high impact force rates4. In theory, greater strength in the calves allows the muscle, and not the bony structure, to absorb the impact, reducing risk of stress fracture. Whatever the reason, calf strength seems to play a protective role in the development of this type of injury, so don’t slack on your weighted calf raises!

  • Know your shoe life.  MilestonePod data shows that on average, rate of impact begins to climb at 65% of shoe life and dramatically increases at 110% of shoe life. Watch your Shoe Odometer to know when to change your shoes before your ROI starts to increase. Check out this popular blog by Dr. Steve to read more about shoe life and ROI.

If you have a low ROI to begin with, great work and I envy you! I do not recommend that runners with low ROI try to change their gait patterns in attempt to lower it even more. It is unlikely to work and unlikely to further reduce risk of injury. I do, however, recommend checking the ROI metric on all types of runs…slow, fast, short, long, track, road, trail. When runners get fatigued, ROI tends to increase, so that could be a thing to watch for in training.

Hit the ground running, lightly, and happy trails!


1 van der Worp H, Vrielink JW, Bredeweg SW.Do runners who suffer injuries have higher vertical ground reaction forces than those who remain injury-free? A systematic review and meta-analysis. Br J Sports Med. 2016; 50(8): 450-7.

2 Willy RW, Willson JD, Clowers K, Baggaley M, Murray N. The effects of body-borne loads and cadence manipulation on patellofemoral and tibiofemoral joint kinetics during running. Journal of Biomechanics.2016; 49(16): 4028-33.

3 Teng HL, Powers CM. Sagittal Plane Trunk Posture Influences Patellofemoral Joint Stress During Running. Journal of Orthopedic and Sports Physical Therapy. 2014; 44(10):785-92.

4 Milner CE, Ferber R, Pollaard C, Hamill J, Davis IS. Biomehanical Factors Associated with Tibial Stress Fracture in Female Runners. Med Sci Sports Exerc.; 2006;38(2):323-8.

Author: Jackie Merritt

Jackie is an accomplished ultra-runner and an avid MilestonePod data fan. She has a Doctorate in Physical Therapy and a PhD in Biomechanics. She lives and trains in Atlanta with her husband Jeff and the Yeti Trail Runners. When not running, Jackie works as a PT and research scientist at Emory University School of Medicine. Jackie also runs for Hoka One One and NATHAN Sports.