The first peak in force is termed the impact peak and results from the collision of the foot with the ground. We can also look at how quickly this force rises and when that peak occurs. This is called the rate of loading.
The second peak is called the active peak. It corresponds to the point when the energy absorption has stopped (the center of mass is at its lowest) and when we start to "push" off against the ground.
Relationship between ground reaction forces (GRFs) and injury
The relationship is contentious. It is simple to assume that less load or less stress on the body leads to a reduction in injury risk. But this does not always pan out. The human body has the ability to adapt and the variability across runners' ability to adapt is huge. Simply, we don't know how much load is bad for an individual person. You can even argue that loading is good in that this is what stimulates an adaptation in the runner (e.g. stronger bones, stronger soft tissue, a better nervous system?). What the cutoff is between good loading leading to adaptation and too much loading leading to injury is our Holy Grail of Injury Prevention. The concept of loading and loading rate on injury in runners could be a long post itself - I will just link to some articles below and briefly touch on this area.
There is some suggestion that the rate of impact loading is related to stress fractures in runners. See this post here and the abstract here. We also have some suggestion that decreasing peak loading can influence stress fracture risk (abstract here and here)
We also have some great research by Dr Irene Davis linking higher impact velocity (e.g rate of loading) with stress fractures (link here). Dr. Davis has also published case series showing changes in loading rates with feedback during running (abstract here).
Some links to follow for more information on this.
Davis and company (here, here, here, an outlier here)
Other researchers showing no relationship with loading and injury ( here and here)
So How Can We Modify Loading During Ground Contact?
In the following sections we will take a look at three methods that are used to change loading during foot strike:
- Changing step rate
- Changing type of foot strike (i.e. rearfoot versus midfoot)
- Changing footwear (or foregoing footwear altogether)
In an upcoming post I will review what other changes occur when we attempt to make these changes to impact loading. Changes don't exist in a vacuum. They could lead to changes in running economy, muscular activation or other unintended consequences that could affect performance and maybe injury risk.
The Bottom Line before the bottom
All three techniques are able to change impact loading and joint loading in some individuals but not in all individuals. What we see in the research is variability and this leads to conflicting results across studies. This is because there are other factors than the three above that influence the ground reaction force. For example, you can change to a midfoot or forefoot strike and still overstride (see a neat video and case on this here). The stiffness in our limbs can all influence impact. This is what happens to some extent with kinematic changes with aging. Older runners can have similar stride rates but still have greater impacts (for a brief review see here). But lets look at some research.
A. Changing step rate
We can increase the number of steps we take while running. Heiderscheit et al (2011) did such a thing (for detailed review of the study look here and here). He increased step rate by 5 and 10%. This is some of what they found:
At both 5 and 10% increases in cadence
- decreased step length
- decreased Center of Mass vertical excursion (less bouncing up and down)
- decreased horizontal distance from the center of mass to the foot (i.e. less overstriding in front of you)
- less knee flexion (excursion) during the foot contact (i.e. increases stiffness)
- decreased energy absorption and energy production at the knee
- decrease in the impact transient occurrence (there were times when runners did not have that sharp spike in ground reaction force plot)
- decreased braking impulse
At a 10% increase in cadence only
- decrease in foot inclination angle at contact (toes point down more)
- decreased stance time duration
- increased rating of perceived exertion
- less hip flexion and adduction
- increased knee flexion at initial contact
- decreased peak vertical ground reaction force
- decreased energy absorption at the hip
A note on the ankle
There was no change in the amount of ankle energy absorption when increasing cadence yet a huge decrease in the amount of energy absorbed at the knee. This is most likely due to a lack of change in the ankle kinematics and how the foot struck the ground. I would guess that changing cadence was not enough to change the type of foot strike. If an individual went from a rearfoot to a mid or forefoot strike we would probably see more energy absorption at the ankle. Take home point, is that a lot variables can influence impact loading.
A note on the initial impact loading and loading rate
The authors found that reducing step length decreased the occurrences of that sharp initial bump in the ground reaction force. This is the impact transient and is what we most see in runners who heel strike - it is essentially the collision force with the ground. However, it was rarely seen (0-1 times in every 5 strides) only 56% of the time when increasing step rate by 10% (this lack of impact transient was seen 22% at preferred step length). So notice this is just a trend in some runners. If you notice with the study the authors did not calculate the average loading rate across all runners. This is how we typically compare loading rates across interventions. I would guess with the variability across the subjects we would not end up with a statistically significant change in loading rate. Presenting the data in this way shows us that sometimes we get a change in the impact transient just not always. As a refresher here is a lovely video of the loss of the impact transient with forefoot running.
Hobara (2012 -abstract here ) had athletes run at 2.5 meters per second and also modified step frequency (increasing huge amounts of 15 and 30%). They found decreases in:
- vertical impact peak (VIP),
- vertical instantaneous loading rate (VILR) and
- vertical average loading rate (VALR).
The only issue with this study is how practical it is to have such a huge change in stride rates. It is great for a proof of principle but we should question whether we want to do this in terms of running economy and even injury risk (i.e. you are taking a lot more strides thus increasing repetitions).
...and the other side of things (nothing is ever that simple)
But, these findings were mildly contrasted with another neat study that looked at changing a number of things (step rate, foot contact style and type of shoe) and the loading response. Giandolini (2012)found the following when increasing cadence 10%:
- no change in the rate of impact loading
- no change in the impact transient
- no change in the time that your foot is on the ground
- a decrease in the aerial time (time you are in flight)
- increase in stiffness (vertical)
These authors looked at the group average for changes in rate of impact loading. They don't show their raw data so it is possible that there may have been some individuals who decreased their rate of impact loading and perhaps lost the impact transient during initial foot strike. This would be consistent with the Heiderscheit study.
In this comprehensive study, these authors were also able to change a number of loading and kinematic variables with other interventions. They did two other things - put runners in a racing shoe (versus a big bulky cushioned shoe) and had the runners switch from a rear foot strike to a mid foot strike. They also combined all three changes (COMBI). So why don't we take a look at this neat-o study along with other relevant studies that look at changing footstrike style.
B. Switching to a Midfoot Strike
The research suggesting changing foot strike influences loading variables
This simple change provided some pretty drastic results. Giandollini et al (2012) found:
- loss of the impact transient when switching (also found in the COMBI)
- greater than 50% decrease in the rate of loading (also found in COMBI)
- interestingly no change in step rate (this is of interest because we often assume that this happens with a midfoot strike. We typically assume that running midfoot versus the heel naturally shortens the stride - suggesting that we can get changes in loading rates without decreasing stride length)
- an increase in Gastroc (calf muscle) and Tibialis Anterior (shin muscle) muscle activity was found just before impact but not during impact. However, the authors did not account for the electromechanical delay (i.e. the muscle turns on immediately but it takes time to take up the slack of the muscle to create force against the bones) that occurs with EMG muscle activity so we shouldn't conclude that the muscles are not creating less force during impact. With the delay these muscles are creating force and are mostly likely contributing to the buffering of the impact loading response.
You can see a video version of this response in the video above from Dr. Lieberman.
...again it is never that simple. Similar research has found different conclusions
Laughton, Davis and Hamill (2003) investigated fifteen habitually rearfoot strike runners and then converted them to a forefoot strike pattern in a single session. The authors found:
- increased average peak vertical ground reaction force
- increased Anterior to Posterior GRF
- Increased Anterior to Posterior loading rates
- no difference in average or instantaneous GRF loading rates
some other findings:
- increased dorsiflexion and calcaneal eversion excursion
- decreased centre of mass excursion during foot contact
- increased knee flexion at initial contact
- decreased knee flexion excursion
- increased knee and leg stiffness
- decreased ankle stiffness
WHOA...this is very conflicting. Sure is. It again stresses that changing a single variable is not sufficient to change other variables. Changing to this type of forefoot strike pattern either increased peak loading, no change in loading rates, increased braking forces and increased knee and leg stiffness. We don't know if stride rate changes but the increased anterior-posterior forces suggest that the forefoot striking may have been related to overstriding. Last, in this particular study the forefoot strikers were not permitted to let their heels hit the ground. They essentially ran on their toes. The Giandollini suggests that converting to midfoot strike can be beneficial (in terms of impact and force variables) but there may be a correct way to do this. i.e. don't just run on your toes
Further research showing variable impact loading with changes in footstrike
Becker et al (2012) in an abstract presented at the ASB 2012 concluded that foot strike pattern does not predict loading rates during shod or barefoot running. With a subject population of 11 (this study was reported as ongoing so it looks like it may be more robust in the future). The authors measured vertical impact loading rate and strike pattern in the runners when they ran either shod or barefoot. What they were able to evaluate was how footstrike pattern related to VILR in quite a novel way. The participants were not told to attempt to change their foot strike pattern. Rather, they had people either run with their shoes or in barefoot and measured their footstrike pattern with something called a Strike Index while also measuring their VILR. With these two measures in hand they determined how foot strike pattern related to VILR because some people would naturally change from a rearfoot pattern while shod to a forefoot pattern while barefoot. So, there were few permutations on what could happen. Here is a sample of what I see as relevant and check out the chart below for all the details.
Lets look at those instances where people RFS while shod but ended up switching to a forefoot strike while barefoot:
- 12/16 switched from a RFS to MFS/FFS while 4/16 remained RFS while barefoot
- of the 12 that switched 5 of them significant increased their VILR while 7 had no change.
- of the 5 that showed changes when going to a MFS/FFS while barefoot all of them showed increases in loading rates while non showed decreases.
- Figure 1 below shows no loss in the impact transient in subjects running with a midfoot strike