Forefoot vs Heel Striking: Muscle Activation Patterns
If you think back to high school Physics lessons, you might remember being taught about forces being transferred, rather than simply disappearing.
In the context of running gait, we can think about this with reference to the way in which we exchange forces all around the body; trading-off stress and strain on one tissue against that applied elsewhere as we alter loading patterns through changes in technique.
Real World Example:
A great example of this type of trade-off is the way in which a runner with an over striding, low cadence, heel striking gait pattern typically applies more torque around the knee joint during early stance phase than we would typically see if a forefoot strike or even simply a (less aggressive heel strike) was applied at a higher cadence.
The trade-off is that as we reduce torque around the knee, we experience increased torque around the ankle joint - specifically an increased external joint moment into ankle dorsiflexion, which is met by an increased internal joint moment into plantarflexion at the ankle, created by the calf-Achilles complex. (Thus the acute calf tightness we often see in runners trying to migrate from a heel striking gait...)
This is one of the less subtle examples of this type of trade-off, which occurs all around the body as we make even the smallest changes to running form!
Rehab Implications
From an injury rehab perspective, this exchanging of forces and loading patterns from one tissue to another is one of the main premises underpinning our use of running gait re-education as a rehabilitation intervention.
As long as you know your functional anatomy, and have a good understanding of running biomechanics; making targeted small but meaningful alterations to running form can reduce stress and strain on a given tissue. However this usually results in increased stress and strain elsewhere. Not a problem as long as you give the runner's body time to develop the required adaptation to the demands of the new loading pattern.
Commonly when thinking about stress and strain, we usually consider the way in which the runner's body loads at initial contact and through into early stance phase of running gait, with the runner weight-bearing on a single limb. Essentially assessing how well the body deals with ground reaction force.
Late Swing & Early Stance Phases of Gait
The following 2014 study however, looks at how habitual forefoot striking runners differ in terms of muscle activity during late swing phase (in preparation for weight-bearing) and early stance phase (as they begin to weight-bear), in comparison to habitual rearfoot (heel) striking runners.
Differences in Muscle Activity between Natural Forefoot and Rearfoot Strikers during Running
Jennifer R. Yong, Amy Silder, Scott L. Delp Journal of Biomechanics; Articles in Press
Abstract:
Running research has focused on reducing injuries by changing running technique. One proposed method is to change from rearfoot striking (RFS) to forefoot striking (FFS) because FFS is thought to be a more natural running pattern that may reduce loading and injury risk. Muscle activity affects loading and influences running patterns; however, the differences in muscle activity between natural FFS runners and natural RFS runners are unknown. The purpose of this study was to measure muscle activity in natural FFS runners and natural RFS runners. We tested the hypotheses that tibialis anterior activity would be significantly lower while activity of the plantarflexors would be significantly greater in FFS runners, compared to RFS runners, during late swing phase and early stance phase. Gait kinematics, ground reaction forces and electromyographic patterns of ten muscles were collected from twelve natural RFS runners and ten natural FFS runners. The root mean square (RMS) of each muscle’s activity was calculated during terminal swing phase and early stance phase. We found significantly lower RMS activity in the tibialis anterior in FFS runners during terminal swing phase, compared to RFS runners. In contrast, the medial and lateral gastrocnemius showed significantly greater RMS activity in terminal swing phase in FFS runners. No significant differences were found during early stance phase for the tibialis anterior or the plantarflexors. Recognizing the differences in muscle activity between FFS and RFS runners is an important step toward understanding how foot strike patterns may contribute to different types of injury.
I found this research particularly interesting in looking at both the differences in kinematics and muscle activity between habitual forefoot strikers and rearfoot strikers, not just from initial contact onwards into stance phase, but considering what happens preceding initial contact.
I frequently meet runners who have recently adopted a forefoot strike (non-habitual by definition) and display a noticeably plantar flexed ankle during late swing phase, just preceding initial contact. See the video below...
It almost looks like they’re "reaching for the ground” with the forefoot.
Interestingly, the more pronounced cases of this are usually the runners complaining of acute calf tightness and discomfort as they make this transition from their habitual heel striking running gait to a forefoot strike.
Cueing these particular runners to remain relatively / gently dorsiflexed (essentially neutral at the ankle in the sagittal plane), rather than plantarflexing usually does wonders in enabling them to to run with much less of a reaction from the calf complex.
The authors of the paper in question perhaps begins to explain the mechanism we’re seeing in action. In the discussion of the full text, the authors cite Winter (2005) in stating that:
"...muscle activity precedes force production"
Knowing that in habitual forefoot striking runners we expect to see an increased internal plantar flexion moment at the ankle joint during early stance phase, the authors expected to observe increased plantar flexor activity immediately preceding initial contact, which is exactly what they saw.
I like to think of this simply as "pre-engagement”.
While we know that in some runners suffering with knee pain, offloading torque at the knee by adopting a forefoot strike pattern can be great for their symptoms; this increases torque at the ankle, and as such demand on the plantarflexor muscles.
What this study suggests to me (but doesn't say explicitly), is that if as coaches / therapists we don't keep an eye on foot position for these new forefoot strikers, and allow them to get too plantarflexed before just initial contact, the calf muscles / Achilles etc... will suffer. Possibly exchanging one problem for another!
Of course, these are just my thoughts and observations in combining this research with what I see from day-to-day...
More research is needed :)
What About Heel Striking Runners?
When comparing the joint kinematics and muscle activity of the rearfoot striking group to the forefoot strikers, the authors noted that heel strikers tend to display less knee flexion at initial contact, meaning that they contact the ground with a 'straighter' knee.
The rearfoot strikers also displayed greater dorsiflexion and tibialis anterior EMG activity than the forefoot strike group during late swing phase.
In addition, these habitual rearfoot strikers displayed more lateral hamstring activity during late swing phase than the forefoot strikers. My feeling is that this is most likely due to the increased eccentric effort needed from biceps femoris to decelerate the tibia, as the lower leg swings forward into the more knee extended position.
The video below gives a good example or a more extended knee at initial contact, with more ankle dorsiflexion.
Of course, in slow-motion we can simplistically 'eyeball' the gross differences in joint kinematics compared to the forefoot strike video, but not see the muscle activation!
I've written before about certain rehab situations where I'll move an runner away from a rearfoot (heel) strike to offload certain structures. A good example of this is Anterior Compartment Syndrome (rationale here).
Looking at the difference in knee joint kinematics and lateral hamstring EMG activity during late swing and early stance between the forefoot strikers and rearfoot strikers, it also appears reasonable to me that some runners with distal Biceps Femoris Tendinopathy might benefit from adapting a forefoot striking pattern... although this is me thinking hypothetically at this point!
One pattern I do see on a fairly regular basis is runners with Hoffa's Pad Impingement who display significant passive knee hyperextension upon examination, who run with a marked rearfoot strike and a relatively extended knee at initial contact.
Once rehab is well under way (lots of neuromuscular control work into terminal knee extension) and running is re-introduced, adopting a more of a forefoot strike pattern seems to help with outcomes.
Food for Thought
That is of course what this all is, food for thought!
No concrete answers to be found here, but a good workout for the brain in understanding what we see from each of our runners, and what they experience in their own n=1 study.
It's worth noting that in this study discussed above, the authors accept the potential limitations that stem from their testing being performed with the subjects treadmill running. Each runner was allowed to run in self selected footwear.
In the full text, they conclude by stating that their data is insufficient to draw conclusions about injury mechanisms, but helps to define the differences between forefoot strike and rearfoot strike patterns.
Let's also not forget that "Not all Heel Strikes are Created Equal"