Brief Thoughts: A critical examination of "Foot Strike and Injury Rates in Endurance Runners: a retrospective study"

Two very different footstrikes captured on high-speed video

This post is a bit behind the times, but I thought I'd get it out there regardless.  This post is a short analysis which takes a look at the latest research from Daniel Lieberman's lab at Harvard University.  Lieberman is, of course, famous for his paper on foot strike in habitually shod and unshod runners which made the cover of Nature magazine and sparked a fierce controversy in the world of biomechanics.  This latest paper, which is available as an "epub" online (it has been accepted and reviewed, but has yet to be printed), turns its attention to foot strike styles and injury rates on the Harvard track and cross country team.  Its title is "Foot Strike and Injury Rates in Endurance Runners: a retrospective study," and I'll reproduce the abstract below for your convenience.

Purpose: This retrospective study tests if runners who habitually forefoot strike have different rates of injury than runners who habitually rearfoot strike.

Methods: We measured the strike characteristics of middle and long distance runners from a collegiate cross country team and quantified their history of injury, including the incidence and rate of specific injuries, the severity of each injury, and the rate of mild, moderate and severe injuries per mile run.

Results: Of the 52 runners studied, 36 (59%) primarily used a rearfoot strike and 16 (31%) primarily used a forefoot strike. Approximately 74% of runners experienced a moderate or severe injury each year, but those who habitually rearfoot strike had approximately twice the rate of repetitive stress injuries than individuals who habitually forefoot strike. Traumatic injury rates were not significantly different between the two groups. A generalized linear model showed that strike type, sex, race distance, and average miles per week each correlate significantly (p<0.01) with repetitive injury rates.

Conclusions: Competitive cross country runners on a college team incur high injury rates, but runners who habitually rearfoot strike have significantly higher rates of repetitive stress injury than those who mostly forefoot strike. This study does not test the causal bases for this general difference. One hypothesis, which requires further research, is that the absence of a marked impact peak in the ground reaction force during a forefoot strike compared to a rearfoot strike may contribute to lower rates of injuries in habitual forefoot strikers.

Brief Thoughts: Are hard workouts counterproductive?

Since my mission to write a set of comprehensive articles detailing the most common running injuries (the “Injury Series” here on RunningWritings) has morphed into a colossal undertaking, I’ve decided to break things up a bit with a new occasional, hopefully weekly series entitled “Brief Thoughts,” which I intend to be shorter, less-scientific musings on training and racing.  Today will be the first installment in that series.

Several world-class runners looking rather relaxed at the 2008 Olympics.  Photo: zimbio.com

One thing that’s been on my mind recently is how often, if ever, an experienced runner should do hard workouts.  This topic crossed my mind while reading this excellent thread on LetsRun, in which several sub-4 milers commented on how their workouts were going when they felt “ready” to break four minutes.

Sub-four miler “usedabe” writes:

There are a few different types of workouts that indicate the type of fitness needed for going under 4:00.

The ability to comfortably run 5-8 miles (true tempo effort) averaging under 5:00 pace is a great indicator of aerobic fitness. Can be one tempo run or long tempo intervals with short rest (like 3x3200 w/2:00 rest).

Something like 20x200 averaging 29.0 - 29.5ish with 200m rest (:50 - :55s) will show you've got good speed endurance.

Being able to go mid/low 50's for a 400m shows you've got the basic speed.

When asked about the “classic” 10x400m at mile pace w/ 60sec rest workout, he writes:

Never did it [10x400m] myself (and I don't think I could have done it if I tried), plenty of people claim it's a good indicator though. Personally, I don't think it's a good idea to run 'indicating' workouts since they tend to amount to a race effort when you could have just run a workout and gotten better - not just tested your fitness. Work on different areas of your overall fitness (aerobic, speed) and race like a madman.

Which is what got me thinking: if super-fit sub-four milers don’t need so many “hard” workouts, are they any good for anybody else? To me, “hard” denotes anything that approaches a race effort.  While some workouts, like 10x400m at mile pace with a minute recovery, are easily delineated as hard, others are not so clear.  The difficulty of an eight mile “tempo run,” for example, is entirely dependent on the effort at which it’s run.  And even the pace is not as informative as you might think—there’s a difference between pace and effort.  It’s very possible to run the right pace but the wrong effort.  I realize this is a controversial proposition, and sounds somewhat surprising coming from Mr. Science here.  But bear with me for a little while and I’ll promise not to tread too deeply into “zen of running” territory.  

Injury Series: Uncovering the role of hip strength and mechanics in the cause and treatment of patellofemoral pain syndrome

Today we are tackling the most common running injury of all: patellofemoral pain syndrome, alternatively known as runner’s knee, anterior knee pain, and (sometimes erroneously) chondromalacia or chondromalacia patellae.  As is often the case, there is some controversy about the naming.  “Patellofemoral pain syndrome” or PFPS is the broadest and most common term, but is often criticized for being a “wastebasket term”—an umbrella definition for any unexplained anterior (frontal) knee pain.  Historically, it was termed chondromalacia patellae, which is a softening of the cartilage under the kneecap.  It turns out that not all cases of PFPS involve softened cartilage, and not all people with softened cartilage have patellofemoral pain! “Runner’s knee” is another common term but is perhaps the most useless.  It doesn’t tell us anything about the problem, other than that you’re a runner and your knee hurts.  Because there is still lack of a specific injury mechanism, virtually all scientific articles use the term “patellofemoral pain syndrome,” as will I.

Symptoms

Patellofemoral pain syndrome is usually characterized by a pain “behind” or “around” the kneecap that is aggravated by squatting, running (particularly downhill), walking down stairs, and prolonged sitting.  In runners it usually starts as a mild irritation and might go away once you’ve gotten warmed up.  But over time it tends to get worse, being aggravated even by short, easy runs.  There is also usually tenderness around the upper edges of the patella and pain if you contract your quadriceps against resistance (on a leg extension weight machine, for example) or compress the patella downward “into” the knee joint. 

Anatomy

“Patella” is the formal name for the kneecap.  The patella itself is a triangular bone attached on one end to the quadriceps muscles via the quadriceps tendon and attached on the other end to the top of the tibia via the patellar tendon.  The patellar tendon is also a common injury location, and patellar tendonitis was covered in an earlier installment in the Injury Series posts.  Note that it’s important to distinguish PFPS from patellar tendonitis,as the ideal treatments are different! The main function of the patella is to give the quadriceps a mechanical advantage in knee extension.  By raising the tendons away from the center of the joint, the knee can generate more power.  The drawback of this is that the patella transmits virtually all of the force to the lower leg during knee extension.  As such, it may be inherently prone to overuse. 

Adopted from Dixit et al.

The backside of the patella slides back and forth during knee flexion and extension in a groove in the femur aptly called the patellar surface.  The assumption is that the development of patellofemoral pain syndrome is due to improper tracking of the patella within this groove.  It’s easy to visualize how a patella that tracks too far towards or away from the center of the body would rub up against the sides of this groove.  Where this improper tracking comes from is an interesting puzzle, and will be dealt with later.  If you’ve read my other Injury Series articles, you know that a fundamental tenant of my approach to managing injuries is that pain is indicative of real, physical damage.  What’s not clear in the case of PFPS is where that damage is occurring.  As mentioned earlier, the common assumption for a long time was that PFPS was caused by chondromalacia  or a softening of the cartilage on the patellar surface brought about by improper patellar tracking.  While chondromalacia is not an uncommon finding among patients with knee pain, PFPS alone is not predictive of cartilage softening or damage, nor is the severity of PFPS pain, as first demonstrated in 1986 by Lindberg, Lysholm, and Gillquist,¹ and confirmed in a 2010 surgical study by Pihlajamaki et al.²  While this is good news, since cartilage is not renowned for its healing abilities, it means that we're not entirely certain where the damage/irritation is occurring.  Essentially, the “syndrome” in “patellofemoral pain syndrome” means that we don’t know exactly why it hurts.