Brief thoughts: On leg stiffness and impact

In the course of writing my previous article on tibial stress fractures, I did a lot of thinking and reading about the role of impact and leg stiffness in the genesis of injury.  There’s a lot of back-and-forth on the subject, with few clear answers from the wealth of research that’s been done.  Before we get too in-depth on the details, let’s go over the basics: 

If we have an athlete step on a force plate while running, we’ll see a very characteristic curve (assuming he’s a heel-striker).  There is a sharp initial spike in force, which drops off a bit and is followed closely by a larger more gradual curve.  This initial spike is the “impact peak,” and it corresponds to the forces generated when your heel strikes the ground.  The second peak is the “active peak,” and it corresponds to the forces that your muscles generate to push off the ground and propel yourself forward.  When we talk about the “impact force,” we are talking about the height of that initial peak.  When we are talking about the “impact loading rate,” we are talking about the slope of the peak, which tells us how rapidly the force increases. 
From Nigg (1)
During the impact peak, your leg is essentially “locked”—that is, your muscles are tensed up and static, and your leg behaves like a passive system of springs and hinges.  Immediately prior to impact, your muscles tense, anticipating the impact with the ground.  You can feel this if you put your hand on your quads the next time you’re out running. 

The degree to which your muscles are stiffened—the “tuning” of the muscles, if you will—depends on the surface you’re running on and the shoes you are wearing (if any).  To maintain optimal running economy, the body attempts to keep the trajectory of your center of mass as constant as possible over a variety of surfaces and footwear conditions.  To do this, the combination of the stiffnesses that act on your center of gravity has to be constant.  Just like your leg behaves like a spring during the impact phase, so too does your shoe and even the ground.  Concrete is like an extremely stiff spring; grass or tartan is like a looser one.  So, as you can infer on your own, if you are out for a run and transition from grass to concrete, your leg stiffness has to drop. 
Because of this change in leg stiffness, the active peak force while running is remarkably stubborn.  No matter the surface or shoe stiffness, a runner maintaining a given pace will always have the same maximum force going into the ground.1  When it comes to peak impact force and impact loading rates, some of the theoretical work jousts with the real-world data, and a lot of the real-world studies don’t agree with each other!
The consensus now seems to be that the impact force is also reasonably stable, as it ought to be, given that the sum of the system’s stiffness (surface + shoes + leg) is a constant (though some differences have showed up, especially in extreme cases, like running barefoot on a hard surface).  Impact loading rate, however, seems to be more variable (or at least controversial).  I am having a hard time working through the physics of the system—given that the overall combination of leg stiffness, shoe stiffness, and ground stiffness is equal, is it necessarily true that the impact loading rate of each of these components must be equal?
Real-world studies on runners with stress fractures have demonstrated that these runners tend to have higher impact loading rates than runners who have not suffered a stress fracture.2, 3
Furthermore, the impact loading rate is strongly correlated with lower leg stiffness and with tibial shock, as measured by an accelerometer taped to the shin.  What’s not clear is how our leg stiffness model fits into these findings.  Are these injured runners suffering because their shoe and surface conditions they run in require high leg stiffness, putting excessive stress on their bones? Or is there leg stiffness mechanism “fried” and not properly responding to input from the body regarding the stiffness of the surface it’s running on, resulting in high leg stiffness all the time?  In the first case, we should recommend that runners with a history of stress fracture and other injuries related to increased lower leg stiffness train in thinner shoes on harder surfaces.  In the second case, we need to figure out a way to “reset” their leg stiffness calibration system, and probably keep them on grass in the meantime, where their abnormally high leg stiffness would be partially nullified by the softer surface.
This wouldn’t be too hard of an experiment to do.  Get a group of injured runners with a high-leg stiffness-related injury (tibial stress fracture and plantar fasciitis are two emerging examples), then examine their leg stiffness during running or hopping (or even using a “human pendulum”) over a range of surface stiffnesses.  Compare their response to that of an age, gender, and training-background-matched control group.  If the two groups display the same change in leg stiffnesses over a range of surface stiffnesses, then the central nervous system is properly adjusting their muscle tuning, and the key to preventing future injury is to avoid situations that demand high leg stiffness, like wearing thick shoes on soft surfaces, and work to identify ways to alter running gait to reduce impact loading rates.  If, however, the injured group displays a dysfunctional muscle tuning system (i.e. their brain/CNS can’t properly calibrate the legs to the surface they are running/hopping on), then we have to figure out why their system is broken and how to fix it.
I suspect it’s unlikely the leg tuning mechanism has failed in most runners with stiffness-related injuries (whatever those end up being).  Problems with leg stiffness tuning would turn up in all sorts of situations—it is, after all, the same mechanism that is responsible for anticipating stairs on a step.  So when you trip over yourself because you thought there was one more step on a staircase, that’s what a failure of the muscle tuning system feels like.  But for now this is just a hypothesis. 
Ultimately, we need a few well-designed studies to nail down once and for all exactly how surface stiffness, impact force, impact loading rate, and leg stiffness are related.  Why have some studies found that harder surfaces increase vertical loading rate, while the model predicts the opposite? Since we are measuring externalforces, does this really tell us about what’s going on inside the body? And most importantly, what’s the role of impact loading rates in the development of injury, and by what mechanism can they be remedied?
1.            Nigg, B., The Role of Impact Forces and Foot Pronation: A New Paradigm. Clinical Journal of Sports Medicine 2001,  (11), 2-9.
2.            Milner, C. E.; Ferber, R.; Pollard, C. D.; Hamill, J.; Davis, I. S., Biomechanical Factors Associated with Tibial Stress Fracture in Female Runners. Medicine & Science in Sports & Exercise 2006, 38 (2), 323-328.
3.            Milner, C. E.; Hamill, J.; Davis, I., Are knee mechanics during early stance related to tibial stress fracture in runners? Clinical Biomechanics 2007, 22 (6), 697-703.

About the Author

John J Davis, PhD

I have been coaching runners and writing about training and injuries for over ten years. I've helped total novices, NXN-qualifying high schoolers, elite-field competitors at major marathons, and runners everywhere in between. I have a Ph.D. in Human Performance, and I do scientific research focused on the biomechanics of overuse injuries in runners. I published my first book, Modern Training and Physiology for Middle and Long-Distance Runners, in 2013.

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