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.
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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 external
forces, 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?
References
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.
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