caitlin pearl - run smart not hard

5/14/2012

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Run Smart not Hard:

The Effect of Stride Cadence on

Impact Variables

Caitlin Pearl, MSClinical Biomechanist

National Institute for Athletic Health & Performance

Sanford Health: Sioux Falls, SD

Outline

BACKGROUND

• Injury Mechanisms

• Impact Force

– Injured vs. Uninjured

– Controlling Impact Force

• Manipulating Step Rate

– Joint mechanics

– Injury rate (?)

APPLICATIONS

FDM-T

– Sanford Services

– Research Opportunities

Running…

• Running is comprised of a series of repetitive

single-limb impacts which requires:

– Sufficient impact force attenuation

– Limb/trunk stability

Powers, 2012

…Is All About Balance

Passive Shock Absorption

Active Shock Absorption

Bone and Cartilage

Eccentric Muscle

Contraction







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Numbers Game

• According to the 2009 National

Runner Demographics, there are

37 million runners in the US

– 11 million run ≥100 times per

year

• About 56% of recreational

runners, and as many as 90% of

runners training for a marathon

will experience some type of

running-related injury each year

http://www.trackshack.com/services/sponsor-demographics.shtml

Running Performance Services:

Sanford Health

• Team of sport rehabilitation

specialists & clinical biomechanist

• Evaluate injured and healthy

runners

• 90-minute evaluation:

– Functional movement screen

with PT: lower extremity

strength, flexibility, posture

– Treadmill running and video

analysis in frontal + sagittal

planes

– Recommendations and

treatment options

Assessing Runners with h/p/cosmos and FDM-T

Image copyright: zebris Medical GmbH, Germany

• Excessive and repetitive impacts (Heiderschiet et al., 2011)

– First 10% of stance phase

• Can reach magnitudes from 1.5 to 5x body weight within 10-30 ms (Hreljac et al. 2004)

– Too much energy for the body to safely absorb

Mechanisms of Injury

Mechanisms of Injury

• Impact forces are associated with overuse running

injuries (Cavanagh et al., 1980, Clement et al., 1980, James et al., 1978, Nigg 1986)

• Overuse injuries, such as stress fractures, are

dependent on both loading magnitude and loading

exposure (Edwards et al., 2009)

– Muscles failing to adequately absorb the energy from impact may

lead to “over-reliance” of passive structures, causing injury (Derrick et al., 1998)

What Determines Impact Force?

• The magnitude of impact force during running is

dependent on: (Heidershchiet et al., 2011)

o Landing mechanics

o Running speed, foot & COM velocity at contact

o Body mass

o Shoe properties

o Surface properties

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vGRF- A Closer Look

Pete Larson, http://www.runblogger.com/2011/02/vertical-impact-loading-rate-in-running.html

vGRF- A Closer Look


vGRF- A Closer Look


vGRF and Footstrike

• Approximately 80% of shod

runners are heelstrikers

• RFS experience higher impact

peaks and loading rates than MFS

and FFS

– -More vulnerable to injuries?

Davis et al., 2010

FDM-T: Heel strike FDM-T: Midfoot Strike

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FDM-T: Forefoot Strike Do Impacts Cause Injury?

• No published data comparing footstrike and injury

• Davis et al. (2010) initiated first prospective study

– Compared the impact loads of rearfoot strike runners who go on to

develop a running injury to their uninjured counterparts.

Comparison of impacts between injured

and non-injured runners

• 57% experienced a prospective injury

• Injured runners had significantly

higher impact loading variables, with

the exception of VILR, than the

uninjured runners

• Peak vertical force was identical

between groups

- Need for further research

Davis et al., 2010

Impact Peak & Loading Rate in Injured vs.

Non-Injured Runners

*

Previously injured runners exhibited a higher impact peak and loading rate than injury-free

runners

Hreljac et al., 2000

*

Kinetic Variables in Subjects with Previous

Lower-Extremity Stress Fractures

PPA (g) vGRF (BW) ILR (BW/s) ALR (BW/s)

Stress Fracture 9.24 3.87 158.61 117.93

Uninjured 7.16 2.48 108.89 77.52

Ferber et al., 2002

22% 36% 32% 34% DIFFERENCE:

Injury Frequency, Impact Peak and Loading Rate

Runners displaying a high impact peak and loading rate did not show an increased number

of running-related injuries over a 6 month monitoring period

Nigg et al., 2001

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How can we control impact?

• Increase step-rate (cadence)

– Decrease step length

• Improve mechanics at initial contact (extended knee, dorsiflexed

ankle)

– Decrease IC COM-heel distance

• Farther the foot strikes the ground in front of COM, greater the

braking impulse

• Most injuries occur during contact. By reducing

contact time, can we reduce the risk of injury?

Manipulating Cadence

Before After

Landing Mechanics

159 steps/min 175 steps/min

Step Frequency and Lower Extremity Loading

VIP= Vertical Impact Peak

VILR= Vertical Instantaneous Loading Rate

VALR= Vertical Average Loading Rate

Lower extremity loading

variables minimized at around

+15% of preferred step

frequency

Hobara et al., 2011

Effects of Step Rate Manipulation on Joint

Mechanics During Running

• Increasing one’s step rate by 10% of greater will result in a reduced

impact load on the body, due to less vertical COM velocity at

landing (Derrick et al. 1998, Hamill et al. 1995)

– As a result, less energy absorption is required by the lower extremity, specifically the

knee

• Can reduce the risk of developing a running-related injury or

facilitating recovery from an existing injury (Heiderscheit 2011)

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Increasing Step Rate Decreases Step Length

* ** *

Heiderscheit et al., 2011

Step Rate and COM Heel Distance

As step rate increased:

• Step length was shorter

• Less COM vertical displacement

• Heel was placed horizontally

closer to COM at touchdown


Increasing Step Rate Decreases Heel-COM

Distance at Contact

**

**


Increasing Step Rate Decreases Braking Impulse

**

* *


Peak vGRF was significantly reduced at +10% preferred step rate

and significantly increased at -10% (±±±± .6 N/kg)

FDM-T: Force Reduction

Peak vGRF 159 steps/min= 1304.2 N

= 18.55 N/kg

Peak vGRF 175 steps/min= 1205.7 N

= 17.15 N/kg

-1.4 N/kg

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Step Rate and Mechanical Energy

Step rate above preferred:

Decrease energy absorption (negative

work) at the knee and hip

Proportional decrease in energy

generation across all joints


Clinical Applications: Increased Cadence

Despite a greater number of loading cycles, running with an increased cadence has

been suggested to reduce the risk of tibial stress fractures. (Edwards et al., 2009)

Clinical Applications: Increased Cadence

• The reduced energy absorption at the hip in knee

when running with an increased cadence may prove

useful in rehabilitation (Heiderscheit, 2011)

– Can injured runners continue to run without aggravating

symptoms?

– Can we facilitate a progressive return to running?

Bottom Line

• It is unclear if, how or why impacts cause injury.

• Discover new meaning for “take a load off”:

increasing cadence may offer an easy, quick solution

for improving large, detrimental forces seen at

impact.

• HOWEVER, it is also important to explore alternative

methods for improving shock/impact absorption,

such as developing hip strength and control, and this

opportunity should be seriously considered for long-

term benefits.

FDM-T: Opportunities for Research?

• Observe the cumulative effect of loading

– Prospective study following loading patterns and injury

occurrence

• Continue seeking cause and effect relationship: impacts vs. injury

rate

– Footstrike patterns and injury occurrence

• MFS/FFS: 1st or 2nd metatarsal stress fracture?

• RFS- Tibial stress fractures, plantar fascitis?

• ITBS, PTF pain?

• Are the biomechanical changes seen with increased

step rate observed beyond short term?

Think about this:

It is estimated a runner sustains 1200-1500 impacts per

mile (each contact= ~1.5-5x BW).

FDM-T provides the opportunity to

look at each one of them.

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Questions?

caitlin pearl - run smart not hard

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