Running with Prosthetics:
Unfair Advantage?
vs
Purpose
• Compare running mechanics in bilateral
transtibial amputees using modern prosthetics to
intact runners to discover any significant
advantage.
– Interpret the findings of 3 studies:
• Point: “Artificial Limbs Do Make Artificially Fast Running
Speeds Possible.”
• Counterpoint: “Artificial Limbs Do Not Make Artificially Fast
Running Speeds Possible.”
• “The Fastest Runner on Artificial Legs: Different Limbs,
Similar Function?”
– Point/counterpoint argument features same authors
Amputation Statistics
• 1.7 million amputees in America
– 1/200 people
– 3,000 people become amputees each week
• 82% due to vascular disease
– Of remaining 18% dysvascular, 97%= lower limb
• Approximately 50% of lower limb amputations
are transtibial
• Prosthetic leg market grows 4% each year
– Ossur and Otto Bock
Prosthetics Terminology
• Residual limb- what’s left of limb after
amputation, “stump”
• Transfemoral/AK- above knee amputation
– 35-60% femur spared
• Knee disarticulation- amputation at the knee joint
with femur still intact
• Transtibial/BK- below knee amputation
– 20-50% of tibia spared
• Socket: connects prosthesis to residual limb,
transfers forces
Running Terminology
• Swing time- measured time (s) between the pushoff and initial foot strike of the same leg
• Stride time- measured time (s) between initial
foot strike of the same leg
• Leg length- measured (m) from the axis of
rotation of hip joint to the ground at the outside
of the heel or prosthetic blade
• Run speed - depends on stride length and rate
• Distance body moves per toe-off
– Depends on takeoff angle - amt of forward lean of
body over takeoff foot)
Case Study: Oscar Pistorius
• “Blade Runner”
– Born without fibulas
– J-shaped carbon fiber bilateral BK prosthetics
• “Cheetahs” by Ossur
– Paralympic Record: 100-11.17s, 200- 22:67s, 400- 47.49s
– 2nd in S. African Nationals able-bodied 400m
• IAAF International Track Organization banned from
Olympic competition Jan. 2008
– Prosthetics more spring than human legs
– Appeal was approved by Court of Arbitration for Sport in
May 2008
– Did not make qualifying time of 45.95s, PR is 46.25
http://www.youtube.com/watch?v=ON4B-fNCvSg
“The Fastest Runner on Artificial Legs:
Different Limbs, Similar Function?”
– Published in June 2009 in the Journal of Applied
Physiology
– Question: Is running with lower-limb prostheses
functionally similar to running with intact, biological
limbs?
• 3 hypotheses: metabolic cost, sprint endurance, mechanics
– Main subject: Oscar Pistorius
– Comparison group: past studies of elite and sub elite
runners, collected data from competitive runners with
similar speeds to Pistorius
1) Metabolic Cost of Running
• Hypothesis: greater than 2 standard
deviations below the mean of intact groups
• Test: interval run on treadmill
– (Rate of Oxygen Uptake/speed of trial)
• Result: 17% lower than able-bodied sprinters
– 2.7 SD lower
• Discussion: inconclusive
– Found research of bilateral amputees having
higher metabolic costs
2) Sprinting Endurance
• Hypothesis: longer duration due to lightweight
carbon-fiber material resisting fatigue
• Test: constant-speed, all-out treadmill trials
• Result: amputee sprint within same range as
intact
3) Running Mechanics
• Hypothesis: Greater than 2 SD below for:
– Foot-ground contact times, aerial times, swing times,
stance-avg vertical rxn forces
• Test: video analysis of 2.5 m/s to 10 m/s, force
software used
• Result: Foot-ground contact 14.1% longer, aerial times
34.3% shorter, swing times 21% shorter, vertical rxn
forces 22.8% less
– All greater than 2 SD away from mean
• Discussion:
– vertical rxn forces less due to absence of several muscles
crossing foot, ankle, and knee joints
– Faster turnover due to weight of prosthetic and residual
limb below the knee being half that of a normal limb
Running Mechanics Graph
A) video images at 10.5 m/s
•solid line shows longer contact,
shorter stride, aerial, and swing
duration
B) Vertical ground forces vs time
•Peaks lower than able-bodied
•Peaks arise faster
C) Horizontal ground forces vs time
•More steady peak than ablebodied
•Peaks arise faster
Black: amputee
Gray: able-body
Solid: right limb
Dotted: left limb
Weyand et. al. The Fastest Runner on Artificial Legs: Different limbs, Similar Function? J Appl Physiol 107: 903-911,
2009; doi:10.1152/japplphysiol.00174.2009
Running Mechanics
Results
A. Contact time vs. Speed
• 3.5 SD higher at 10m/s
B. Aerial time vs. Speed
• 4.4 SD lower at 10m/s
C. Swing time vs. Speed
• 3.4 SD lower at 10m/s
D. Vertical Force vs. Speed
• 5.2 SD lower at 10m/s
•
Comparison of Differences
• minimal at 2.5-3m/s
• Modest 4-5m/s
• Pronounced 6-10m/s
Conclusion
• Bilateral transtibial amputee running is
physiologically similar to intact runners but
mechanically not.
– Physiological relation may be due to heavy use of
extensor muscles crossing hip and knee in both groups
– Weight of prosthetic and residual limb below knee:
2.5 kg, approximately half weight of normal limb
– More research needed
• Study was not in race setting: fast closing times may be due
to slow acceleration b/c no ankle muscles
“Point: Artificial Limbs Do Make
Artificially Fast Running Speeds Possible”
• Published in Journal of Applied Physiology
Nov. 19, 2009 by Peter Weyand and Matthew
Bundle
• Subjects: compare double amputee sprint
runner to four track athletes and two elite
male sprinters
Overview
• Mechanical variables determine run speed
– Quickness of reposition of limbs, forward distance
while foot touching ground, force applied to ground
– Speed=step freq*forward distance during contact*avg
vertical force
• Primary requirement to run: apply ground forces
large enough to get the aerial time needed for
next step
• Average Vertical Force= total step time/contact
time
Artificial Limbs and Performance
• Amputee stride frequencies
– 15.8% greater than athletes in lab
– 9.3% greater than elite sprinters overground
• Short swing times=reposition limbs faster
– 21% shorter than athletes in lab
– 17.4% shorter than top two finishers in 1987
World Track Championships 100 m - (0.344s)
• Contact length to leg length
– 9.6% higher than athletes in lab
– Due to high compliancy of artificial limb
Artificial Limbs and Performance
• Stanced average vertical force
– Lower by 0.46Wb than track athletes
– Fall within range of 1.65-2.52 Wb
• Adjusted swing times and contact lengths
similar to able-body athletes
– Speed=step freq*forward distance during
contact*avg vertical force
– speed decreased from 10.8 to 8.3m/s
10m/s
• Leg compression insert: midstance, maximum
limb compression
– External moment arm at knee 40% less
– External moment arm at hip 65% less
Conclusion
• Two modifications that would increase
bilateral transtibial amputee sprint speed:
– Reduction in mass-> reduce swing time
– Increase length-> increase contact time
• Finding: Artificial limbs out perform biological
Counterpoint: “Artificial Limbs Do Not Make
Artificially Fast Running Speeds Possible.”
• Published in June 2009 in the Journal of Applied
Physiology by Kram, Grabowski, McGowan, Brown,
and Herr
• Question: Do modern running prostheses provide a
significant advantage over biological legs
• Overview: only one amputee tested, Oscar Pistoriusno advantage or disadvantage, give experiments
needed for future
Artificial Limbs vs Able-bodied
• Amputee Running Economy Higher: False
– Worse for amputees, but did not meet p<0.05 significance
– Only two reported bilateral transtibial: Pistorius and another
runner
• Pistorius 1.15 SD below mean, other runner 1.92 SD above mean
• Short leg swing due to prosthetic weight: False
– Highly neurologically trained
– Compensates for force limitations from prosthetics
• Amputees have lower ground reaction forces: False
– No published GRF data for unilateral at top speed
– Only one bilateral subject published: Pistorius
– Pistorius was found to exert lower vertical force
• Due to prosthetic or weak legs
• Prosthetics must have some give and Pistorius is highly trained
Suggestion: Compare Unilateral
Amputees
• Vertical force of affected to unaffected?
– If greater vertical force with unaffected, than
prosthetics are disadvantageous b/c force limitation
• Unilateral amputees have same leg speed times
between legs?
– If true: Leg speed advantage NOT due to lightweight
prosthetics
• Adding mass will not increase leg swing or
decrease time?
– If true: weight of prosthetic is not a factor
Who is correct?
Only time can tell…
Resources
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“Amputation Statistics by Cause: Limb Loss in the United States.” National Limb
Loss Information Center. Revised 2008. http://www.amputeecoalition.org/fact_sheets/amp_stats_cause.pdf.
Epstein, D. “New Study, For Better or Worse, Puts Pistorius’ Trial in Limelight.”
Inside Olympic Sports: Sports Illustrated.
http://sportsillustrated.cnn.com/2009/writers/david_epstein/11/19/oscar.pistoriu
s/1.html
Hamilton, N. et al. “Kinesiology: Scientific Basis of Human Motion.” 11 ed. 2008.
McGrawHill Companies. New York, NY.
O’Sullivan, S. and Sielgman, R. “National Physical Therapy Exam Review and Study
Guide.” 2009. International Education Resources. Concord, MA.
Weyand, P. et al. “The Fastest Runner on Artificial Legs: Different Limbs, Similar
Function?” J Applied Physiology. 2009 Sep;107(3):903-11. Epub 2009 Jun 18.
Weyand, P. and Bundle, Kram, R. et al. “Point: Counterpoint ‘Artificial limbs do / do
not make artificial running speeds possible.’” J Applied Physiology. 2009 Nov 19.