Of Course Your Knees And Quads
Hurt More After Running Than Walking: You're Only Human
BETHESDA, MD (August 23, 2004) – Your knees take
the brunt of the increased demands on your lower body in terms of the amount
of muscle mass used and joint flexion when you compare walking to running.
By a lot. Why? Because you’re human.
Though humans share a lot of qualities with other
mammals, we are unique in terms of posture, locomotion and gait. (In fact,
we’re among the only two-legged mammals who walk and run.) For instance,
horses consume about the same amount of energy to cover a mile when running
or walking, while humans consume substantially more energy when they run
than when they walk.
But with our unique patterns of limb kinematics, a
group of scientists wanted to study exactly how that affects how we use our
muscles while walking and running, and to better understand why it’s more
“efficient” to walk than to run.
Harvard research finds five-fold increase in knee
torque, muscle force
The researchers, most of whom at one time were graduate
students of the late C. Richard Taylor at Harvard University, filmed four
healthy males walking and running at six self-selected speeds. They measured
vertical force on the ground and velocity as the subjects chose “slow,”
“preferred” and “fast” speeds for both running and walking.
They found that with an increase of speed and gait, the
maximum muscle force increased steadily at the hip, remained fairly constant
at the ankle, but increased sharply at the knee when the subjects changed
from a walk to a run. In most instances (except for the hip at a run), they
found that limb muscles were primarily acting to generate force on the
ground and the muscle’s role in overcoming inertia and gravity was minimal.
Results of the research are reported in a paper
entitled “Muscle mechanical advantage of human walking and running:
implications for energy cost,” which is online at the Journal of
Applied Physiology (July 2004 Article in Press), one of 14 peer-reviewed journals published by
the American Physiological Society.
Lead author Andrew A. Biewener is at Department of
Organismic and Evolutionary Biology at Harvard University, Boston; Claire T.
Farley is at the Dept. of Integrative Physiology at the University of
Colorado, Boulder; Thomas J. Roberts was at the Dept. of Zoology, Oregon
State University, Corvallis; and Marco Temaner is at the Dept. of Organismal
Biology & Anatomy, University of Chicago, Illinois. Since completion of the
paper, Thomas Roberts has moved to Brown University.
Change in posture while running reduces mechanical
advantage
Since the knees are more bent during running than
during walking, the researchers found that the amount of force generated by
the knee extensors (quadriceps muscles) rose almost 5-fold when walking
humans broke into a run, a somewhat confusing idea for the non-expert. These
high forces generated by the knee extensors cause running to be aerobically
more demanding than walking. Consider this example: when a person tries to
stand with their knees bent to around 90°, their quads fatigue very rapidly.
In contrast, if they stand with their legs straight, they don’t notice any
fatigue in their quads. This is similar to the running vs. walking
differences. In running, the knee is much more bent when the foot is on the
ground than in walking. For this reason, the quads generate much higher
forces during running and consume much more energy.
The study identifies this single difference between
walking and running as playing an important role in causing running to be
less economical than walking. The researchers note that it’s not the entire
reason, but it’s important.
By contrast to the knee extensors, the ankle and hip
extensors don’t have a large change in posture or force generation at the
gait transition, so the energy consumption by those muscle groups doesn’t
increase substantially at the gait transition. Due to the contrast between
the knee extensors and the other limb muscle groups, they identified the
high forces generated by knee extensors as the primary reason for the high
energy cost of running.
The researchers also looked at the active muscle volume
needed to generate force on the ground, and here, too, the knee extensors
sprung way past the hip and ankle. Whereas all three joints increased the
active muscle volume as speed increased and gait changed, the knee extensors
increased 4.9-fold during running (to 49% of the three extensor groups
combined, vs. 23% at a walk). This compared with a 1.77-fold increase for
the hip extensors (to 36% of the aggregate total while running, from 46%
walking) and a 1.10-fold increase for ankle extensors (way down to 16% of
the total while running from 36% at a walk).
They warn, however, that “the interacting effects of
increased muscle recruitment but decreased activation duration on energy
cost, when humans increase speed and change gait from a walk to a run,
remains an important challenge to sort out.”
The researchers conclude that “greater energy cost
during running in humans may be explained in part by the decrease in limb
mechanical advantage results from the use of more flexed knee joint during
running versus walking [and speculate that this] may reflect the evolution
of a unique erect bipedal gait within hominids which distinguishes modern
humans from avian bipeds and mammalian quadrupeds.”
Source and funding: The article, “Muscle
mechanical advantage of human walking and running: implications for energy
cost,” is online in the Journal of Applied Physiology, published by
the American Physiological Society. A copy of the abstract is
available to the public at
www.the-aps.org.
This study was supported by NSF grant IBN-930763 and
NIH grants AR-046499 and AR-047679.
Editors note: A copy of the research paper by
Biewener et al. is available to the media. Members of the media are
encouraged to obtain an electronic version and to interview members of the
research team. To do so, please contact Donna Krupa at APS (301) 634-7209,
cell (703) 967-2751 or
dkrupa@the-aps.org.
- APS Intersociety meeting on the INTEGRATIVE BIOLOGY
OF EXERCISE
- Co-sponsored by the American Physiological Society,
Canadian Society for
- Exercise Physiology and the American College of Sports
Medicine
- Oct. 6-9, 2004, Austin, Texas
The
American Physiological Society was founded in 1887 to foster basic and
applied bioscience. The Bethesda, Maryland-based society has more than
10,000 members and publishes 14 peer-reviewed journals containing almost
4,000 articles annually.
APS
provides a wide range of research, educational and career support and
programming to further the contributions of physiology to understanding the
mechanisms of diseased and healthy states. In May, APS received
the Presidential Award for Excellence in Science,
Mathematics and Engineering Mentoring (PAESMEM).
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