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, 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.