December 2, 2008 | David F. Coppedge

That Spring in Your Step Is Semi-Automatic

Cross-country runners know the challenge of running on uneven terrain.  What they may not know is that they are executing one of the most difficult operations for robot designers: how to make an upright, walking machine make rapid decisions on irregular surfaces without falling.
    Monica Daley of the Royal Veterinary College wrote about this last week in Current Biology.1

We know quite a lot about how humans and animals run over completely level, uniform surfaces – conditions that can be easily studied on a track or treadmill.  Yet, the real world is much more complex, requiring frequent stride-to-stride adjustments to deal with bumps, holes and obstacles in the road.  What strategies do runners use to keep moving forward when the going gets rough?  Only recently has biomechanics research begun to turn to this challenging question.  New research by Grimmer and colleagues reveals that the answer may be a lot simpler than you might think.

She must have said “simpler” with tongue in cheek, because her next sentence said, “Running involves a cascade of systems working together, including the brain, spinal cord, sensory organs, muscles and bone.”  Earlier she had noted, “We often take our own impressive stability for granted, but if you watch a toddler learn to walk and run, you can see that it can be a challenging task.”
    Scientists try to model running with images of a bouncing ball, where tension on landing is released like a spring on the way up.  “Similarly, by using springs in their legs, animals can passively cycle energy through spring recoil, reducing the need for muscle work,” she explained.  Seems simple so far.  But tendons, muscles and joints differ in their springiness.  Did you know that humans have exceptional spring in their step, comparable to horses and kangaroos?
    The situation gets harder to model when the cross-country runner gets off the treadmill and onto the trail.  Rocks, ditches, bumps and other obstacles require constant monitoring and adjustment.  The speed of signals in the nerves, though, is finite from toe to brain.  How can the brain keep up with a flood of constantly changing data from the extremities in time to adjust?
    Here’s where Grimmer’s theory comes into play: the brain may be ordering a basic bouncing pattern with slight modifications.  This way, decisions are not required for every motion – just minor adjustments as needed.  “This allows the body to keep moving in its simple bouncing pattern without a stumble or fall.”
    If true, this means that most of the running motion occurs passively without active brain signaling.  An analogy from business might help:

That is not to say that neural control is not required for running.  To change speed, direction, or switch from a run to a walk, active control and path planning is certainly involved.  However, tuning your leg to behave like a simple mass-spring system may allow the brain and spinal cord to worry only about this higher level control, leaving within stride adjustments to the mechanical system.  Think of it as the difference between a ‘micro-managing’ supervisor and one who delegates responsibility and checks in now and then.  Overall, the latter strategy is considered more effective, because it frees the manager to pay attention to the big picture.  However, for this approach to succeed, things must not fall apart when the supervisor is not looking.

Notice that this strategy would not work without prior systems being in place.  Legs, muscles, tendons, bones and all the other components have to be able to run the strategy with minimal supervision.
    Daley asked a question midway through her article that tempted one to ask a big-picture question: “So, is the spring-like behaviour of human and animal legs an accident of nature, or a strategy to simplify the job of the central nervous system?”  The remaining paragraphs, in which she described the benefits of the delegating-manager strategy, suggested the latter.  She did not, however, use the word evolution anywhere in the article.  If it is not an accident of nature, one can draw one’s own conclusions about where strategies come from.


1.  Monica A. Daley, “Biomechanics: Running Over Uneven Terrain Is a No-Brainer,” Current Biology, Volume 18, Issue 22, 25 November 2008, pp. R1064-R1066, doi:10.1016/j.cub.2008.09.050.

Have you ever done rock-hopping in a river?  It’s fun and challenging, but the slightest mistake and you could be in for a dunk or broken leg.  Slippery rock, distracting currents, and uneven surfaces galore – did you think about how much calculus your brain has to do on the sensory inputs to do this?  Even a child can manage the task fairly well.  Dr. Daley did not extend her simplified analysis to this and other complex tasks humans perform (think balance beam).
    The wonders of the human body should inspire awe and respect.  How could you mistreat your machine?  If you owned a Ferrari, would you not give it special care?  How much more should we care about the tremendous gift the Creator has given us in the body we inhabit.  For more on the unique human capabilities involved in running, read the memorable entry from 11/14/2004, “The Evolution of Marathon Man.”

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