Pre-Programmed Reactions: Part II

One type of pre-programmed reaction is the corrective stumbling reaction.  Some times when we are walking, we encounter some type of obstacle or perturbation that affects our balance and may cause us to fall.  The corrective stumbling reaction is present to help prevent us from falling, that is, to maintain our balance until the nervous system has to prepare and initiate a voluntary response to help correct for the perturbation. 

If we are walking and our swing leg (the one that is off the ground) hits something (a curb, step, object, etc.), the correct stumbling reaction creates a flexor response in the leg muscles of the swing leg that will lift the leg up over the obstacle.  If the stance leg (the one that is on the ground) steps on something uneven, or hits something, it creates an extensor response in the muscles of that leg that will shorten the amount of time it is on the ground, allowing the person to place the other leg (the swing leg) on the ground quickly, which will increase the chances of maintaining balance and not falling down.  This pre-programmed reaction will take place approximately 50-100 ms after the person encounters the perturbation, and hopefully will prevent the person from falling.  In the picture above, this reaction helped prevent some of the runners from falling, but did not occur fast enough or cause a great enough response to prevent some of the other runners from falling.