These 'Bots Are Made for Walking
Walking seems so simple: Just put one foot in front of the other. Yet every step you take is a precarious act. When you walk, your body’s center of mass is rarely located over one of your feet. Each step forward is really a fall that you must catch by swinging your leg forward so that your foot lands on the floor just in the nick of time. The successful execution and timing of each step is aided by the way that the leg swings like a pendulum, but it still requires precise muscular coordination and excellent balance.
People who suffer a stroke, brain injury, or a partial spinal cord injury lose some of these essential abilities. Damage to the brain itself, or interference in the communication between the brain and the muscles, results in weakness or unwanted muscle activity, which limits the person’s ability to simultaneously propel and support the body during locomotion.
Fortunately, it turns out that the body has some built-in redundancy, at least in some species. Although the brain normally directs bodily motions, animal studies show that complex movements of the limbs are possible even after the spinal cord has been completely cut off from the brain. Such independence was first documented more than a century ago by British neurophysiologist Sir Charles Sherrington.
In the early 1900s Sherrington and others experimented on dogs and cats whose spinal cords had been severed (referred to as “spinal” animals). The studies revealed that the stepping motions of walking can be produced by spinal reflexes alone. The stepping reflexes discovered in those experiments were much more complicated and coordinated than the simple knee jerk that is elicited by tapping a rubber hammer on the tendon below the kneecap. Sherrington observed rhythmic stepping motions in which one limb of the animal was drawn toward the body while the limb on the opposite side of the body was pushed away—all without any master coordination by the brain.
Those findings in spinal cats and dogs suggested that control of locomotion in healthy, intact animals originates at least in part in the spinal cord rather than in the brain. But it took many decades before anyone found practical applications for that insight. In the 1970s researchers showed that if the body of a spinal cat were supported over a treadmill, the cat’s legs would step in time with the moving belt. When the same types of experiments were performed on spinal kittens also given a drug to enhance nerve signal transmission, the animals’ stepping motions would even adapt to changes in the speed of the treadmill.
More amazing, the stepping behavior of spinal cats improves with practice, implying that their spinal cords, not their brains, have done the learning. Daily training on the treadmill causes spinal cats to take longer steps, achieve more normal muscle excitation patterns, and support more of their own weight, although learning appears to be specific to the activity that is practiced: Cats that are trained to stand do poorly on locomotion tests.
The animal results inevitably raised human questions: Do people have the same neural circuitry that permits reflexive locomotion in cats? And if so, is recovery of walking ability possible for patients with injuries to the brain or spinal cord? Anecdotal evidence points to such “central pattern generators” for locomotion in humans. For instance, there have been reports of involuntary stepping motions in patients who suffered incomplete spinal cord injury. Also, newborn babies make alternating stepping motions with their legs, displaying timing patterns that roughly correspond to those seen years later in their mature walking.