Wednesday, September 12, 2012
Still Cross About the Cross-Crawl: Some Messages from Mother Nature
As I commented some months ago, there are still a number of Internet sites that promote what they call the “cross-crawl”-- treatment that encourages the use of crawling on all fours and other movements that use the two sides of the body reciprocally, and whose proponents claim that such movements recapitulate and improve on early brain development. (If this reminds anyone of the “patterning” method that has twice been rejected in policy statements by the American Academy of Pediatrics, that’s correct, it’s the same basic idea.) The “cross-crawl’ has been presented as a treatment for cerebral palsy, for autism, for dyslexia, and for all kinds of other problems that might be attributable to early brain damage.
There are two problems with the “cross-crawl” idea. One is that nobody has presented any systematic evidence (i.e., not anecdotes) that supports the effectiveness of such a treatment for anything. The second difficulty is that it’s simply implausible that years of brain development could be reversed in this way, whether they began with injury or not. The assumption made by “cross-crawl” advocates is that because certain motor and perceptual changes are associated with events in brain development, one must cause the other-- and that the motor events cause the brain development, not the other way around.
In spite of marvelous modern brain imaging techniques, we can’t observe events in children’s brains as they crawl, or for that matter events in the brains of child or adult patients as they go through “cross-crawl” treatment. Maybe, maybe, important events are happening there, and showing that would go a long way toward making outcome evidence less necessary. We don’t have either kind of evidence, though.
Is there anything about the development and behavior of other mammals that would help us accept or reject the claims put forward by “cross-crawl” proponents? Well, we can look around us and see that many animals do use that reciprocal movement, moving the front left and back right legs together, then the front right and back left legs together. This movement resembles what adult humans do in walking, as we counter-rotate the trunk to balance as we take a step, and make our left hands swing forward as the right legs go forward. Lions do this-- but tigers don’t. Instead of walking at a trot, like lions, tigers pace, with the two left legs working together, then the two right legs. It looks as though normal development and effective functioning can take place whether the right and left sides work in a “crossed” fashion or otherwise.
If you’ve ever gone to a trotting race, you probably know that some horses trot, using the “crossed” pattern. Others pace, with front and back legs working together like a tiger’s. These are not completely natural gaits for either one, and you may see pacers wearing “hopples” that associate front and back legs on each side.
A recent article (www.nytimes.com/2012/09/11/sports/scientists-find-gene-could-explain-horses-ability-to-pace.html) suggests that specific genes, which are active in neurons in the spinal cord, determine what movement patterns are possible and their absence makes training of certain movements difficult or impossible. This is especially true of the trot, in which coordinated diagonal limb movements are essential (and which is comparable to the “cross-crawl”). The pace is not a naturally occurring gait for horses and must be trained; although the trot occurs naturally, horses must be trained to continue to use trot movements at high speeds and not to break into a gallop, which involves a “four-beat” move as each hoof strikes the ground at a separate time. In trotting races, horses are disqualified if they break from the trot to the gallop as they move faster, so their ability to control the movement pattern is important---and under genetic control. A genetic test can predict which horses have the genetic control of motor neurons that will enable the trained horse to trot or pace even at higher speeds. The genes and the nervous system control the movements, rather than the other way around, as “cross-crawl” therapists would claim.
I would be the first to point out that people are not horses, and that what is true of one species may not be generalizable to another. However, in the absence of other kinds of evidence, it does seem that we might pay attention to what Mother Nature has to show us-- and in this case it seems to be that other mammals can naturally either trot or pace, or be trained to do either, rather than having normal development exclusively involving the “crossed” movement pattern. In addition, it seems to be a mistake to think that movement experience necessarily alters the brain. It’s the brain (actually, the spinal cord) that causes the movement and associated experience.
Hmm, maybe teaching people to pace will be the next big thing---