Hello.
In a nutshell, it has to do with the way our brains are wired to our
limbs. Researchers have found that there is "parallel activation"
between ipsilateral limbs (i.e., the arm and leg on the same side of
your body) that causes the brain to have a much easier time moving
those limbs in the same direction (called "isodirectional" motion)
than in opposite directions ("non-isodirectional").
Brain scans done by researchers have confirmed a certain region of the
brain called the "supplementary motor area" (SMA) shows increased
activity when a person attempts to move ipsilateral limbs (e.g., right
hand and right foot) in opposite ("non-isodirectional") ways.
I've located two scientific journal articles that discuss this
phenomenon.
"Brain Areas Involved in Interlimb Coordination" (2000) hosted by
Johns Hopkins University:
http://www.psy.jhu.edu/~courtney/200.370/0892a.pdf
[This document is in PDF format, so the Adobe Acrobat
Reader is required. If you don't have that, please visit Adobe's web
site:
http://www.adobe.com/products/acrobat/readstep2.html ]
Key points from the article:
"Comparisons were made between activations during the isolated
movements of wrist and foot segments (WRIST or FOOT) and their
coordination whereby the limbs moved in either the same
(isodirectional, ISODIR) or in opposite directions (nonisodirectional,
NON-ISODIR). Previous behavioral work has revealed that these two
coordination modes are not equally difficult. Whereas movements of
the limb segments in the same direction in extrinsic space can be
accomplished easily in normal and pathological groups without much
effort, the synchronization of the limb motions in opposite directions
is more difficult and is characterized by a lower degree of accuracy
and stability."
Also:
"The comparison of both modes revealed that the ipsilateral SMA (and
cingulate motor cortex) showed an increased level of activation during
the more difficult non-isodirectional mode as compared to the
isodirectional coordination mode.
This suggests that the SMA and cingulate are becoming increasingly
important when more difficult spatial relations between simultaneous
limb movements have to be monitored, particularly when the
coordination pattern is inherently unstable."
--------
The other article is:
"Excitability changes in human corticospinal projections to forearm
muscles during voluntary movement of ipsilateral foot" (2002) Journal
of Physiology
http://www.jphysiol.org/cgi/content/full/539/3/903
The article confirms the same relationship mentioned above concerning
ipsilateral limbs, but also offers some intriguing theories about how
this mechanism evolved. The authors point out there's no really
obvious behavior that requires the hand and foot to move in the same
direction. In other words, under ordinary circumstances, you don't to
go around making the same circular motions with your hand and foot, so
why does the brain insist on link the two? The authors suggest that
the mechanism may be related to posture and balance (i.e., by
discouraging you to move your limbs in contradictory directions
perhaps an unbalanced state is avoided).
From the article:
"It might finally be argued that preferential coupling of
isodirectional movements of the hand and foot has hardly any obvious
purpose in ordinary behaviour. This is probably the reason why such a
coupling is usually described as a constraint, rather than as an
organisation pattern, as happens, for example, for the various types
of limb coupling related to locomotion (cf. Orlovsky et al. 1999). An
economy principle would predict that any motor organisation should not
have developed without some functional pressure. Thus it seems
interesting to speculate how to categorise hand-foot coupling within
the context of motor control. In this regard, the 'anticipatory
postural activities'... For instance, they are characterised by the
parallel activation of muscles in different body segments, they are
scaled with the intensity of the prime movement (Aruin & Latash, 1996)
and can be reduced or abolished when the biomechanical context is
modified (Aruin et al. 1998). Their scope is either to prepare a
fixation chain connecting the moving segment to a firm support, or to
produce a motor action that contrasts the postural unbalance produced
by the main body action. It is of interest that when performing wrist
flexion-extension while trying to maintain a constant limb posture,
APAs develop in the upper limb which are characterised by directional
postural synergies (Chabran et al. 1999)... This allows us to
postulate that even when a manifest intervention of the APA is not
required, as in our experimental condition (a sitting subject with the
foot supported by the oscillating platform), subthreshold effects may
nevertheless take place. According to this view, the positive and
negative constraints characterising ipsilateral limb coupling might
indeed be an expression of some underlying postural mechanism."
http://www.jphysiol.org/cgi/content/full/539/3/903
search terms:
"hand foot coordination"
isodirectional coordination
"ipsilateral limbs" coordination
I hope this helps.
And thank you for the interesting question! That email has been
circulating over the past few months and a lot of folks have been
talking about it. I encountered it myself a few weeks ago and was
intrigued by it, so thanks for giving me the opportunity to research
this! |
