Objective
When
a stroke damages the corticospinal tract (CST), it has been
hypothesized that the motor system switches to using the
corticoreticulospinal tract (CRST) resulting in abnormal arm synergies.
Is use of these tracts mutually exclusive, or can the motor system
spontaneously switch between them depending on the type of movement it
wants to make? If the motor system can share control at will, then
people with a rudimentary ability to make dexterous movements should be
able to perform synergistic arm movements as well.
Methods
We
analyzed clinical assessments of 319 persons’ abilities to perform
“out-of-synergy” and “in-synergy” arm movements after chronic stroke
using the Upper Extremity Fugl-Meyer (UEFM) scale.
Results
We
identified a moderate range of arm impairment (UEFM = ~30-40) where
subjects had a rudimentary ability to make out-of-synergy (~23%-50% on
the out-of-synergy score) and dexterous hand movements (~3-10 blocks on
Box and Blocks Test). Below this range persons could perform in-synergy
but not out-of-synergy or dexterous movements. In the moderate range,
however, scoring better on out-of-synergy movements correlated with
scoring worse on in-synergy movements (P = .001, r ≈ −0.6).
Conclusion
Rudimentary
dexterity corresponded with reduced ability to move the arm in-synergy.
This finding supports the idea that CST and CRST compete and has
implications for rehabilitation therapy.
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