Continuum of reflex excitability in hemiplegia: influence of muscle lengthand muscular transformation after heel-cord lengthening and immobilizationon the pathophysiology of spasticity and clonus
Jp. Lin et al., Continuum of reflex excitability in hemiplegia: influence of muscle lengthand muscular transformation after heel-cord lengthening and immobilizationon the pathophysiology of spasticity and clonus, DEVELOP MED, 41(8), 1999, pp. 534-548
The electromyographic (EMG) responses to tendon taps at the ankle and ensui
ng muscular twitch forces and temporal parameters were studied at varying a
ngles across the joint range in 18 children, aged 3 to 14.9 years, with con
genital hemiparetic cerebral palsy and 22 healthy (control) children, aged
3 to 13.6 years. Those subjects with hemiparesis were community ambulators
without assistance. In all subjects, passive muscle stretch caused a waxing
of the reflex EMG and twitch force near neutral (with the sole of the foot
at right angles to the tibia) and a diminution of these with further dorsi
flexion. Twitch times increased with each dorsiflexing increment, being slo
west at maximum dorsiflexion and fastest at the resting plantarflexion angl
e. Heterogeneity of the hemiparetic-limb data is evident when compared with
data of non-paretic and unaffected limbs, with clear differences in the cl
onic (fast twitch) as opposed to non-clonic (slow twitch) muscles. In four
cases with clinical clonus, clonus frequency was reduced by passive dorsifl
exion. Plaster immobilization for 1 month produced clonus which was previou
sly absent in one subject, and caused a fast-twitch phenotype to emerge in
two subjects. Follow-up after heel-cord lengthening in one subject showed t
hat clonus frequency diminished from 9 to 3 Hz with slowing and strengtheni
ng of muscle-twitch phenotype. Short- and long-term peripheral manipulation
s appear to regulate neuromuscular excitability according to whether muscle
s are loaded or unloaded. Although damage to the nervous system provides th
e setting for reflex excitability, the data suggest that the muscle length
(which specifies the joint angle) and the muscle-twitch phenotype of any gi
ven limb for any given case appear to dictate the actual speed and strength
of reflex muscle-twitch and clonus profiles. This study illustrates how pe
ripheral manipulations of muscles and tendons may alter the expression of w
hat have hitherto been considered as exclusively central phenomena.