Lack of on-going adaptations in the soleus muscle activity during walking in patients affected by large-fiber neuropathy.

The aim of this study
was to investigate the contribution of feedback from large-diameter
sensory fibers to the adaptation of soleus muscle activity after small
ankle trajectory modifications during human walking. Small-amplitude
and slow-velocity ankle dorsiflexion enhancements and reductions
were applied during the stance phase of the gait cycle to mimic
the normal variability of the ankle trajectory during walking. Patients
with demyelination of large sensory fibers (Charcot-Marie-Tooth type
1A and antibodies to myelin-associated glycoprotein neuropathy) and
age-matched controls participated in this study. The patients had
absent light-touch sense in the toes and feet and absent quadriceps and
Achilles tendon reflexes, indicating functional loss of large sensory
fibers. Moreover, their soleus stretch reflex response consisted of a
single electromyographic (EMG) burst with delayed onset and longer
duration (P 0.01) than the short- and medium-latency reflex
responses observed in healthy subjects. In healthy subjects, the soleus
EMG gradually increased or decreased when the ankle dorsiflexion
was, respectively, enhanced or reduced. In the patients, the soleus
EMG increased during the dorsiflexion enhancements; however, the
velocity sensitivity of this response was decreased compared with the
healthy volunteers. When the dorsiflexion was reduced, the soleus
EMG was unchanged. These results indicate that the enhancement of
the soleus EMG is mainly sensitive to feedback from primary and
secondary muscle spindle afferents and that the reduction may be
mediated by feedback from the group Ib pathways. This study provides
evidence for the role of sensory feedback in the continuous
adaptation of the soleus activity during the stance phase of human
walking.