Operant conditioning of the soleus H-reflex does not induce long-term changes in the gastrocnemius H-reflexes and does not disturb normal locomotion in humans

Abstract

In normal animals, operant conditioning of the spinal stretch reflex or the H-reflex has lesser effects on synergist muscle reflexes. In rats and people with incomplete spinal cord injury (SCI), soleus H-reflex operant conditioning can improve locomotion. We studied in normal humans the impact of soleus H-reflex down-conditioning on medial (MG) and lateral gastrocnemius (LG) H-reflexes and on locomotion. Subjects completed 6 baseline and 30 conditioning sessions. During conditioning trials, the subject was encouraged to decrease soleus H-reflex size with the aid of visual feedback. Every sixth session, MG and LG H-reflexes were measured. Locomotion was assessed before and after conditioning. In successfully conditioned subjects, the soleus H-reflex decreased 27.2%. This was the sum of within-session (task dependent) adaptation (13.2%) and across-session (long term) change (14%). The MG H-reflex decreased 14.5%, due mainly to task-dependent adaptation (13.4%). The LG H-reflex showed no task-dependent adaptation or long-term change. No consistent changes were detected across subjects in locomotor H-reflexes, EMG activity, joint angles, or step symmetry. Thus, in normal humans, soleus H-reflex down-conditioning does not induce long-term changes in MG/LG H-reflexes and does not change locomotion. In these subjects, task-dependent adaptation of the soleus H-reflex is greater than it is in people with SCI, whereas long-term change is less. This difference from results in people with SCI is consistent with the fact that long-term change is beneficial in people with SCI, since it improves locomotion. In contrast, in normal subjects, long-term change is not beneficial and may necessitate compensatory plasticity to preserve satisfactory locomotion.

Keywords: learning; plasticity; rehabilitation; spinal cord; synergists.

Copyright © 2014 the American Physiological Society.

Figures

Fig. 1.

Average conditioned H-reflexes (left; 225 trials) and control H-reflexes (right; 20 trials) in soleus, medial gastrocnemius (MG), and lateral gastrocnemius (LG) in the sixth baseline session (solid line) and the last conditioning session (dotted line) from a subject whose H-reflex decreased significantly. The conditioned H-reflex size decreased after soleus H-reflex down-conditioning in soleus and MG, but not in LG. The control H-reflex decreased after conditioning only in soleus. Background electromyography (EMG) and M-wave size did not change between the baseline and last conditioning sessions. Sol, soleus.

Fig. 2.

Average locomotor EMG activity for all 7 successfully conditioned subjects from soleus, tibialis anterior (TA), MG, LG, vastus lateralis (VL), and biceps femoris (BF) of both legs before (solid line) and after (dotted line) the 30 conditioning sessions. For each muscle, EMG amplitude in each of the 12 equal bins was normalized by the amplitude in the bin with the highest amplitude. There were no significant differences between pre- and postconditioning in any of the 12 bins in any of the 12 muscles.

Fig. 3.

Average locomotor H-reflexes for all 7 successfully conditioned subjects from soleus, MG, and LG in the conditioned leg before (solid line) and after (dotted line) the 30 conditioning sessions. H-reflex size is expressed as a percentage of the maximum M-wave (%Mmax). As for locomotor EMG activity in Fig. 2, the step cycle was divided into 12 equal bins and an average reflex size for each bin was calculated. There were no significant differences between pre- and postconditioning in any of the 12 bins in any of the 3 muscles.

Fig. 4.

Average ankle, knee, and hip joint angles for all 7 successfully conditioned subjects in the conditioned leg over the 12 bins of the step cycle before (solid line) and after (dotted line) the 30 conditioning sessions. There were no significant differences between pre- and postconditioning in any of the 12 bins in any of the three joints. Ankle: +, dorsiflexion; −, plantarflexion; knee: +, flexion; −, extension; Hip: +, flexion; −, extension.