Rhythmic arm cycling induces short-term plasticity of the soleus H-reflex amplitude
Date
2007-11-30T00:22:28Z
Authors
Javanrohbakhsh, Fatemeh Bahar
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Abstract
Plasticity in spinal networks has been proposed as a means to permit motor skill
learning and recovery after central nervous system disorders. This plasticity is
significantly driven by input from the periphery (Wolpaw & Carp, 2006). For instance,
attenuation of soleus Hoffmann (H) reflex can last beyond the period of different types of
conditioning via putative presynaptic inhibition (Brooke et al., 1997). Interestingly,
rhythmic arm cycling can also attenuate soleus H-reflex via interlimb connections and
presynaptic pathways (Frigon, Collins, & Zehr, 2004). However, it remains to be studied
if this attenuation is maintained beyond the period of arm cycling. In this study, we
hypothesized that excitability of H-reflex pathway would remain suppressed after
cessation of arm cycling. Subjects were seated with their trunk and feet fixed at a neutral
position. Using an arm ergometer, they cycled at 1Hz for 30min. H-reflexes were evoked
via stimulation of the tibial nerve in the popliteal fossa at 5 minute intervals. These
intervals began prior to the cycling and continued during cycling and up to 30 minutes
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after termination of cycling (n=12). Besides soleus muscle, electromyography was
recorded from tibialis anterior, vastus lateralis and biceps femoris. Stimulation was set to
evoke an M-wave which evoked an H-reflex on the ascending limb of the recruitment
curve (size was 75% Hmax) obtained prior to cycling. The M-wave amplitude was
maintained throughout all trials by monitoring and adjusting the level of stimulation
intensity. All H-reflex and M-wave data were normalized to the averaged Mmax to reduce
inter –subject variability. The main result was that the suppression of H-reflex amplitude
persisted beyond the period of arm cycling. H-reflex amplitudes were significantly
(p<0.05) smaller up to 20 min after arm cycling had stopped. This suggests that arm
cycling can induce plastic adaptation in the soleus H-reflex pathway that persists well
beyond the period of conditioning. Also, in an additional experiment (n=8), the prolonged
effect of arm cycling combined with superficial radial (SR) nerve stimulation was
investigated. Interestingly, this cutaneous nerve stimulation cancelled out the prolonged
suppression of H-reflex amplitude induced by arm cycling. Since SR nerve stimulation
facilitates soleus H-reflex via reductions in the level of Ia presynaptic inhibition (Zehr,
Hoogenboom, Frigon, & Collins, 2004), persistence in presynaptic inhibitory pathways is
suggested as an underlying neural mechanism. These results have relevance for
optimizing rehabilitation techniques in the treatment of spasticity which is known to be
related to the H-reflex size (Levin & Hui-Chan, 1993).
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Keywords
H-reflex, rhythmic movements, plasticity, arm cycling