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* To whom correspondence should be addressed. E-mail: bcclar01{at}syr.edu.
The loss of strength and function following prolonged unweighting may result from alterations in skeletal muscle and/or neurological properties. In this paper (Part II) we report our findings on plantarflexor neurological properties following 4-weeks of unilateral lower limb suspension (ULLS), along with the efficacy of motor imagery (MI) training on these properties. In the companion paper (Part I) we report our findings on the changes in skeletal muscle properties and the effectiveness of a novel intervention (ischemia). Additionally, in the present paper we collectively analyze our findings from both papers to determine the relative contribution of neural and muscular factors in strength loss. Measurements of central activation (CA), the H-reflex, and nerve conduction velocity were made before and after 4-weeks of ULLS in healthy adults (n=18; 19-28 years). A subset of the subjects (n=6) performed plantarflexor (PF) MI training 4 days/week. Following ULLS, we observed a significant increase in the soleus H-reflex (45.4% + 4.0 to 51.9% + 3.7 expressed relative to the maximal muscle action potential). Additionally, there was a longer interval between the delivery of an electrical stimulus to the tibial nerve and the corresponding muscle action potential (M-wave latency; 6.18 + 0.62 to 6.67 + 0.36 msec) and H-reflex wave (H-wave latency; 29.20 + 1.43 to 29.66 + 1.41 msec). The efficacy of MI was ambiguous, with no significant effect detected (although a modest effect size was observed [eta-squared=0.18]). These findings suggest that unweighting induces plastic changes in neural function that appear to be spatially distributed throughout the nervous system. In terms of the relative contribution of neural and muscular factors regulating strength loss, we observed that neural factors (primarily deficits in CA) explained 48% of the variability in strength loss, while muscular factors (primarily sarcolemma function) explained 39% of the variability.
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