Limb segment load inhibits post activation depression of soleus H-reflex in humans

Abstract

Objective We investigated the effect of various doses of limb segment load on soleus H-reflex amplitude and post activation depression in healthy individuals. We also explored the influence of limb segment load on spinal circuitry in one individual with chronic SCI. Methods 28 healthy adults and one SCI subject received compressive loads applied to the top of their knee at varied doses of load (10%, 25%, and 50% of the body weight). Soleus H-reflexes were measured before (baseline) and during the loading phase. Results There were no significant differences in H-reflex amplitudes during the 50 % BW load-on phase as compared to either baseline session or the load-off phase. However, the post activation depression was decreased over 9% (p < 0.05) during the load-on phase compared to the load-off phase and scaled according to load (50%>25%>10%). The post activation depression ratio also appears less responsive to varying loads after chronic SCI. Conclusions Limb segment load decreases post-activation depression in humans. These findings suggest that the mechanism associated with post activation depression is modulated by limb segment load, and may be influenced by spinal reorganization after SCI. Significance Future studies will determine if various levels of spasticity modulate the response of limb segment load on post activation depression in those with acute and chronic SCI.

Limb compressive load does not inhibit post activation depression of soleus H-reflex in indiviudals with chronic spinal cord injury

“…The findings from healthy adults in the current study are consistent with our previous work, showing that segment compressive load significantly decreases post activation depression of the soleus H-reflex (Tseng and Shields, 2012). Specifically, an increase in the conditioned H2 amplitude was observed during loading while the tested H1 amplitude remained constant, resulting in a reduction of post activation depression ratio.…”

“…Furthermore, the magnitude of the loss of post activation depression is scaled according to level of load, with the greatest reduction of post activation depression during the 50% of BW compressive load and the least reduction during the 10% of BW compressive load. Importantly, findings from our studies in healthy adults suggest that load diminished the capacity to gate afferent input via pre-synaptic inhibitory pathways (Tseng and Shields, 2012). Load-induced H-reflex modulation is likely attributed to sensory afferents at multiple sites.…”

“…Each session was separated by 1 min. This paradigm was previously assessed for the reproducibility of soleus H-reflexes in healthy adults and was used to establish a dose (load)-response (H-reflex) relationship (Tseng and Shields, 2012). Briefly, the protocol always tested the soleus H-reflexes in a 1.25-min baseline session followed by three 2.5-min loading sessions in a random order.…”

“…In the absence of muscle contractions, segment compressive load significantly decreased post activation depression of the soleus H-reflex in healthy adults (Tseng and Shields, 2012). Importantly, the magnitude of the loss of post activation depression was scaled according to the dose of load delivered to the limb segment (50% body weight (BW) > 10% BW).…”

“…In preparation for this study, we developed a method to deliver compressive loads to a single limb segment to minimize supra spinal drive for healthy subjects (Tseng and Shields, 2012). Accordingly, the purpose of this study is to test if segment compressive load increases the loss of post activation potentiation in people with chronic SCI as compared to a group without SCI.…”