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Purpose: Impaired reflex regulation is assumed to contribute to upper-extremity motor impairment poststroke; however, the relationship between reflex inhibition and motor function remains unclear. To address this question, it is first necessary to determine the reproducibility of reflex responses. The objective of this study was to establish the test-retest reliability of flexor carpi radialis H-reflex inhibition in healthy control and stroke participants and investigate the correlation between H-reflex inhibition and grip strength., Materials and Methods: Eighteen persons poststroke (mean ± SD: age 63 ± 13 years; 6 ± 5 years poststroke; 13 males) and 16 healthy controls (age: 62 ± 12 years) participated. Reflex inhibition was tested on 2 separate days by conditioning the H-reflex with radial nerve stimulation at two different interstimulus intervals: 13 ms (presynaptic Ia inhibition-PSI) and 0 ms (disynaptic inhibition). Pearson's and intraclass correlation coefficients [two-way mixed model-ICC (1, 2)], and standard error of measurement (SEM) were calculated., Results: Relative reliability (ICCs) ranged from good to excellent (0.61-0.78). SEM was low (range 10-19%, stroke; 15-20%, healthy controls). Paretic grip strength and paretic limb PSI revealed a positive correlation (r = 0.70; p < 0.0125). Disynaptic inhibition and paretic grip strength were not correlated., Conclusions: To our knowledge, this is the first study to demonstrate reproducibility of reflex inhibition in individuals poststroke. Furthermore, we quantify smallest real differences, which provide an estimate of the magnitude of effect required to determine a meaningful change, exceeding measurement error. The correlation between PSI and grip strength suggests the potential contribution of PSI to grip force production and upper-extremity motor function.

Synaptic efficacy associated with muscle spindle feedback is partly regulated via depression at the Ia-motorneuron synapse through paired reflex depression (PRD) and presynaptic inhibition (PI). The purpose of this study was to examine PRD and PI of the soleus H-reflex at rest and with a background voluntary muscle contraction. The experiment was conducted on 10 healthy males with no history of neurological deficits. Soleus H-reflex and M-wave curves were elicited in three conditions: unconditioned, PRD (two consecutive H-reflexes with 100 ms interval), and PI (1.2 × MT to tibialis anterior 100 ms prior to soleus H-reflex). Each condition was tested at rest and with a 10% soleus contraction. PRD and PI both produced a pronounced inhibition to the soleus motor pool at rest, with a significant difference observed between threshold values (78.9, 89.3, and 90.4% for unconditioned, PRD, and PI reflexes, respectively). During the voluntary contraction the threshold for both inhibitory mechanisms was significantly reduced, and were not different from the unconditioned H-reflex (74.5, 78.9, and 77.0% for unconditioned, PRD, and PI reflexes, respectively). The slope of PI and the PI Hmax/Mmax ratio were significantly altered during contraction whereas no differences were observed for PRD. The results suggest these inhibitory mechanisms depend on the interaction between background voluntary activation and stimulus intensity. This behavior of these inhibitory mechanisms underscores the specificity of spinal circuitry in the control of motor behaviors.