Effect of paired-pulse stimulus parameters on the two phases of short interval intracortical inhibition in the quadriceps muscle group

Abstract

Background: Short interval intracortical inhibition (SICI) is commonly used to assess inhibition in the motor cortex and is known to be affected by the paired-pulse stimulus parameters (i.e., interstimulus interval [ISI], conditioning stimulus intensity [CSI] and test stimulus intensity [TSI]) used during testing. While the effects of stimulus parameters are well-studied in the upper-extremity, evidence in the lower-extremity is lacking.

Objective: To comprehensively examine the effects of alterations in paired-pulse stimulus parameters on the two phases of SICI in the quadriceps muscle group.

Methods: Seventeen adults (8 males, 9 females) volunteered to participate in this study. SICI was examined over a range of CSIs (70-90% active motor threshold [AMT]), TSIs (100-140% AMT), and ISIs (1.0-3.0 ms) using both EMG and torque responses elicited by transcranial magnetic stimulation (TMS).

Results: The results indicated that SICI at 1.0 ms ISI was best revealed with a CSI of 70% and TSI ≥110% AMT, whereas SICI at 2.5 ms ISI was best revealed with a CSI of 80-90% and a TSI of ≥130% AMT. Unlike upper-extremity muscles, evaluating SICI with a CSI of 70% AMT and an ISI of 1.0 ms produced the greatest inhibition for all TSIs. In general, inhibitory effects were contaminated by facilitatory effects when using a TSI of 100% AMT.

Conclusions: The amount of inhibition was dependent on the stimulation parameters used during testing. A CSI of 70% AMT, ISI of 1.0 ms, and TSI of ≥110% AMT appear to be optimal for measuring SICI in the quadriceps muscle; however, other parameters can be used if careful consideration is given to the described interaction between the parameters.

Keywords: Cortical excitability; bayesian; knee; paired-pulse stimulation; twitch; two phases.

Fig. 1:

A schematic of the (A) experimental set-up and (B) protocol.

Fig. 2:

3D surface plots showing the relationship between interstimulus interval (ISI), test stimulus intensity (TSI), and short interval intracortical inhibition (SICI) for the motor evoked quadriceps torque responses at various conditioning stimulus intensities (CSIs). Note the colors are only for illustration purposes.

Fig. 3:

3D surface plots showing the relationship between interstimulus interval (ISI), test stimulus intensity (TSI), and short interval intracortical inhibition (SICI) for the EMG-based motor evoked potentials at various conditioning stimulus intensities (CSIs). Note the colors are only for illustration purposes.

Fig. 4:

Scatterplots demonstrating the relationship between short interval intracortical inhibition (SICI) ratios obtained from motor evoked torque responses (MEP torque) and motor evoked potentials (MEP EMG) of vastus medialis (VM), rectus femoris (RF), vastus lateralis (VL), and quadriceps muscles (average of VM, RF, and VL).