The Effect of Transcranial Magnetic Stimulation Inter-Pulse Interval on Corticospinal Excitability of the Biceps Brachii

Research Background

Transcranial Magnetic Stimulation (TMS) is a non-invasive neurophysiological research technique widely used to assess corticospinal excitability in both healthy individuals and clinical patients. By applying electromagnetic pulses over the primary motor cortex, TMS indirectly activates the descending corticospinal pathway, thereby generating Motor Evoked Potentials (MEPs) in target muscles. The amplitude of MEPs is generally interpreted as reflecting the state of corticospinal excitability, with larger amplitudes indicating higher excitability.

However, a significant drawback of TMS is the variability of MEPs. Even under strictly controlled conditions, MEP amplitudes elicited within seconds of each other can differ. To mitigate the impact of this variability on research outcomes, TMS studies typically elicit multiple MEPs (often 8-10 or more) under a single experimental condition and then calculate the average of these measurements as a more reliable indicator of corticospinal excitability. Nevertheless, when selecting the time interval between adjacent stimulations (i.e., Inter-Pulse Interval, IPI), TMS researchers often make arbitrary choices, sometimes even altering the IPI within a single experiment. Previous studies have shown that IPI may influence MEP amplitudes, particularly in resting muscles. However, the effect of IPI on MEP amplitudes in actively contracting muscles remains less understood and inconclusive.

Therefore, this study aimed to investigate the influence of IPI on MEP amplitudes during submaximal isometric elbow flexion of the biceps and triceps brachii. The hypothesis was that IPI would not affect MEP amplitudes in either the biceps or triceps brachii.

Research Source

This study was conducted by David H. Imeson, Lea Gerditschke, Liana E. Brown, and Davis A. Forman, affiliated with the Department of Kinesiology and Psychology at Trent University, Canada. The research was published in 2025 in the European Journal of Neuroscience, under the title “Transcranial Magnetic Stimulation Inter-Pulse Interval Does Not Influence Corticospinal Excitability to the Biceps Brachii During Submaximal Isometric Elbow Flexion.”

Research Process

1. Participant Recruitment and Experimental Setup

The study recruited 12 right-handed participants (mean age: 22.1 years), all of whom passed TMS safety checks and physical activity suitability assessments. During the experiment, participants were seated upright with their right forearm supported on a foam pad, and their wrists were secured with a Velcro strap to perform isometric elbow flexion. Real-time feedback on biceps brachii muscle activity was provided via a computer monitor, and participants were required to maintain muscle activity within the target range (10% of maximum muscle activity).

2. Electromyography (EMG) Recording

Bipolar surface electrodes were used to record muscle activity in the biceps and triceps brachii. Electrodes were placed along the direction of muscle fibers, and signals were recorded at a sampling rate of 5 kHz using a CED Micro 1401-4 device, with analysis performed using Signal 8 software. Signals were amplified and filtered to ensure data quality.

3. Transcranial Magnetic Stimulation (TMS) Setup

TMS was delivered using a circular coil positioned over the vertex, with stimulation intensity set to 120% of the biceps brachii’s active motor threshold. Six different IPI conditions (4, 6, 8, 10, 12, and 14 seconds) were designed, with five stimulations delivered per condition, totaling five trains of stimulations (25 stimulations per condition). Participants were instructed to initiate muscle contraction 1 second before each stimulation and maintain it for 0.5 seconds after stimulation.

4. Data Analysis

Peak-to-peak MEP amplitudes were measured from the initial deflection of the background EMG signal, and all MEP amplitudes were normalized to baseline values. Additionally, pre-stimulus muscle activity was measured and normalized to the muscle activity during maximal voluntary contraction (MVC). Statistical analysis was performed using SPSS software, with two-way repeated measures ANOVA used to assess the effects of IPI and MEP number on MEP amplitudes and pre-stimulus muscle activity.

Research Results

1. Pre-Stimulus Muscle Activity

Pre-stimulus muscle activity in the biceps and triceps brachii showed no significant differences across IPI conditions or MEP numbers. This indicates that muscle activity remained consistent throughout the experiment and did not influence corticospinal excitability.

2. Corticospinal Excitability

MEP amplitudes in the biceps and triceps brachii showed no significant differences across IPI conditions. However, when MEP amplitudes were expressed as a ratio to pre-stimulus muscle activity, the biceps brachii exhibited a significant decrease in MEP amplitude after the first stimulation (MEP 1: 32.8 ± 5.9; MEP 5: 27.7 ± 4.3, p < 0.05). This result suggests that while IPI does not affect MEP amplitudes, continuous stimulation may lead to a gradual decline in MEP amplitudes.

Conclusions and Significance

The findings of this study indicate that during submaximal isometric elbow flexion, IPI (4-14 seconds) does not significantly influence MEP amplitudes in the biceps and triceps brachii. This contrasts with previous findings in resting muscles, suggesting that the effect of IPI on MEP amplitudes may depend on the muscle’s contractile state. Additionally, the study found that MEP amplitudes decreased significantly after the first stimulation, indicating that continuous stimulation may lead to a gradual reduction in MEP amplitudes.

Research Highlights

1. First Study on IPI Effects in Actively Contracting Muscles: This study is the first to investigate the influence of IPI on MEP amplitudes in the biceps and triceps brachii, addressing a gap in the field.
2. Multiple IPI Conditions: The study designed six different IPI conditions, providing a comprehensive evaluation of IPI’s impact on MEP amplitudes.
3. Effect of Continuous Stimulation on MEP Amplitudes: The finding that continuous stimulation may lead to a gradual decline in MEP amplitudes has important implications for the design of TMS studies.

Future Research Directions

Future research could explore the effects of longer IPIs (e.g., 20 seconds or more) on MEP amplitudes, as well as the role of IPI under different neuromuscular states (e.g., fatigue, injury, or higher contraction intensities). Additionally, studies could investigate the influence of higher stimulation intensities on MEP amplitudes.