Changes in Nerve Excitability Caused by Nerve Stretch and Nerve Glide

Abstract

Purpose : When nerve damage occurs, it can lead to functional impairment and pain, often accompanied by changes in the surrounding tissues. Among the various treatment methods for neurogenic disorders, nerve mobilization has been widely used as a non-invasive physical approach. The purpose of this study is to compare and analyze changes in nerve excitability induced by proximal glide, distal glide, and stretch through nerve conduction velocity testing, in order to provide reference data for the application of nerve mobilization techniques. Methods : The sensory nerve conduction velocity of the median nerve was evaluated in 30 healthy adults in their 20s and 30s. Based on the median sensory nerve conduction velocity measured at rest, measurements were taken to assess changes in nerve excitability under the following conditions: supramaximal stimulus (baseline 1), resting phase (1/3 of the supramaximal stimulus, baseline 2), proximal glide (1/3 of the supramaximal stimulus), distal glide (1/3 of the supramaximal stimulus), and stretch (1/3 of the supramaximal stimulus). The measurements were performed in a random order using an electromyography (EMG) device (Keypoint, Danteck, Denmark). Results : There was a statistically significant difference in the proximal glide during the terminal latency than during the resting period; however, no clinically significant difference was observed. The results of the nerve conduction velocity test showed that the amplitudes were significantly increased in the distal glide and stretch of the nerve. Conclusion : Statistically significant differences in amplitude were observed during distal glide and stretch, suggesting a potential influence on nerve excitability. Furthermore, as these techniques may induce excessive nerve excitability, it is considered that careful attention should be paid in clinical settings when applying distal glide or nerve stretching to patients, in order to avoid overstimulation of the nerve.

Ⅰ. Introduction

The nervous system transmits and analyzes the information received from inside and outside the body to trigger appropriate responses. Therefore, the maintenance of appropriate nerve length that is able to adapt well to the various body movements performed in everyday life is crucial (Santana et al., 2015). However, neurologic dysfunction caused by nerve damage can lead to neurological deterioration and pain, accompanied by changes in the relevant tissues (Doneddu et al., 2023). Therefore, for a long time, the nervous system has been examined using sensory tests and parameters such as reflex and muscle strength. However, more recently, neurodynamic tests such as the stimulation of the peripheral nerves have been performed (Ciuffreda et al., 2024). Neurodynamicis the integration of the mechanics and physiology of the nervous system according to the neural mobility concept that states that the nervous system should be appropriately stretched and contracted to ensure the appropriate mobility range of the nerves (Ellis et al., 2022). Many non-invasive physical methods have been used to treat neurogenic disorders. Nerve mobilization is one of these methods used for pain relief by reducing the scar tissues within the nerve tissues (Turl & George, 1998). In addition, given that the peripheral nervous system must be stretched properly to maintain normal muscle tension and mobility range (Martínez-Jiménez et al., 2024), nerve mobilization can increase the compliance of the peripheral nerves, reducing nerve compression and excessive friction (Salniccia et al., 2024).

The concepts of nerve stretch, tension, and gliding in nerve mobilization play a major role in treatment planning. Nerve stretch can cause the nerves to become sensitive and cause pain. It may also cause problems in nerve conduction, leading to sensory or motor disorders (Heimburg, 2022). Sonawane et al. (2023) stated that from a neurological viewpoint, muscle weakness and neurological symptoms result from neuronal ischemia or hypertension caused by mechanical stress. Liu et al. (2018) also reported that blood-nerve barrier dysfunctions are caused by repetitive pulling of the nerve. In contrast, glide, referring to a relative movement of the nerve tissues against adjacent tissues involving tension on one side of the nerve and relaxation on the other side, plays a role in evenly distributing the tension of the nerve system to prevent hypertension in certain area (Baptista et al., 2024). The resulting increase in the blood circulation and axonal trait migration between the inside and outside of the nervous system is crucial for the functional or structural aspects of the neurons (Korr, 1978). These processes facilitate axonal trait migration, increasing the nerve conduction velocity, and the decreased nerve compression facilitates an increase in the blood flow to the nerve; this is closely associated with the recovery of soft tissues, including neurons and muscles (Menorca et al., 2013). Several previous studies have demonstrated the efficacy of nerve mobilization on peripheral nerve lesions associated with carpal tunnel syndrome (Ijaz et al., 2022; Jiménez-del-Barrio et al., 2022).

Elvey (1986) reported that by adding the nerve gliding therapy to the conservative treatment of carpal tunnel syndrome, the need for surgery was reduced by up to 29.8 %. To summarize, previous studies have shown that performance of nerve mobilization for patients with neurological diseases helps in achieving normal movements through the interactions of each element involved in the mechanical movements of the nerves such as stretch, glide, and pressure. However, in certain instances, a particular element of the nervous system has a dominant influence on the clinical problem. Nevertheless, to our knowledge, no data pertaining to a comparative analysis of the effects of these elements are currently available.

The purpose of this study was to conduct a comparative analysis of the changes in nerve excitability following nerve stretch and glide using a median nerve conduction velocity test, and thereby provide data that can be used for performing nerve mobilization effectively.

Ⅱ. Methods

1. Subjects

This study was conducted on 30 healthy male and female adults aged 20~30 years who were living a normal daily life. The subjects were informed about the details of the experimental procedures, following which they agreed to participate in the study. This study was approved by the Bioethics Committee of the Catholic University of Pusan (CUPIRB-2015-051).

2. Intervention program

To compare the nerve conduction velocity of the median nerve by nerve stretch and nerve glide, all study subjects were made to perform stretching (shoulder depression, abduction 90 °~110 °, lateral rotation 90 °, elbow extension, forearm supination, wrist and thumb extension, contralateral cervical lateral flexion) (Fig 1), proximal gliding (shoulder depression, abduction 90 °~110 °, lateral rotation 90 °, elbow extension, forearm supination, wrist and thumb flexion, contralateral cervical lateral flexion) (Fig 2), and distal gliding (shoulder depression, abduction 90 °~110 °, lateral rotation 90 °, elbow extension, forearm supination, wrist and thumb extension, ipsilateral lateral flexion) (Fig 3) of the median nerve during the experimental period. First, in order to measure the intensity of stimulation that induces the maximum excitability of the median sensory nerve, latency and amplitude were measured by supramaximal stimulation (baseline 1) in a resting position without stretching the median sensory nerve. In the same posture, latency and amplitude of the resting phase (baseline 2) were measured with 1/3 of supramaximal stimulation. After applying proximal gliding, distal gliding, and stretching for 1 minute each in the same posture, latency and amplitude were measured at 1/3 stimulation intensity of supramaximal stimulation. Resting phase (baseline 2), proximal gliding, distal gliding, and stretching were measured. The sequence was randomized, and measurements were taken with a break of at least 1 minute after intervention and measurement.

Fig 1. Median nerve stretch

Fig 2. Median nerve proximal glide

Fig 3. Median nerve distal glide

3. Measurement method

An EMG instrument (Keypoint, Danteck, Denmark) was used to comparatively monitor the degree of nerve excitability changes induced by the stretching, proximal gliding, and distal gliding of the median nerve. The results of the sensory nerve conduction velocity test were analyzed using a retrograde measurement method. To perform the test of median sensory nerve conduction velocity, the recording electrodes and associated electrodes were attached to the metacarpophalangeal joint and the proximal interphalangeal joint of the second finger using ring electrodes, and the grounding electrodes were attached to the lower arm. Electrical stimulation was applied to the median nerve beneath the wrist crease using a bipolar electrode (Preston & Shapiro, 2024). The stimulus with a stimulation frequency of 1 ㎐ was applied in the forward direction; the low-pass filtering, high-pass filtering, sensitivity, and sweep speed were set at 1 ㎑, 50 ㎐, 20 ㎶, and 1 ㎳, respectively. The terminal latency of the median sensory nerve conduction velocity test was measured at the first time point when the compound nerve action potential began to change to a negative phase, and the amplitude from the positive peak to the negative peak was measured (Fig 4).

Fig 4. Example of latency and amplitude onecach conditions, BL 1; baseline 1 test (supra maximal stimulus), BL 2; baseline 2 test (⅓supra maximal stimulus), ST; stretching (⅓supra maximal stimulus), DG; distal gliding (⅓supra maximal stimulus), PG; proximal gliding (⅓supra maximal stimulus), T/L; terminal latency, Amp; amplitude

4. Analytical methods

The results of this study were analyzed using the SPSS program for windows, version 20.0. The general characteristics of the subjects were analyzed using descriptive statistics, and the results of the measurement of the sensory nerve conduction velocity were comparatively analyzed using one-way ANOVA. Tamhane’s test was performed as a post-hoc test. The significance level (α) was set at .05.

Ⅲ. Results

1. General characteristics of the subjects

This study included 30 healthy adults (12 men and 18 women) who agreed to participate in the study after being explained the purpose and design of the study. The sample size was calculated using G*Power with an effect size of 0.25, a power of 0.80, and a significance level of .05. Their mean age was 26.73±4.57 years, mean height was 172.90±8.45 ㎝, mean body weight was 68.93±12.78 ㎏, and mean body mass index (BMI) was 22.91±3.02 ㎏/㎡ (Table 1).

Table 1. General characteristics of subjects (n= 30)

BMI; body mass index

2. Comparison of the terminal latency and amplitude by proximal gliding, distal gliding, and stretching

There was no significant difference among the values of terminal latency at baseline 2 (B2L), terminal latency during proximal gliding (PGL), terminal latency during distal gliding (DGL), and terminal latency during stretching (STL) (p>.05). However, the amplitude at baseline 2 (B2A), amplitude during proximal gliding (PGA), amplitude during distal gliding (DGA), and amplitude during stretching (STA) were analyzed. The results showed that the amplitude at each condition was significantly different (p<.05). The terminal latency and amplitude values for the different intervention conditions are presented Table 2, Fig 5.

Table 2. Mean of terminal latency and amplitude under each conditions (n= 30)

B2; baseline 2, PG; proximal gliding, DG; distal gliding, ST; stretching, values within a column with different superscript letters are significantly each other at p<.05

Fig 5. Comparison of the amplitude between the B2A, PGA, DGA and STA on each condition

Ⅳ. Discussion

Nerve mobilization can be considered as an important treatment method for neuromuscular diseases in clinical practice and in recent years, nerve mobilization has been widely used in clinical practice to improve the functions of the peripheral nerves in patients with nerve injury. From among all the peripheral nerve disorders, median nerve compression can cause carpal tunnel syndrome (Joshi et al., 2022), the most common single neuropathy commonly observed in clinical practice, and may compromise the ability to perform activities of daily living because of damage-related sensory and motor disturbances (Davies, 1994). Therefore, the mobilization of the median nerve is commonly performed in clinical practice. According to a previous study by Boudier-Revéret et al. (2017), when used for patients with carpal tunnel syndrome, nerve mobilization alleviated swelling, one of the causes of carpal tunnel syndrome, and improved the nerve stretch ability and circulation, leading to a significant internal pressure-lowering effect in the carpal tunnel. Furthermore, Jiménez-del-Barrio et al. (2022) reported that continuous mobilization of the median nerve in patients with carpal tunnel had a pressure-lowering effect in the carpal tunnel region, leading to an improvement in nerve conduction velocity, pain intensity.

This study aimed to measure the nerve excitability by evaluating the nerve conduction velocity based on the results of nerve mobilization, which has been reported in previous studies, and to comparatively analyze the data to improve treatment efficacy. In the nerve conduction velocity test, the terminal latency, defined as the time required for the fastest nerve fiber to conduct the signal from the stimulation point to the recording point (Kiene & Hiett, 2015). Between the resting period and each intervention was not significantly different during distal gliding and stretching (p>.05), while it was significantly different during proximal gliding (p<.05). In previous research, Preston and Shapiro (2020) reported that the normal terminal latency values of the median sensory nerve conduction velocity were ≤3.5 ㎳. Singh et al. (2017) also reported that the mean terminal latency of the median sensory nerve was 2.80±0.56 ㎳ in males and 2.40±0.33 ㎳ in females. Therefore, 2.17±0.39 ㎳, the measured value at latency of proximal gliding in this study, is within the standard deviation, so it is considered not to be a clinically significant difference.

The amplitude represents the sum of the nerve fibers activated by nerve stimulation (Barkhaus et al., 2024), and the results of this study showed that there was no significant difference in the amplitude during proximal gliding of the nerve (p>.05). However, the amplitude was significantly different for distal gliding and stretching of the nerve (p<.05). This result suggests that distal gliding or stretching of the nerve has a greater impact on nerve excitability than proximal gliding of the nerve, which can be explained in relation to the nerve tension point. The tension point is an area in the nervous system with less movement than the adjacent tissues surrounding the nerves or the area with minimal movement, characterized by the adaptation of the nervous system to movement. During the movement of the body, the movement of the nerves does not always occur in one direction; it is directed by the stimulus around the tension point. When the elbow joint is extended, the median nerve is pulled toward the body if the wrist and finger joints are bent in a contralateral lateral bending position of the neck. Conversely, the medial nerve is pulled away from the body if the wrist and the finger joints are extended in an ipsilateral lateral bending position of the neck. In addition, when the elbow joint is extended, the median nerve is pulled toward the body and away from the body at the same time if the wrist and the finger joints are extended in a contralateral lateral bending position of the neck, leading to increased tension during proximal and distal gliding. Choi et al. (1989) reported that the general amplitude obtained from the measurement of the sensory nerve conduction velocity was 28.50±17.60 ㎶. However, in our study, the amplitudes obtained from the measurements of the sensory nerve conduction velocity during distal gliding and stretching were 37.35±23.10 ㎶ and 40.27±19.11 ㎶, respectively. Therefore, our study indicated that distal gliding and stretching of the nerve significantly increased the amplitude in the sensory nerve conduction velocity test. These results are consistent with the result of Ha (2013) that is nerve conduction velocity was significantly changed only in the distal segment when self-median nerve mobilization was performed. This result can be attributed to the increased neural activation by mechanical receptors that respond sensitively to the pressure changes in the distal segmental joint pockets of the human body (Brody & Hall, 2018). In addition, the results showed that the amplitudes during distal gliding and stretching increased by 65.19 % and 70.11 %, respectively, compared to the amplitude during the resting period. And amplitude during the supramaximal stimulation was 55.10±27.11 ㎶, and the amplitude value during nerve stretching was close to that during supramaximal stimulation, suggesting that care should be taken to avoid overactivation of the nerve during stretching.

The limitations of this study include the fact that healthy adults aged 20~30 years were selected as subjects and that the nerve excitability changes were examined only in a single nerve. Moreover, although the nerve mobilization should ideally be performed for 15 min (Ferragut-Garcías et al., 2017), in this study, the sensory nerve conduction velocity was measured after a 1 min period. Therefore, further studies on nerve mobilization involving various age groups and several nerve types are required in the future in consideration of the time and duration for performing nerve mobilization.

Ⅴ. Conclusion

As a result of this study, excessive excitation was induced during distal gliding and stretching of the nerve. Therefore, when distal gliding or nerve stretching is appliedto patients in clinical practice, it is considered that careful attention is needed to avoid excessive excitation of the nerve.