Effects of gait speed on paraspinal muscle activation: an sEMG analysis of the multifidus and erector spinae

Participants

Forty people were initially enrolled in the study, of whom nine did not meet the inclusion criteria and were excluded from further analysis. The final study group consisted of 31 healthy subjects, students of physiotherapy at the Pomeranian University in Słupsk (20 women (64.51%) and 11 men (35.48%)). The mean age of the study participants was 22.5 years; (SD 2.4) years. The data were collected between 05.04.2023 and 22.06.2023. The purpose and protocol of the study were explained and each participant signed an informed consent form before participating. This study was approved by the Bioethics Committee at the District Medical Chambers in Gdańsk (KB-33/23).

Selection criteria

Inclusion criteria for the study involved being over 18 years of age, being a physiotherapy student at the Pomeranian University in Słupsk, and signing an informed consent to participate in the study. Exclusion criteria for the study included individuals with a history of spinal injury or after spinal surgery, people with spinal and/or limb deformities, people with neurological diseases, pregnant women (due to static changes in the musculoskeletal system during pregnancy and possible resulting spinal pain), people who exceeded the safe maximum heart rate for the physical activity performed during the study.

Research method

The study was based on the author’s interview questionnaire and sEMG. These procedures were conducted at the Biomechanics Laboratory of the Department of Physiotherapy at the Pomeranian University in Słupsk. The duration of the examination of one person was approximately 20–25 min.

The interview questionnaire includes questions on anthropometric data such as age, gender, height (cm), weight (kg) and BMI (calculated as weight divided by the square of height) (kg/m²). The interview questionnaire also includes questions relating to study inclusion and exclusion criteria.

The sEMG test was performed with an eMotion EMG device, equipped with 4 MT-WBA-1-EMG sensors and 12 Ambu Blue Sensor R (Ag/AgCl) electrodes measuring 57 × 48 mm. The sensors were connected wirelessly to the eMotion EMG receiver (Bluetooth module). The EMG signals were sampled at 1 kHz, with a common mode rejection ratio (CMRR) of 104 dB and a resolution of 14 bits. The sensor sensitivity was 1 µV/bit. We applied a band-pass filter between 20 and 450 Hz to reduce noise and isolate the relevant muscle activation frequencies. Electrodes were placed according to the recommendations of the European Project Surface EMG for Non-Invasive Assessment of Muscles (SENIAM) with amendments after (Okubo et al., 2010; Soer et al., 2022). The areas on the patient’s body where the electrodes were placed were washed with soapy water and then disinfected with a 70% alcohol solution. If hair was present at the electrode sites, hair removal was performed. Once the electrodes were attached, the device was calibrated.

The location of the electrodes for the multifidus muscle (MM) was at the initial attachment near the L5/S1 vertebral transition (due to the most superficial location of the muscle in its course) at an electrode edge distance of 2 cm lateral to the spine, with a 2 cm gap between the electrode edges, in line with the course of the muscle fibres. The electrodes for the erector spinae muscles (MES) were placed 2 cm above the line defined by the most cranial (cephalic) vertices of the edge of the iliac crest, at a distance of the electrode edge 2 cm lateral to the spine, with a 2 cm gap between the electrode edges, following the course of the muscle fibres (Fig. 1).

Test procedure

Patients underwent a single examination session consisting of three interconnected phases (Fig. 1). Firstly, participants were introduced to the course, indications and contraindications and the purpose of the study, and after giving their informed and written consent, they were interviewed.

Secondly, participants were prepared for surface electromyography (sEMG). Further in this phase, initial sEMG measurements of the postural paraspinal muscles were taken in three positions: (1) at rest while lying prone with a bolster under the participant’s abdomen to relax the examined structures; (2) while standing; and (3) during maximum voluntary isometric contraction (MVIC) (Meldrum et al., 2007; Norasi, Koenig & Mirka, 2022). When performing MVIC, the patient lay forward, with restraining straps around the shoulder girdle and shin, and performed a maximal torso extension. MVIC with simultaneous sEMG recording was performed to normalize the sEMG data. Each measurement lasted 60 s. Prior to commencing formal testing, all participants underwent a treadmill familiarization phase. During this phase, participants were allowed to walk on the treadmill at a self-selected comfortable pace for 5 min to acclimate themselves to the treadmill and the mechanics of walking.

In the third phase, sEMG was recorded during treadmill walking at speeds of 1 km/h; 3 km/h; 5 km/h and 6 km/h. The test time was 60 s for each speed. Muscle activity was measured only during gait. A 30-second rest was taken between testing at 3 km/h and 5 km/h and 5 km/h and 6 km/h. The subject stood on a stationary treadmill before each sEMG measurement. Before the sEMG measurement, the walking speed on the treadmill was increased until the target walking speed was reached. The sEMG measurement was performed only when the target speed was reached. The measurement lasted 60 s. After the 60-second measurement was completed, the treadmill speed was reduced to zero. After reaching zero, there was a 30-second break. Then, the procedures were repeated for each target walking speed. Heart rate was monitored at rest and during physical activity performed during the study using a medical finger pulse oximeter.

Comparative methods

Following data acquisition during the test procedure, a series of comparative methods were employed to analyze and interpret the sEMG signals. These methods focused on quantifying muscle activity levels and their relationship to different gait speeds.

The data were converted to a percentage value of the amplitude of the sEMG recording in relation to maximum voluntary isometric contraction (%MVIC) according to the following relationship: (1)${\displaystyle \text{\%}MVIC=y_{signal\forall v^{\to }}\cdot y_{MVIC}^{-1}\left[\text{\%}\right]}$ where: %MVIC - current percentage value of MVIC,

y_(signal ∀v^→) - signal value of the examined muscle for each specified speed,

y_MVIC - signal value of the examined muscle during MVIC.

Due to the presence of outliers, the median was used to represent the relationship between the growth of the arguments—expressed as the rate of change in muscle activity—and the analysis of %MVIC changes across the given activity types and locomotion speeds. Lagrange polynomial interpolation from this relationship was then used to analyze the trend in more depth: (2)${\displaystyle W_n\left(x\right)=\sum _{j=0}^n{y}_j\frac{\left(x-x_0\right)\left(x-x_n\right)}{\left(x_j-x_0\right)\left(x_j-x_n\right)}\Rightarrow W_n\left(x\right)=\sum _{j=0}^n{y}_j\prod _{k=1;k\ne j}^n\frac{x-x_k}{x_j-x_k}}$ (3)${\displaystyle W\left(x\right)=-\frac{y_{1\mathrm{km/h}}\left(x-3\right)\left(x-5\right)\left(x-6\right)}{40}+\frac{y_{3\mathrm{km/h}}\left(x-1\right)\left(x-5\right)\left(x-6\right)}{12}-\frac{y_{5\mathrm{km/h}}\left(x-1\right)\left(x-3\right)\left(x-6\right)}{8}+\frac{y_{6\mathrm{km/h}}\left(x-1\right)\left(x-3\right)\left(x-5\right)}{15}}$

where: y_xkm/h—the median value of the received signal for a given muscle during the walking trial on a treadmill at a speed of x km/h.

Equations (2) and (3) were used to determine the functions of the given muscles and to establish equivalent values. Finally, the relationships between argument increments for a given discharge, expressed as the rate of change in muscle activity, were also examined based on the established relationship: (4)${\displaystyle {\Delta }_M=1+\left(y_{MVIC\text{\%}\forall \left(v^{\to }+n\right)}-y_{MVIC\text{\%}\forall v^{\to }}\right)\cdot y_{MVIC\text{\%}\forall v^{\to }}^{-1}}$ where: M—rate of change in muscle activity of the examined muscle;

y_{MVIC%∀v^→}—percentage value of MVIC for each specified speed

y_{MVIC%∀(v^→+n)}—percentage value of MVIC for each subsequent speed.

Statistical analysis

Statistical analysis was conducted using IBM SPSS Statistics (version 29, IBM Corp., Armonk, NY, USA) and MedCalc Statistical Software (version 22.016, MedCalc Software Ltd, Ostend, Belgium). Graphical representations were generated using GraphPad Prism (version 10.1.0, GraphPad Software, Boston, USA). Quantitative variables are presented as the mean (standard deviation) [95% confidence interval], median, and range. Qualitative variables are summarized as frequencies and percentages. The normality of variable distributions was assessed using the Shapiro–Wilk, Anderson-Darling, and Kolmogorov–Smirnov tests with Lilliefors correction. Levene’s test was used to evaluate the homogeneity of variances. For comparisons between two groups, an independent samples t-test was used when the assumptions of normality and homogeneity of variance were met; otherwise, the Mann–Whitney U test was applied. Differences in the same variable across multiple groups, assuming a non-normal distribution, were assessed using the Friedman test, followed by post-hoc analysis if significant differences were found. A significance level of p < 0.05 was applied to all statistical tests.