Reliability of mechanomyographic amplitude measurements for trunk muscles during maximal voluntary isometric contraction

Abstract

BACKGROUND:

Mechanomyography (MMG) has been used to investigate mechanical characteristics of muscle contraction in clinical and experimental settings.

OBJECTIVE:

The aim of this study was to determine the test-retest reliability of mechanomyographic amplitude (MMG ${}_{\text{RMS}}$ ) measurements as a tool for measuring the maximal voluntary isometric contractions (MVICs) of trunk muscles in healthy participants.

METHODS:

There were ten young adults participating in this study. Accelerometers were used to detect surface MMG signals from three trials of 5-s MVICs of the rectus abdominis, external obliques, erector spinae, and multifidus in the vertical, transverse, and longitudinal directions. Intraclass correlation coefficient (ICC), standard error of measurement (SEM), and minimum detectable change were calculated.

RESULTS:

Good to excellent test-retest reliability of mechanomyographic amplitude (MMG ${}_{\text{RMS}}$ ) measurements was achieved for all MVICs of trunk muscles in healthy participants, as indicated by ICCs ranging from 0.99 to 0.64 for MMG ${}_{\text{RMS}}$ of the trunk muscles during MVIC.

CONCLUSIONS:

This study demonstrates that MMG is a reliable measurement to detect the activation amplitudes of trunk muscles during MVIC.

Keywords

Test-retest reliability mechanomyography trunk muscle maximal voluntary isometric contraction

1. Introduction

Mechanomyography or the mechanomyogram (MMG) is a method used to record dimensional changes in active skeletal muscle fibre contractions at low frequency oscillations [1, 2]. Analysis of MMG signals allows examination of various aspects of muscle function, such as neuromuscular fatigue [3] and neuromuscular disorders in adult and paediatric populations [4], the control of external prostheses, and measuring the effectiveness of anaesthesia [5]. Currently the standard tool for measuring muscle contraction is electromyography (EMG). When recorded simultaneously with EMG from the same muscle, MMG exhibits a similar pattern [6]. While EMG amplitudes measure the motor unit action potentials during muscle contraction, MMG detects the mechanical oscillations of contracting muscle fibres [7].

MMG is a mechanical signal that can be recorded from the surface of a contracting muscle when the movements of muscle fibers cause vibrations as the source of the wave generation [8]. A number of sensors can be used to record MMG, including accelerometers, microphones, piezoelectric sensors, and laser sensors [9]. In general, the wave propagation has been reported as muscle oscillations in the lateral [10, 11, 12, 13], longitudinal [14] and transverse [14, 15] directions. Additionally the transverse direction of MMG signal propagation could be produced when the oscillation frequency less than 25 Hz, otherwise it would be shown in the longitudinal direction [16]. Thus a triaxial accelerometer has been used for MMG recordings of muscle oscillation amplitude [17].

Reliability refers to the reproducibility of measurements when repeated at random in the same subject, representing one of the principal characteristics of an outcome measure in clinical trials. Measurement reliability is important to consider when choosing the primary outcome measure for clinical trials and should be assessed and assured through a quality control program based on randomly selected duplicate assessments [18]. Reliability is a method to measure the consistency or repeatability of measurements that includes relative reliability and absolute reliability. Relative reliability represents the relative consistency of measurements that can be explained using an intraclass correlation coefficient (ICC) [19]. Moreover, absolute reliability is the degree of the repeated measurements that uses standard error of measurement (SEM) and Bland and Altman’s 95% limits of agreement to represent outcomes. SEM also represents the absolute measurement error expressed as a percentage of the grand mean (SEM%) with an acceptance level of $<$ 10%. The Bland and Altman 95% limits of agreement test is used to assess systematic errors [20]. In the clinical measure, the minimal detectable change (MDC) is important for estimating the smallest variation. MDC represented the absolute, minimal detectable difference as a percentage of the grand mean (%MDC) with an acceptance level of 30% [21]. Therefore, ICC, SEM, and MDC are usually used to quantify measurement reliability.

The reliability of MMG has been tested on limb muscles. Akataki et al. [22] confirmed the reliability (average ICC $=$ 0.83) of MMG ${}_{\text{RMS}}$ of the biceps brachii during submaximal isometric step muscle actions. For the submaximal isometric force of the vastus lateralis, ICCs of the MMG amplitude ranged from 0.36 to 0.93 over two trials [23, 24]. The surface MMG amplitude can be affected by the location of the accelerometer on the muscle [25, 26, 27, 28, 29]. Stokes [2] reported that greater MMG activity was recorded over the belly muscle than over the fascia at the muscle borders, which may be related to differences in thickness of the adipose tissue, the amount of muscle mass, motor unit discharge pattern, muscle architecture, and/or motor unit territorial distribution [25, 26, 29]. Current research has focused primarily on limb muscles, with relatively little known about the reliability of using MMG signals for detecting the isometric contractions of trunk muscles. Dankaerts’ research [30] focused on the reliability of EMG for rectus abdominis (RA), external oblique (EO), multifidus (MF), and erectus spinae (ES) of trunk muscles during maximal voluntary isometric contractions (MVICs). Therefore, the purpose of this study was to examine the test-retest reliability of MMG for trunk muscles during MVICs in healthy individuals.

2. Methods

2.1 Participants

Ten healthy volunteers (5 men and 5 women, 23.5 $\pm$ 1.1 years, 59.7 $\pm$ 9.8 kg, 164.8 $\pm$ 6.0 cm) participated in this study. None of the participants reported any current neuromusculoskeletal disorders involving the trunk. The Chang Gung Medical Foundation Institutional Review Board approved this study (IRB: 102-3103B). Written informed consent was obtained from all participants prior to data collection.

2.2 Instruments

The MMG signal was collected using a set of triaxial accelerometers (LIS331DLH; STMicroelectronics, Inc., Geneva, Switzerland, 3 $\times$ 3 $\times$ 1 mm, weight: 80 mg) that were pre-amplified with a gain of 200, frequency of 0–200 Hz, sensitivity of 10 mV/ms ${}^{-2}$ , and range of $\pm$ 2 g. The X-axis was defined as transverse movement along the medial-lateral direction of muscle fibre; the Y-axis was defined as longitudinal movement along the proximal to distal of muscle fibre, and the Z-axis was defined as vertical movement along the anterior-posterior direction of muscle fibre. The accelerometer was attached to the skin using double-sided adhesive tape. It was placed 2 cm above the umbilicus and 3 cm lateral to the midline for RA measurements, 10 cm lateral to the midline and 45 ${}^{\circ}$ adjacent to the horizontal plane for EO measurements, at level 5 parallel to the line between the posterior superior iliac spine and the L1–L2 interspinous space for MF measurements, and 4 cm lateral to L3 of the spinous process for ES measurements. The overall mass of the accelerometer was set at $<$ 2 g to avoid substantial attenuation of the MMG signal [19].

2.3 Experimental protocol

For MVIC measurements of trunk muscles, each participant lay on the bed with legs held immobile with a belt in the supine position for abdominal muscles and in the prone position for back muscles. The investigator applied resistance on the shoulders for 5 s during MVICs. The participants performed three trials, one for familiarization and two experimental trials. The order of testing for each muscle was randomized. A 5-min rest was allowed between each muscle trial, whether practice or test trial, and a 1 h rest was allowed between test and retest to avoid fatigue.

The postures held for evoking MVIC of related trunk muscles included the following: (1) RA by a resisted curl-up with resistance applied in a symmetrical manner on the shoulders of the participant by the investigator; (2) right EO by a resisted crossed curl-up with the right shoulder towards the left and resistance applied on the right shoulder by the investigator; left EO was in an opposite direction; and (4) MF and ES, let the participant be in prone position with hands on the neck to lift the head, shoulders and elbows just off the examination table in the meanwhile symmetrical manual resistance was provided to the scapular region by the investigator.

2.4 Signal processing

MMG signals (ms ${}^{-2}$ , g) recorded from the trunk muscle were sampled at 1 kHz and stored on a personal computer. The signal was processed using custom-written software with MATLAB Version 7.10.0 (R2010a) (Mathworks, Natick, MA, USA). The MMG signals were filtered (fourth-order Butterworth filter) with a pass band of 5–100 Hz. Three-second epochs were selected from the MMG signal of the trunk muscle to calculate the average amplitude value expressed as the root-mean-square value (MMG ${}_{\text{RMS}}$ ).

Table 1
ICC, SEM, %SEM, MDC, %MDC for MMG ${}_{\text{RMS}}$ measures of trunk muscles during MVIC

Muscles	Directions	Test (g)	Retest (g)	ICC	95% CI	SEM (g)	SEM/mean (%)	MDC (g)	MDC/mean (%)
RA	Transverse	0.069 $\pm$ 0.042	0.074 $\pm$ 0.042	0.97 ${}^{**}$	0.87–0.99	0.008	10.7	0.021	29.8
	Longitudinal	0.088 $\pm$ 0.056	0.091 $\pm$ 0.057	0.90 ${}^{**}$	0.60–0.98	0.017	19.3	0.048	53.4
	Vertical	0.075 $\pm$ 0.036	0.080 $\pm$ 0.034	0.89 ${}^{**}$	0.57–0.97	0.011	14.4	0.031	40.0
EO	Transverse	0.057 $\pm$ 0.016	0.056 $\pm$ 0.020	0.94 ${}^{**}$	0.77–0.99	0.004	7.9	0.012	21.9
	Longitudinal	0.053 $\pm$ 0.020	0.048 $\pm$ 0.011	0.92 ${}^{**}$	0.67–0.98	0.003	6.8	0.009	18.9
	Vertical	0.055 $\pm$ 0.012	0.055 $\pm$ 0.014	0.87 ${}^{**}$	0.47–0.97	0.005	8.2	0.013	22.8
ES	Transverse	0.026 $\pm$ 0.004	0.026 $\pm$ 0.004	0.97 ${}^{**}$	0.90–0.99	0.001	2.6	0.002	7.1
	Longitudinal	0.029 $\pm$ 0.012	0.031 $\pm$ 0.009	0.95 ${}^{**}$	0.79–0.99	0.002	7.9	0.007	21.9
	Vertical	0.025 $\pm$ 0.008	0.028 $\pm$ 0.013	0.93 ${}^{**}$	0.72–0.98	0.003	10.5	0.008	29.1
MF	Transverse	0.025 $\pm$ 0.005	0.025 $\pm$ 0.004	0.94 ${}^{**}$	0.77–0.99	0.001	3.9	0.003	10.9
	Longitudinal	0.028 $\pm$ 0.005	0.029 $\pm$ 0.004	0.73 ${}^{*}$	0.64–0.93	0.002	7.9	0.006	22.0
	Vertical	0.018 $\pm$ 0.002	0.018 $\pm$ 0.004	0.88 ${}^{**}$	0.53–0.97	0.001	5.4	0.003	15.1

Note. MMG, mechanomyography; EO, external obliques; RA, rectus abdominis; ES, erector spinae; MF, multifidus; ICC, intraclass correlation coefficient; SEM, standard error of measurement; %SEM, standard error of measurement as a percentage of the grand mean; MDC, minimal detectable difference; %MDC, minimal detectable difference as a percentage of the grand mean. A paired t-test was performed to determine the differences in MMG ${}_{\text{RMS}}$ between the test and retest of the trunk muscles, ${}^{*}$ $p<$ 0.05, ${}^{**}$ $p<$ 0.01.

Figure 1.

The raw data of MMG and RMS data during MVIC from one participant.

Table 2

Absolute reliability of the MMG ${}_{\text{RMS}}$ measures of trunk muscles during MVIC

Muscles	Directions	$d$	SE of $d$	95% CI of $d$	Limits of agreement
RA	Transverse	0.0053	0.0154	$-$ 0.0059–0.0165	$-$ 0.0254–0.0360
	Longitudinal	0.0027	0.0340	$-$ 0.0221–0.0275	$-$ 0.0652–0.0706
	Vertical	0.0053	0.0219	$-$ 0.0107–0.0212	$-$ 0.0385–0.0490
EO	Transverse	$-$ 0.0034	0.0086	$-$ 0.0028–0.0096	$-$ 0.0137–0.0205
	Longitudinal	0.0045	0.0066	$-$ 0.0093–0.0004	$-$ 0.0177–0.0088
	Vertical	0.0014	0.0089	$-$ 0.0079–0.0051	$-$ 0.0192–0.0164
ES	Transverse	0.0005	0.0013	$-$ 0.0005–0.0015	$-$ 0.0022–0.0031
	Longitudinal	0.0021	0.0047	$-$ 0.0013–0.0055	$-$ 0.0073–0.0115
	Vertical	0.0032	0.0054	$-$ 0.0008–0.0071	$-$ 0.0077–0.0140
MF	Transverse	0.0001	0.0020	$-$ 0.0015–0.0014	$-$ 0.0040–0.0039
	Longitudinal	$-$ 0.0016	0.0041	$-$ 0.0013–0.0046	$-$ 0.0065–0.0098
	Vertical	$-$ 0.0003	0.0019	$-$ 0.0011–0.0017	$-$ 0.0034–0.0040

Note. MMG, mechanomyography; $d$ , the mean of difference between the two test sessions; SE, standard error; CI, confidence interval.

Figure 2.

The comparison of trunk muscle MMG ${}_{\text{RMS}}$ during MVIC between trunk flexors and extensors.

2.5 Statistical analysis

Descriptive statistics included mean and standard deviation for age, body height, body weight, and MMG ${}_{\text{RMS}}$ . The intraclass correlation coefficient (ICC), standard error of measurement (SEM), and minimal detectable change (MDC) were used for test-retest reliability. ICC was used to determine relative consistency and was classified as “excellent” ( $\geqslant$ 0.81), “good” (0.61–0.80), “moderate” (0.41–0.60), or “poor” ( $\leqslant$ 0.40). SEM represented the absolute measurement error expressed as a percentage of the grand mean (SEM%) with an acceptance level of $<$ 10%. SEM was calculated as the following equation:

$\displaystyle\text{SEM}=\frac{S_{X}\sqrt{1-\textit{ICC}}}{{\overline{X}_{1}}+{% \overline{X}_{2}}}\times 100,$

where $S_{X}$ was the pooled standard deviation, ${\overline{X}_{1}}$ was the mean MMG amplitude during the test trial, and ${\overline{X}_{2}}$ was the mean MMG amplitude during the retest trial. Smaller SEMs represent greater measurement reliability. MDC was used to determine the minimal change in MMG measurement during MVIC. MDC was calculated as the following equation: MDC $=$ SEM $\times$ $\sqrt{2}$ $\times$ 1.96. MDC represented the absolute minimal detectable difference as a percentage of the grand mean (%MDC) with an acceptance level of 30%. Bland-Altman plots were used to visually compare measurements of estimated MMG amplitudes between the test and retest trials and were obtained for each test by plotting the difference ( $d$ ) between test and retest versus the mean MMG amplitudes of the test and retest trials. In this study, we also calculated standard error (SE) of $d$ , where a 95% confidence interval (CI) of $d$ for the mean difference indicates systematic bias in measurements [20] and limits of agreement from Bland-Altman plots [21]. A paired t-test was performed to determine the differences in MMG ${}_{\text{RMS}}$ between the test and retest of the trunk muscles. The significance level was set at $\alpha$ of 0.05. All data were analysed using SPSS for Windows, version 19.0 (SPSS, Chicago, IL, USA).

3. Results

The original data of MMG and MVIC from one participant was demonstrated as Fig. 1. The reliability and measurement variability statistics for MMG ${}_{\text{RMS}}$ of trunk muscles during MVIC are shown in Table 1. The result of this study indicated that there was no differences in MMG ${}_{\text{RMS}}$ between the test and retest of the trunk muscles Additionally, the MMG ${}_{\text{RMS}}$ amplitudes of trunk extensors were less than those of trunk flexors for all the axes that was illustrated as Fig. 2. ICCs are ranged from 0.73 to 0.97 with statistical significance, in which MMG along all directions referred as excellent reliability except longitudinal direction of MF ( $r=$ 0.73) referred as good reliability. While SEM ranged from 2.6 to 19.3% and MDC ranged from 7.1 to 53.4%. Absolute reliability of the MMG ${}_{\text{RMS}}$ measures of trunk muscles during MVIC are presented in Table 2. The $d$ values were close to zero with a narrow 95% confidence interval (CI). No systematic bias could be seen because the 95% CI of $d$ includes the zero.

4. Discussion

In this study, we assessed the test-retest reliability of MMG for MVIC of trunk muscles. To measure the level of agreement between test-retest measures, indices such as ICC were used. ICCs found in this study indicate good to excellent relative reliability of MMG signals of trunk muscles during MVIC. Relative reliability is the degree to which individuals maintain their position in a sample with repeated measurements, which are affected by between-subject variability. Compared with studies of limb movements, which have reported poor to good reliability for ICCs within the 0.36–0.93 range [21], this study clearly showed that MMG for trunk muscle activation during MVIC is a detectable and reliable measurement.

Using ICCs alone does not reflect the comprehensive features of reliability. The Bland and Altman method for absolute reliability provides an alternative approach and supplements the ICC measurements for assessing reliability in clinical research. Absolute reliability is the degree to which repeated measurements vary for individuals [20]. Apart from the Bland and Altman method, SEM and MDC can also be used to quantify measurement errors caused by repeated measurements. The Bland-Altman plot provides a visual interpretation to inspect the size and range differences in measurements and any bias or outliers. The 95% CI for the mean difference indicates systematic bias in measurements [18]. Furthermore, we also found good to excellent reliability with good agreement in measurements from the Bland-Altman plots. We have demonstrated that MMG is a reliable measurement to detect oscillations of trunk muscles during MVIC with an acceptable SEM, except for the RA muscle.

There are several limitations in the study. First of all, a limited study population of healthy young adults was recruited in this study; therefore, the test-retest reliability of MMG measurements for trunk muscles in this study cannot be generalized for the entire population. In addition, we did not determine the torque produced by the torque transducer synchronized with MMG during MVIC quantitatively because of the limitation of measurement setup for MVIC of trunk muscles. And the manual resistance from the operator during MVIC could introduce artefacts due to movement such as limb tremors, which are difficult to identify and remove from MMG signals as a source of variability [30].

In addition, MMG ${}_{\text{RMS}}$ of RA did not show an acceptable level of SEM or MDC in the vertical and longitudinal directions, which we suggest using MMG ${}_{\text{RMS}}$ measurement over RA with caution in practice The difference in MMG ${}_{\text{RMS}}$ amplitude between the trunk extensors and flesors could be determined by the motor unit location and the transverse position of the accelerometer as the reported by Farina et al. [31]. In additions, The MMG ${}_{\text{RMS}}$ of MF muscle along the longitudinal direction in this study was the only one with good reliability different from the other trunk muscles with excellent reliability, which may be related to that is the only deep extensor muscle group and could be affected by the site and orientation of muscle fibers.

These findings indicate that MMG amplitude measurements show good test-retest reliability for MVIC of trunk muscles with good to excellent ICCs and acceptable SEM% and MDC. MMG appears to be a reliable tool for investigating trunk muscle responses during isometric contraction. The MMG in this study is a reliable measurement for detecting the activation amplitudes of trunk muscles during MVIC; for clinical relevance, we suggest using a lightweight mechanomyographic sensor as a wearable sensor for measuring not only mechanical characteristics of muscle contraction levels during dynamic activities, but also for rehabilitation as a biofeedback tool to monitor muscle activities of the trunk muscles Furthermore, future work applying MMG ought to further investigate the mechanical characteristics of trunk muscles and establish the reliability of MMG measurements of trunk muscles in individuals with low back pain.

Conflict of interest

None to report.

Footnotes

Acknowledgments

The authors acknowledge the grant (BMRP349, CMRPD1B0221, EMRPD1G0241, 98-2410-H-182- 022-MY2, 99-2410-H-182-039) from Chang Gung Memorial Hospital and Healthy Aging Research Center of Chang Gung University in Taiwan, and thank all participants in this study.

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Reliability of mechanomyographic amplitude measurements for trunk muscles during maximal voluntary isometric contraction

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.2 Instruments

2.3 Experimental protocol

2.4 Signal processing

Table 1 ICC, SEM, %SEM, MDC, %MDC for MMG RMS measures of trunk muscles during MVIC

3. Results

4. Discussion

Conflict of interest

Footnotes

Acknowledgments

References

Table 1
ICC, SEM, %SEM, MDC, %MDC for MMG ${}_{\text{RMS}}$ measures of trunk muscles during MVIC