Electromyographic assessment of back muscle weakness and muscle composition: reliability and validity issues

Age-specific differences in the time-frequency representation of surface electromyographic data recorded during a submaximal cyclic back extension exercise: a promising biomarker to detect early signs of sarcopenia

PURPOSE

Motivated by the goal of developing new methods to detect early signs of sarcopenia, we investigated if surface electromyographic (SEMG) data recorded during the performance of cyclic, submaximal back extensions are marked by age-specific differences in their time and frequency characteristics. Furthermore, day-to-day retest reliability of the EMG measures was examined.

METHODS

A total of 86 healthy volunteers used a back dynamometer to perform a series of three maximal voluntary contractions (MVC) consisting of isometric back extensions, followed by an isometric back extension at 80% MVC, and finally 25 slow cyclic back extensions at 50% MVC. SEMG data was recorded bilaterally at L1, L2, and L5 from the iliocostalis lumborum, longissimus, and multifidus muscles, respectively. Tests were repeated two days and six weeks later. A linear mixed-effects model with fixed effects "age, sex, test number" and the random effect "person" was performed to investigate age-specific differences in both the initial value and the time-course (as defined by the slope of the regression line) of the root mean square (RMS-SEMG) values and instantaneous median frequency (IMDF-SEMG) values calculated separately for the shortening and lengthening phases of the exercise cycles. Generalizability Theory was used to examine reliability of the EMG measures.

RESULTS

Back extensor strength was comparable in younger and older adults. The initial value of RMS-SEMG and IMDF-SEMG as well as the RMS-SEMG time-course did not significantly differ between the two age groups. Conversely, the IMDF-SEMG time-course showed more rapid changes in younger than in older individuals. Absolute and relative reliability of the SEMG time-frequency representations were comparable in older and younger individuals with good to excellent relative reliability but variable absolute reliability levels.

CONCLUSIONS

The IMDF-SEMG time-course derived from submaximal, cyclic back extension exercises performed at moderate effort showed significant differences in younger vs. older adults even though back extension strength was found to be comparable in the two age groups. We conclude that the SEMG method proposed in this study has great potential to be used as a biomarker to detect early signs of sarcopenic back muscle function.

Trunk Muscle Activation Patterns Differ Between Those With Low and High Back Extensor Strength During a Controlled Dynamic Task

It is proposed that reduced function in one of the spinal systems (active, passive, and neural) outlined by Panjabi could increase the risk of experiencing a low back injury (LBI). Also proposed is that reduced function in any one system can be compensated for by adjusting the time-varying recruitment of trunk muscles. This study addressed whether those with reduced active system function (WEAK), measured as back extensor strength, would have different trunk muscle activation patterns than those with higher function (STRONG), and secondly whether this relationship would be modified following recovery from a LBI. Sixty men participated, 30 recently recovered from LBI (rLBI, 4–12 weeks post injury) and 30 who had not had a LBI in the last year (ASYM). ASYM and rLBI participants were separated into STRONG and WEAK sub-groups if their isometric back extensor strength was above or below their group median, respectively. Trunk electromyograms from 24 muscle sites were recorded during a highly controlled horizontal transfer task. Principal component analysis captured key muscle activation patterns (amplitude and temporal); then analysis of variance models tested for strength or group*strength effects on these patterns consistent with the two main objectives. Significant strength, or group by strength effects were found for 3/4 electromyographic comparisons. In general, the WEAK group required higher activation amplitudes of abdominal and back extensor muscles, and greater temporal responsiveness of back extensor muscles only to the changing external moments than those who were STRONG. Group by strength interactions found that participants in the rLBI group had greater differences between WEAK and STRONG participants for overall muscle activation amplitudes in both abdominal and back extensor muscles. This increase in muscle activation was interpreted as compensation for lower maximum force properties whereas the increased temporal responsiveness captured a greater need to modify the agonist back extensors muscle activation patterns only in response to changes in the dynamic moments. Interactions captured that the recent experience of pain (rLBI) modified the magnitude of adjustment in muscle activation patterns potentially adapting to an increased risk of instability (painful flare) events associated with a deficit (lower strength) of the active system.

Age- and sex-specific effects in paravertebral surface electromyographic back extensor muscle fatigue in chronic low back pain

The impact of aging on the back muscles is not well understood, yet may hold clues to both normal aging and chronic low back pain (cLBP). This study sought to investigate whether the median frequency (MF) surface electromyographic (SEMG) back muscle fatigue method—a proxy for glycolytic muscle metabolism—would be able to detect age- and sex-specific differences in neuromuscular and muscle metabolic functions in individuals with cLBP in a reliable way, and whether it would be as sensitive as when used on healthy individuals. With participants seated on a dynamometer (20° trunk anteflexion), paraspinal SEMG activity was recorded bilaterally from the multifidus (L5), longissimus (L2), and iliolumbalis (L1) muscles during isometric, sustained back extensions loaded at 80% of maximum from 117 younger (58 females) and 112 older (56 female) cLBP individuals. Tests were repeated after 1–2 days and 6 weeks. Median frequency, the SEMG variable indicating neuromuscular fatigue, was analyzed. Maximum back extensor strength was comparable between younger and older participants. Significantly less MF-SEMG back muscle fatigue was observed in older as compared to younger, and in older female as compared to older male cLBP individuals. Relative reliability was excellent, but absolute reliability appeared large for this SEMG-fatigue measure. Findings suggest that cLBP likely does not mask the age-specific diagnostic potential of the MF-SEMG back extensor fatigue method. Thus, this method possesses a great potential to be further developed into a valuable biomarker capable of detecting back muscle function at risk of sarcopenia at very early stages.

The effect of extensible and non-extensible lumbar belts on trunk muscle activity and lumbar stiffness in subjects with and without low-back pain

BACKGROUND

Lumbar belts have been shown to increase lumbar stiffness, but it is unclear if this is associated with trunk muscle co-contraction, which would increase the compression on the spine. It has been hypothesized that lumbar belts increase lumbar stiffness by increasing intra-abdominal pressure, which would increase spinal stability without increasing the compressive load on the spine.

METHODS

Trunk muscle activity and lumbar stiffness and damping were measured in healthy and low-back pain subjects during three conditions: no lumbar belt; wearing an extensible lumbar belt; wearing a non-extensible lumbar belt. Muscle activity was measured while subjects performed controlled forward and backward 20° trunk sways. Lumbar stiffness and damping were measured by applying random continuous perturbation to the chest.

FINDINGS

External oblique activity was decreased when wearing either lumbar belt during all phases of movement, while rectus abdominis and iliocostalis activity were decreased during the phase of movement where the muscles were maximally active while wearing either belt. Trunk stiffness was greatly increased by wearing either belt. There were no consistent differences in either lumbar stiffness or muscle activity between the two belts. Wearing a lumbar belt had little to no effect on damping. There were no group differences in any of the measures between healthy and low-back pain populations.

INTERPRETATION

The findings are consistent with the hypothesis that lumbar belts can increase spinal stability by increasing intra-abdominal pressure, without any increase in the compressive load on the spine. The findings can also be generalized, for the first time, to subjects with low-back pain.

The potential use of spectral electromyographic fatigue as a screening and outcome monitoring tool of sarcopenic back muscle alterations

Background

To examine whether or not median frequency surface electromyographic (MF-EMG) back muscle fatigue monitoring would be able to identify alterations in back muscle function in elderly muscles, if a protocol was used that allowed optimum standardization of the processes underlying electromyographic fatigue, and whether these tests were reliable from day to day.

Methods

A total of 42 older (21 females; 67 (±10.5) years old) and 44 younger persons (19 females; 33 (±10) years) performed maximum isometric back extensions which were followed by one 30 s lasting 80% submaximum extension. Participants were seated on a dynamometer with their trunks 30° anteflexed, and they repeated all tests after 1-2 days and 6 weeks. SEMG was recorded bilaterally from the L1 (iliocostalis lumborum), L2 (longissimus), and L5 (multifidus) recording sites. Outcome variables included maximum back extension torque, initial MF-EMG (IMF-EMG), MF-EMG slope declines, and individual MF-EMG muscular imbalance scores. Two-factorial ANOVAs served to examine the age and gender-specific effects, and models from Generalizability Theory (G-Theory) were used for assessing retest-reliability.

Results

Maximum back extension moment was non-significantly smaller in elders. IMF-EMG was overall higher in elders, with significant differences at the L5 recordings sites. In the elderly, MF-EMG fatigue declines were significantly smaller in L5, in the recording with the most negative slope, or if the slope of all electrodes was considered. Retest reliability was unanimous in young and older persons. ICC-type measurements from G-Theory of both the IMF and the fatigue slopes ranged from 0.7 to 0.85. Absolute SEM values were found clinically acceptable for the IMF-EMG, but relatively high for the fatigue slope declines.

Conclusions

The MF-EMG fatigue method is able to elucidate alterations of aging back muscles. This method, thus, might be suggested as a potential biomarker to objectively identify persons at risk for sarcopenia. Considering the clinical relevance of the IMF-EMG relative to the MF-EMG slope declines, spectral EMG may also be used as an outcome monitoring tool in elderly populations.

The association of physical deconditioning and chronic low back pain: A hypothesis-oriented systematic review

PURPOSE

Does physical deconditioning (loss of cardiovascular capacity and strength/endurance of paraspinal muscles) exist in patients with chronic low back pain (CLBP) and are treatments specifically aimed to reduce these signs effective?

METHOD

Systematic literature search in PUBMED, MEDLINE, EMBASE and PsycINFO until December 2004 to identify observational studies regarding deconditioning signs and high quality RCTs regarding the effectiveness of cardiovascular and/or muscle strengthening exercises. Internal validity of the RCTs was assessed by using a checklist of nine methodology criteria in accordance with the Cochrane Collaboration.

RESULTS

There is conflicting evidence that cardiovascular deconditioning is present in CLBP and limited evidence for wasting of the multifidus muscle. No study examined the effectiveness of cardiovascular training specifically. General and lumbar muscle strengthening are equally effective as other active treatments. Only moderate evidence is available for the effectiveness of intensive low back extensor muscle strengthening compared to less intensive strengthening.

CONCLUSION

Probably reactivation caused by active treatment and not the reconditioning itself is the important factor in the reduction of disability. Further prospective and evaluative research into the role of physical deconditioning is necessary. It seems more promising to further explore the interplay between biological, social and psychological factors.

Physical capacity tasks in chronic low back pain: What is the contributing role of cardiovascular capacity, pain and psychological factors?

PURPOSE

To explore the association of personal and body functions with physical capacity tasks used in the evaluation of chronic low back pain (CLBP) treatment.

METHOD

Cross-sectional study in which 221 patients with non-specific CLBP participated. Physical capacity was assessed by six capacity tasks, and several personal and body functions were assessed by questionnaires (age, gender, pain intensity, duration of pain, radiating pain to leg, fear of injury/movement, depression, pain catastrophizing and internal control of pain). Cardiovascular capacity was measured by a modified Astrand submaximal bicycle test. The association of these hypothetically influential personal and body function factors with capacity tasks was examined with multiple linear regression analyses.

RESULTS

The total explained variance was low to moderate (9 - 19%), except for stair climbing for which the variance just reached 30%. Many putative factors did not contribute significantly to the level of outcome of several different tasks.

CONCLUSIONS

Cardiovascular capacity, pain intensity, fear of injury/movement, cognitions, and depression had statistically significant but clinically minor effects on several, but not all capacity tasks. Radiating pain, age and duration of complaints had no significant influence at all. Due to anthropometric differences men outperformed women on most tasks. Apparently the influence of many personal, physical but especially psychological factors on the selected capacity tasks is not high at all. This might indicate that these tasks are more objectively measuring physical capacity than expected.

On the use of EMG-ratios to assess the coordination of back muscles

BACKGROUND

Electromyographic (EMG) amplitude ratios (EMG-ratios) have been proposed to assess back muscle coordination in chronic low back pain patients to avoid the normalization of EMG using maximal contractions. The aim of this study was to test the relevance of this type of EMG analysis.

METHODS

Healthy subjects (44 men and 13 women) and patients with chronic low back pain (57 men) performed three 7s static ramp extension contractions ranging from 0% to 100% of the maximal voluntary contraction while standing in a static dynamometer. A subgroup of 20 healthy men also performed 5s step contractions at 10%, 20%, 40%, 60% and 80% of the maximal voluntary contraction. Finally, to assess reliability, another subgroup (n=20 healthy and 20 men with chronic low back pain) performed the protocol three times, on different days. Surface EMG signals were collected from four pairs of back muscles and subcutaneous tissue thickness was measured at the corresponding electrode sites. EMG amplitude values were computed at each 5% force level from 10% to 80% of the maximal voluntary contraction. Then, EMG-ratios were computed between different electrode sites and averaged bilaterally.

FINDINGS

All EMG-ratios were affected by the force level and the contraction type (ramp vs step contractions). Statistically significant Pearson's correlations (r=-0.38 to -0.57) were obtained between some EMG-ratios and their corresponding subcutaneous tissue thickness ratios. The reliability of the EMG-ratio variables ranged from moderate to excellent (intra-class correlation coefficients between 0.50 and 0.91). Comparisons between 12 men and 13 women and between 24 healthy men and 57 men with chronic low back pain showed that EMG-ratios were sensitive to sex but not to pain status. Multivariate analyses applied on the EMG-ratios identified clusters of subjects but none of the main clinical variables were able to clearly characterize these clusters.

INTERPRETATION

Overall, even though additional research is warranted to further substantiate some important psychometric characteristics of the EMG-ratios as well as their biomechanical and clinical significance, these results support their use for assessing the coordination patterns of back muscles, provided that confounding variables such as the force level, the contraction type, and subcutaneous tissue thickness are accounted for.

Intra-session repeatability of lower limb muscles activation pattern during pedaling

Assessment of intra-session repeatability of muscle activation pattern is of considerable relevance for research settings, especially when used to determine changes over time. However, the repeatability of lower limb muscles activation pattern during pedaling is not fully established. Thus, we tested the intra-session repeatability of the activation pattern of 10 lower limb muscles during a sub-maximal cycling exercise. Eleven triathletes participated to this study. The experimental session consisted in a reference sub-maximal cycling exercise (i.e. 150 W) performed before and after a 53-min simulated training session (mean power output=200+/-12 W). Repeatability of EMG patterns was assessed in terms of muscle activity level (i.e. RMS of the mean pedaling cycle and burst) and muscle activation timing (i.e. onset and offset of the EMG burst) for the 10 following lower limb muscles: gluteus maximus (GMax), semimembranosus (SM), Biceps femoris (BF), vastus medialis (VM), rectus femoris (RF), vastus lateralis (VL), gastrocnemius medianus (GM) and lateralis (GL), soleus (SOL) and tibialis anterior (TA). No significant differences concerning the muscle activation level were found between test and retest for all the muscles investigated. Only VM, SOL and TA showed significant differences in muscle activation timing parameters. Whereas ICC and SEM values confirmed this weak repeatability, cross-correlation coefficients suggest a good repeatability of the activation timing parameters for all the studied muscles. Overall, the main finding of this work is the good repeatability of the EMG pattern during pedaling both in term of muscle activity level and muscle activation timing.

Back and hip extensor muscles fatigue in healthy subjects: task-dependency effect of two variants of the Sorensen test

Paraspinal muscle fatigability during various trunk extension tests has been widely investigated by electromyography (EMG), and its task-dependency is established recently. Hip extensor muscle fatigability during the Sorensen test has been reported. The aim of the present experiments was to evaluate the task-dependency of back and hip extensor muscle fatigue during two variants of the Sorensen test. We hypothesized that the rate of muscular fatigue of the hip and back extensor muscles varies according to the test position. Twenty healthy young males with no history of low back pain volunteered to participate in this cross-sectional study. They were asked to perform two body weight-dependent isometric back extension tests (S1 = Sorensen test; S2 = modified Sorensen on a 45 degrees Roman chair). Surface EMG activity of the paraspinal muscles (T10 and L5 levels) and hip extensor muscles (gluteus maximus; biceps femoris) was recorded, and muscular fatigue was assessed through power spectral analysis of the EMG data by calculating the rate of median power frequency change. We observed hip extensor muscle fatigue simultaneously with paraspinal muscle fatigue during both Sorensen variants. However, only L5 level EMG fatigue indices showed a task-dependency effect between S1 and S2. Hip extensor muscles appear to contribute to load sharing of the upper body mass during both Sorensen variants, but to a different extent because L5 level fatigue differs between the Sorensen variants. Our findings suggest that task-dependency has to be considered when EMG variables are compared between two types of lumbar muscle-fatiguing tasks.

Issues in relation to the repeatability of and correlation between EMG and Borg scale assessments of neck muscle fatigue

INTRODUCTION

The repeatability of subjective and objective assessments of neck muscle fatigue is very important with regard to the clinical applicability of these methods.

METHOD

To establish between-days reliability, 33 healthy volunteers performed a 60% maximum voluntary isometric contraction test from a standing position in all neck movements. Cervical muscle fatigue was assessed on three separate occasions from the spectral (median frequency, MF) and amplitude (root mean square, RMS) analysis of the electromyogram (EMG) signal recorded from the cervical paraspinal group, splenius capitis, levator scapulae and sternocleidomastoid. Subjective assessment of fatigue was rated by employing the Borg scale. Intraclass correlation coefficient ICC((1,1)), standard error of measurement (SEM), smallest detectable difference (SDD) indices and Pearson's correlation co-efficient were calculated for the analysis of the results.

RESULTS

Normalised median frequency (MF) slope had low repeatability and large between-day error (ICC((1,1))=0.28-0.61; SEM=0.33-0.60%/s; SDD=132.7-703.2%) for the protagonist muscles of each movement. Initial median frequency (IMF) had moderate to good reliability and small error (ICC((1,1))=0.64-0.81; SEM=2.8-8.8Hz; SDD=19.9-38.5%). The RMS slope yielded also poor repeatability. The Borg assessment was more reliable than the EMG estimate though variability between sessions was still quite high (SDD=29.2-136.5%). No correlation was found between the EMG and Borg assessment of neck muscle fatigue (r=-0.01-0.39).

CONCLUSION

The protocol used for assessing neck muscle fatigue proved to be reliable only for the IMF but the clinical usefulness of this measure remains questionable. The lack of correlation between objective and subjective estimation of neck muscle fatigue was possibly a consequence of the poor measurement repeatability. Further research is needed to identify the factors responsible for these results on neck area.

The influence of non-specific low back pain on pressure pain thresholds and disability

The purpose of this study was to investigate the pressure pain thresholds (PPTs) with respect to the Erector spinae and the hip muscles in 87 patients with subacute non-specific low back pain (LBP) and to evaluate the relationship between the PPTs and disability. In order to establish reference values, 64 healthy subjects were examined with respect to PPTs and used as a control group against the group of LBP patients. The mean PPT values of the Erector spinae and the hip at all examined points of the LBP group were significantly lower (p<0.001) in comparison to the PPT values of the healthy group. An exceptionally high difference (2.7 kg/cm2) was found at the L3 Erector spinae level. The correlation between having LBP or not in the whole group (n=151) and PPT, was highest at the L3 level of the Erector spinae (r=-0.710, p<0.001). When the group of patients with LBP was divided into two subgroups in terms of having an Oswestry disability index (ODI) lower than 40 ("moderate LBP disability") or an ODI higher than 40 ("severe LBP disability") it was surprising to notice that there was no significant difference between the PPTs of the Erector spinae and the hip musculature. This study has shown the possibility of the existence of muscular disorder in the lumbar part of the Erector spinae in patients with non-specific low back pain, but also reveals the strong inter-individual differences in muscular fibrosis sensitivity and pain behaviour related to gender.

Reliability of EMG measurements for trunk muscles during maximal and sub-maximal voluntary isometric contractions in healthy controls and CLBP patients

The purpose of this study was to compare the reliability of trunk muscle activity measured by means of surface electromyography (EMG) during maximal and sub-maximal voluntary isometric contractions (MVC/sub-MVC) over repeated trials within-day and between-days in healthy controls and patients with chronic low back pain (CLBP). Eleven volunteers (six controls and five CLBP patients) were assessed twice with a 1-week interval. Surface EMG signals were recorded bilaterally from six trunk muscles. Intra-class correlation coefficients (ICC) and standard error of measurement as a percentage of the grand mean (%SEM) were calculated. MVC and sub-MVC showed excellent within-day reliability in both healthy controls and CLBP patients (ICC mean 0.91; range 0.75-0.98; %SEM mean 4%; range 1-12%). Sub-MVC for both groups between-days showed excellent reliability (ICC mean 0.88; range 0.78-0.97; %SEM mean 7%; range 3-11%). The between-days MVC for both groups showed trends towards lower levels of reliability (ICC mean 0.70; range 0.19-0.99; %SEM mean 17%; range 4-36%) when compared to sub-MVC. Findings of the study provide evidence that sub-MVC are preferable for amplitude normalisation when assessing EMG signals of trunk muscles between-days.

Surface electromyography assessment of back muscle intrinsic properties

The purpose of this study was to assess (1) the reliability and (2) the sensitivity to low back pain status and gender of different EMG indices developed for the assessment of back muscle weakness, muscle fiber composition and fatigability. Healthy subjects (men and women) and chronic low back pain patients (men only) performed, in a static dynamometer, maximal and submaximal static trunk extension tasks (short and long duration) to assess weakness, fiber composition and fatigue. Surface EMG signals were recorded from four (bilateral) pairs of back muscles and three pairs of abdominal muscles. To assess reliability of the different EMG parameters, 40 male volunteers (20 controls and 20 chronic low back pain patients) were assessed on three occasions. Reliable EMG indices were achieved for both healthy and chronic low back pain subjects when specific measurement strategies were applied. The EMG parameters used to quantify weakness and fiber composition were insensitive to low back status and gender. The EMG fatigue parameters did not detect differences between genders but unexpectedly, healthy men showed higher fatigability than back pain patients. This result was attributed to the smaller absolute load that was attributed to the patients, a load that was defined relative to their maximal strength, a problematic measure with this population. An attempt was made to predict maximal back strength from anthropometric measurements but this prediction was prone to errors. The main difficulties and some potential solutions related to the assessment of back muscle intrinsic properties were discussed.