Median Frequency Slope Research Articles

Gender differences in spectral and entropic measures of erector spinae muscle fatigue.

Electromyographic power spectral analysis is a valuable measurement; however, conflicting results have been reported for amplitude and frequency changes during a fatiguing submaximal muscle contraction. This study compared gender differences for two analyses in subjects with low back pain (LBP). Distinct gender differences are found in musculoskeletal illness/dysfunction, and we examined the effect of gender on entropy and median frequency (MF) slope in a cohort of subjects with LBP. A total of 44 subjects (24 female and 20 male) completed the modified Sorenson test. These subjects ranged in age from 26 to 64 years old, with an average age of 49.9 +/- 9.4 years. Overall, a significant fatigability difference was found based on MF slope (F = 21.33, p = 0.001) and entropy measures (F = 68.26, p = 0.001) of the back muscles. While the MF slope was not different (F = 0.44, p = 0.51) between genders, the entropy values were higher for the male subjects than for the female subjects (F = 6.70, p = 0.01). These results indicate that the Shannon entropy measure differentiates between genders. Further studies are needed to evaluate the effectiveness of using nonlinear analysis as a measurement tool.

The reliability of surface electromyography to assess quadriceps fatigue during multi joint tasks in healthy and painful knees

This study’s aim was to determine the between days reliability of surface EMG recordings from the superficial quadriceps during a multi joint sub-maximal fatiguing protocol. Three subject groups (healthy n = 29; patellofemoral pain syndrome n = 74; knee osteoarthritis n = 55) performed the task at 60 maximum voluntary isometric contraction on three separate days. Spectral and amplitude EMG parameters were recorded from vastus medialis oblique, vastus lateralis and rectus femoris and were analysed for between days reliability using intraclass correlation coefficient (ICC (2,1)), the standard errors of measure and smallest detectable differences. For frequency results, initial and final frequency values had ‘good’ or ‘excellent’ reliability in all groups for all muscles. ICCs for median frequency slopes for vastus medialis oblique, vastus lateralis, and rectus femoris respectively, in the osteoarthritis group were 0.04, 0.55, and 0.72; in the patellofemoral pain group were 0.41, 0.17, and 0.33; in the healthy group were 0.68, 0.64, and 0.31. The standard errors of measurement and smallest detectable differences for all groups and for all muscles were unacceptably high. For amplitude results, ICC root mean squared initial and final values were ‘good’ to ‘excellent’ for all groups and all muscles, albeit with high measurement error. The ICCs for root mean squared slopes in all tests were ‘poor’ with extremely high measurement error. The poor between days reliability and high measurement error suggests that surface EMG should not be adopted to assess fatigue during multi joint sub-maximal isometric quadriceps testing.

Upper limb muscle imbalance in tennis elbow: A functional and electromyographic assessment

The purpose of this study was to investigate strength, fatigability, and activity of upper limb musculature to elucidate the role of muscular imbalance in the pathophysiology of tennis elbow. Sixteen patients clinically diagnosed with tennis elbow, recruited from a university hospital upper limb orthopedic clinic, were compared with 16 control subjects with no history of upper limb musculoskeletal problem, recruited from university students and staff. Muscle strength was measured for grip, metacarpophalangeal, wrist, and shoulder on both sides. Electromyographic activity (RMS amplitude) and fatigue characteristics (median frequency slope) of five forearm and two shoulder muscles were measured during isometric contraction at 50% maximum voluntary contraction. All strength measurements showed dominance difference in C, but none in TE. In tennis elbow compared to controls, hand/wrist and shoulder strength and extensor carpi radialis (ECR) activity were reduced (p < 0.05), while fatigue was normal. A global upper limb weakness exists in tennis elbow. This may be due to disuse and deconditioning syndrome caused by fear avoidance, and needs to be addressed in prevention and treatment. Activation imbalance among forearm muscles (reduced extensor carpi radialis activity) in tennis elbow, probably due to protective pain-related inhibition, could lead to a widespread upper limb muscle imbalance.

Muscle Maintenance by Volitional Contraction against Applied Electrical Stimulation

Muscle training exercises are needed for muscular endurance during spaceflight. This study was designed to investigate effects of volitional contraction against applied electrical stimulation on the muscular endurance of the proximal upper extremity. Thirteen healthy sedentary men were allocated into two groups. One group participated in a hybrid (HYB) exercise regimen in which the biceps brachii was stimulated as he volitionally extended his elbow, and the triceps brachii was stimulated as the volitionally flexed his elbow. The second group underwent a similar regimen in which the electrical stimulation (ELS) was alternatively delivered to the biceps brachii and then to the triceps brachii with the limb fixed. Forty-second surface electromyography (EMG) recordings at 50% maximum voluntary contraction (MVC) were made as baseline data at just before starting the training regimen, and again conclusion. The median frequency (MF) and mean power frequency (MPF) slopes with time were determined using power spectrum analysis. There were statistical significance only for the triceps in which the MF and MPF slopes in the HYB Group became less negative over the period of study (from -45.7+/-14.7 and -47.0+/-8.6%/min at baseline to -36.9+/-10.7 and -36.8+/-7.0%/min at the end of training, respectively). The corresponding values for these slopes in the ELS Group showed opposite tends with less marked changes of borderline significance for MF and of statistical significance for MPF. These results suggested that the HYB exercise regimen was capable of producing an improvement in triceps but not biceps brachii.

A proposta biomecânica para a avaliação de sobrecarga na coluna lombar: efeito de diferentes variáveis demográficas na fadiga muscular

OBJETIVOS: Analisar a fadiga de músculos lombares e determinar as variáveis demográficas relacionadas com a fadiga destes músculos. MÉTODOS: A atividade eletromiográfica (EMG) dos músculos iliocostal direito (IL-D), iliocostal esquerdo (IL-E), multífido direito (MU-D) e multífido esquerdo (MU-E) de 18 voluntários foi captada durante contrações isométricas sub-máximas. Valores de root mean square (RMS) e freqüência mediana (FM) foram correlacionados com o tempo de resistência isométrica (TRI). Slopes de RMS positivos e de FM negativos indicaram a ocorrência da fadiga muscular. Procedimentos de regressão múltipla foram realizados para verificar as variáveis demográficas relacionadas com a fadiga muscular. RESULTADOS: A fadiga foi identificada em todos os músculos e intensidades de contração (p&lt;0.01), exceto no MU-E em 5% na análise do slope de RMS. Diferenças significantes foram encontradas entre o TRI de 5% e 15% (p=0.01), 5% e 20% (p=0.0002). Níveis de fadiga mais elevados estiveram bilateralmente presentes nos músculos multífidos na análise do slopes de FM. A associação do TRI, da idade e da massa corporal dos voluntários foi identificada como fator determinante para a ocorrência da fadiga nos músculos avaliados. CONCLUSÕES: Intervenções destinadas ao tratamento de problemas na coluna lombar devem considerar os vários fatores responsáveis pela fadiga dos músculos desta região.

Strength and fatigability of selected muscles in upper limb: Assessing muscle imbalance relevant to tennis elbow

Purpose The aetiology of tennis elbow has remained uncertain for more than a century. To examine muscle imbalance as a possible pathophysiological factor requires a reliable method of assessment. This paper describes the development of such a method and its performance in healthy subjects. We propose a combination of surface and fine-wire EMG of shoulder and forearm muscles and wrist strength measurements as a reliable tool for assessing muscle imbalance relevant to the pathophysiology of tennis elbow. Methods Six healthy volunteers participated. EMG data were acquired at 50% maximal voluntary isometric contraction from five forearm muscles during grip and three shoulder muscles during external rotation and abduction, and analysed using normalized median frequency slope as a fatigue index. Wrist extension/flexion strength was measured using a purpose-built dynamometer. Results Significant negative slope of median frequency was found for all muscles, with good reproducibility, and no significant difference in slope between the different muscles of the shoulder and the wrist. (Amplitude slope showed high variability and was therefore unsuitable for this purpose.) Wrist flexion was 27 ± 8% stronger than extension (mean ± SEM, p = 0.006). Conclusion This is a reliable method for measuring muscle fatigue in forearm and shoulder. EMG and wrist strength studies together can be used for assessing and identifying the muscle balance in the wrist–forearm–shoulder chain.

Assessment of neuromuscular and haemodynamic activity in individuals with and without chronic low back pain

BackgroundBiering-Sørenson (1984) found that individuals with less lumbar extensor muscle endurance had an increased occurrence of first episode low back pain. As a result, back endurance tests have been recommended for inclusion in health assessment protocols. However, different studies have reported markedly different values for endurance times, leading some researchers to believe that the back is receiving support from the biceps femoris and gluteus maximus. Therefore, this study was designed to examine the haemodynamic and neuromuscular activity of the erector spinae, biceps femoris, and gluteus maximus musculature during the Biering-Sørenson Muscular Endurance Test (BSME).MethodsSeventeen healthy individuals and 46 individuals with chronic low back pain performed the Biering-Sørenson Muscular Endurance Test while surface electromyography was used to quantify neuromuscular activity. Disposable silver-silver-chloride electrodes were placed in a bipolar arrangement over the right or left biceps femoris, gluteus maximus, and the lumbosacral paraspinal muscles at the level of L3. Near Infrared Spectroscopy was used simultaneously to measure tissue oxygenation and blood volume changes of the erector spinae and biceps femoris.ResultsThe healthy group displayed a significantly longer time to fatigue (Healthy: 168.5s, LBP: 111.1s; p ≤ 0.05). Significant differences were shown in the median frequency slope of the erector spinae between the two groups at 90–100% of the time to fatigue while no significant differences were noted in the haemodynamic data for the two groups.ConclusionAlthough the BSME has been recognized as a test for back endurance, individuals with chronic LBP appear to incorporate a strategy that may help support the back musculature by utilizing the biceps femoris and gluteus maximus to a greater degree than their healthy counterparts.

A prospective randomised controlled trial of spinal manipulation and ultrasound in the treatment of chronic low back pain

Objectives To assess the short- and long-term effectiveness of spinal manipulation therapy, and to identify the effect of manipulation on lumbar muscle endurance in patients with chronic low back pain (LBP). Design A randomised controlled trial comparing manipulation and exercise treatment with ultrasound and exercise treatment. Setting An outpatient physiotherapy department. Participants One hundred and twenty patients with chronic LBP were allocated at random into the manipulation/exercise group or the ultrasound/exercise group. Interventions Both groups were given a programme of exercises. In addition, one group received spinal manipulation therapy and the other group received therapeutic ultrasound. Main outcome measures Pain intensity, functional disability, lumbar movements and muscle endurance were measured shortly before treatment, at the end of the treatment programme and 6 months after randomisation using surface electromyography. Results Following treatment, the manipulation/exercise group showed a statistically significant improvement ( P = 0.001) in pain intensity [mean 16.4 mm, 95% confidence interval (CI) 6.1–26.8], functional disability (mean 8%, 95% CI 2–13) and spinal mobility (flexion: mean 9.4 mm, 95% CI 5.5–13.4; extension: mean 3.4 mm, 95% CI 1.0–5.8). There was no significant difference ( P = 0.068) between the two groups in the median frequency of surface electromyography (multifidus: mean 6.8 Hz, 95% CI 1.24–14.91; iliocostalis: mean 2.4 Hz, 95% CI 2.5–7.1), although a significant difference ( P = 0.013) was found in the median frequency slope of surface electromyography in favour of spinal manipulation for multifidus alone (mean 0.3, 95% CI 0.1–0.5). A significant difference was also found between the two groups in favour of the manipulation/exercise group at 6-month follow-up. Conclusions Although improvements were recorded in both groups, patients receiving manipulation/exercise showed a greater improvement compared with those receiving ultrasound/exercise at both the end of the treatment period and at 6-month follow-up.

Recruitment Of Lower Extremity Musculature During The Biering-Sørenson Muscle Endurance Test

In 1984, Biering-Sørenson found that individuals with less lumbar extensor muscle endurance had an increased occurrence of first episode low back pain. As a result, back endurance tests have recently been recommended for inclusion in health assessment protocols. However, different studies have reported markedly different values for endurance times both in healthy individuals as well as those suffering from chronic low back pain (LBP). The contribution of the Biceps Femoris and Gluteus Maximus musculature during this test may affect the endurance times and has not been reported to date. PURPOSE To examine the neuromuscular activity and fatigue characteristics of the Erector Spinae, Biceps Femoris, and Gluteus Maximus musculature during the Biering-Sørenson Muscular Endurance Test (BSME) performed by a LBP population. METHODS Twenty-seven males (n = 14) and females (n = 13) reporting chronic LBP performed the BSME while surface electromyography was used to quantify neuromuscular activity. Disposable silver-silver-chloride electrodes were placed in a bipolar arrangement over the right Biceps Femoris, Gluteus Maximus, and Erector Spinae muscles. Average median frequency (MF) and root mean square (RMS) was calculated for each 1 second epoch throughout the muscular endurance test. RESULTS The average endurance time was 115.5 seconds, with no significant difference found between males and females. Using a Kruskal-Wallis test, significant fatigue was shown in all muscle groups with a univariate analysis displaying a mean gender difference in the median frequency slope of the Gluteus Maximus muscle. RMS values were not found to increase with time to exhaustion in any of the three muscle groups. CONCLUSIONS These preliminary data indicate that the chronic LBP population recruited the Erector Spinae, Gluteus Maximus and Biceps Femoris muscles from the start of the BSME, a test designed to assess back endurance.

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 = 2.8–8.8 Hz; 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.

Supplementation of general endurance exercise with stabilisation training versus general exercise only: Physiological and functional outcomes of a randomised controlled trial of patients with recurrent low back pain

Background. Determination of the mode of action of new exercise techniques in different back pain populations is lacking. The effectiveness of supplementing an exercise programme with stabilisation exercises concerning physiological and functional parameters in non-specific back pain patients is unknown. Methods. Randomised controlled trial, comparing a general trunk muscle endurance exercise approach enhanced with specific muscle stabilisation exercises (S&G group) with a general exercise approach only (G group). 55 patients with recurrent back pain were randomised in S&G group ( n = 29) and G group ( n = 26). Both groups received an 8-week exercise intervention and written advice. Paraspinal muscle strength and electromyographic fatigue of the erector spinae and multifidus were measured. Additionally, 3 functional speed tests were assessed. Outcomes were collected pre- and post-intervention. Findings. No differences were detected for any of the paraspinal fatigue characteristics either within or between groups, apart from a significant decrease in normalised median frequency slope of the erector spinae for the G group. Paraspinal muscle strength and all functional tests have demonstrated significant within-group improvements for both groups, without any between-group differences. Interpretation. An 8-week stabilisation exercise-enhanced approach presented equal benefits to a general endurance-based exercise programme for patients with recurrent non-specific back pain. A slightly steeper slope for the erector spinae in the G group was the only electromyographic fatigue alteration noted. Concomitant strength improvement probably reflects neural input changes rather than histochemical muscle changes. Physical exercise alone and not the exercise type was the key determinant for improvement in this patient group.

The value of electromyography of the lumbar paraspinal muscles in discriminating between chronic-low-back-pain sufferers and normal subjects.

The authors studied the surface electromyographic (EMG) spectrum of the paraspinal muscles of 350 subjects. They were classified by their history as normal (n=175), chronic low back pain (n=145), or past history (n=30). They pulled upwards on a floor-mounted load cell at two-thirds of their maximum voluntary contraction for 30 s, while the EMG was measured from the paraspinal muscles at the L4/L5 level. From the EMG signal the root-mean-square (RMS) was calculated. Power spectrum analysis allowed calculation of the median frequency slope, the initial median frequency (IMF), modal frequency, peak amplitude and spectral width at half peak amplitude (half-width). All of the variables of the chronic group were significantly different from the normal group, except the median frequency slope, RMS slope and mode. Half-width, age and maximum voluntary contraction were shown to be independent predictors of back pain classification. Half-width classified the subjects with a sensitivity of 0.65 and a specificity of 0.75.

Reliability of surface EMG during sustained contractions of the quadriceps

The purpose of this study was to determine test–retest reliability for median frequency (MDF) and amplitude of surface EMG during sustained fatiguing contractions of the quadriceps. Twenty-two healthy subjects (11 males and 11 females) were tested on two days held one week apart. Surface EMG was recorded from rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) during sustained isometric contractions at 80% and 20% of maximal voluntary contraction (MVC) held to exhaustion. Quadriceps fatigue was described using four measures for both MDF and amplitude of EMG: initial, final, normalized final and slope. For both MDF and amplitude, the initial, final and normalized EMG showed moderate to high reliability for all three muscle groups at both contraction levels (ICC = 0.59–0.88 for MDF; ICC = 0.58–0.99 for amplitude). Slope of MDF and amplitude was associated with a large degree of variability and low ICCs for the 80% but not the 20% MVC. MDF and amplitude of EMG during sustained contractions of the quadriceps are reproducible; normalized final values of MDF and amplitude show better reliability than slope.

Lumbar muscle fatigue and recovery in patients with long-term low-back trouble––electromyography and health-related factors

Objective. The aim was to explore the validity and reliability of EMG for assessing lumbar muscle fatigue. Design. Patients with long-term low-back trouble ( n=57) were compared to a healthy reference group ( n=55). Back muscle fatigue and recovery were studied in relation to health-related factors. Background. EMG spectral variables are important tools in the assessment of patients with low-back trouble. The influence of disability on these variables needs further investigation. Methods. EMG from the lower back muscles was recorded during a 45 s trunk extension at 80% of maximal voluntary contraction torque and during recovery. Disability was studied using questionnaires. Results. The reliability was high for maximal voluntary contraction torque and EMG initial median frequency, lower for the median frequency slope, and insufficient for median frequency recovery half-time. The patients had lower maximal voluntary contraction torque, higher initial median frequency at L5 level, flatter slope, and longer recovery half-time than the healthy subjects did. However, for subjects with significantly negative slope, indicating fatigue, there was no significant difference in slope between patients and healthy subjects, while, for subjects without such fatigue, patients showed significantly flatter slopes at L5. The sensitivity/specificity of the test was 86%/78%. The most significant variables selected with logistic regression were maximal voluntary contraction torque and initial median frequency at L5. Patients without significantly negative slopes during contraction and/or not exponential-like EMG recovery scored worse on several items concerning disability and self-efficacy. Conclusions. EMG spectral variables in combination with torque might be used for classification. For patients with long-term low-back trouble, the ability to fatigue the lumbar muscles sufficiently to obtain a significantly negative slope during an 80% maximal voluntary contraction may be a sign of better functioning. Relevance The ability to fatigue the back muscles during a test requiring a high force output might be achieved with back muscle training focused on increasing strength and self-efficacy.

Effect of fatigue on torque output and electromyographic measures of trunk muscles during isometric axial rotation

Ng JK-F, Parnianpour M, Richardson CA, Kippers V. Effect of fatigue on torque output and electromyographic measures of trunk muscles during isometric axial rotation. Arch Phys Med Rehabil 2003;84:374-81. Objectives: To examine the changes in torque output resulting from fatigue, as well as changes in electromyographic measures of trunk muscles during isometric axial rotation and to compare these changes between directions of axial rotation. Design: Subjects performed fatiguing right and left isometric axial rotation of the trunk at 80% of maximum voluntary contraction while standing upright. Setting: A rehabilitation center. Participants: Twenty-three men with no history of back pain. Interventions: Not applicable. Main Outcome Measures: Surface electromyographic signals were recorded from 6 trunk muscles bilaterally. The primary torque in the transverse plane and the coupling torques in sagittal and coronal planes were also measured. Results: During the fatiguing axial rotation contraction, coupling torques of both sagittal and coronal planes were slightly decreased and no difference was found between directions of axial rotation. Decreasing median frequency and an increase in electromyographic amplitude were also found in trunk muscles with different degrees of changes in individual muscles. There were significant differences ( P[lt ].05) between right and left axial rotation exertions in median frequency slope of external oblique, internal oblique, latissimus dorsi, and iliocostalis lumborum muscles, but no such difference was found in median frequency slope of rectus abdominis and multifidus muscles. This could be attributed to different functional roles among the muscles. Similar differences ( P[lt ].05) between right and left axial rotation in median frequency slope were also detected in the electromyographic amplitude slope of the trunk muscles. Coefficient of variation of the torque output and electromyographic activation in most of the trunk muscles increased during the fatigue process. Conclusion: The changing coupling torque, different fatigue rate, and activation changes of trunk muscles, as well as the increase in variability during fatiguing axial rotation exertion, could affect the internal loading and stability of the spine; this needs to be further quantified in future studies. [copy ] 2003 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

Between-days reliability of electromyographic measures of paraspinal muscle fatigue at 40, 50 and 60% levels of maximal voluntary contractile force.

To ascertain which percentage of maximal voluntary contractile force of the paraspinal muscles, when tested in a functional position, is most reliable for assessing electromyographic (EMG) fatigue changes. Ten healthy volunteers with no history of low back pain (six males). The surface EMG signal during 60-second isometric contractions of the paraspinal muscles at 40, 50 and 60% levels of maximal voluntary contractile force was captured and analysed. Each contraction level was assessed on two occasions, at least three days apart. The initial median frequency, the decline in median frequency slope and the increase in root mean square values were assessed for between-days reliability, using intraclass correlation coefficients (ICCs) and standard errors of measurements (SEM). Normalized median frequency and root mean square values were also assessed. At 40% of maximal voluntary contraction, little or no EMG fatigue changes occurred in any of the observed parameters. At 50% maximal voluntary contraction the initial mean frequency and root mean square changes proved highly reliable, with ICCs ranging from 0.74 to 0.86 and 0.75 to 1.00 respectively. Normalizing the root mean square data reduced the reliability, but this was still acceptable with ICCs 0.70-0.83. The median frequency decline slope proved less reliable with ICCs 0.24-0.74 for raw and 0.26-0.77 for normalized data. At 60% maximal voluntary contraction the initial mean frequency proved as reliable as initial median frequency at 50% with ICCs 0.70-0.89. The raw and normalized root mean squares (ICCs 0.43-0.89 and 0.30-0.87 respectively) and raw and normalized median frequency (ICCs 0.27-0.51 and 0.24-0.53 respectively) changes were less reliable than at 50% MVC. Overall, the reliability is better at the L4/5 than at the L2/3 level. Outcome measures taken at 50% maximal voluntary contraction are the most reliable in functional testing the paraspinal muscles of healthy volunteers. With initial median frequency and root mean square values being more reliable parameters than median frequency decline. At the L4/5 level, however, all parameters were acceptably reliable at 50% of maximum effort. However the between-subject variability of the median frequency decline and root mean square incline slopes suggest that these parameters are not yet fully suitable for monitoring fatigue changes during prolonged isometric contraction.

Recovery of electromyograph median frequency after lumbar muscle fatigue analysed using an exponential time dependence model.

The recovery of the median frequency of the power spectrum of the electromyogram (EMG) after fatigue has been studied to obtain reference data for healthy subjects (n = 55). In a seated position, the subjects performed a 45 s isometric back muscle contraction at 80% of maximal voluntary contraction, followed by 5 s contractions after 1, 2, 3, and 5 min in the recovery phase. In an additional reliability study (n = 11), this was performed six times. Surface EMG was recorded on four sites, namely, bilaterally from the lumbar muscles at the level of the first (L1) and fifth (L5) vertebrae. By non-linear regression, an exponential time dependence model was used to analyse the recovery of median frequency, giving recovery half-time as a resulting measure. Agreement with exponential time dependence was very good (coefficient of determination r(2) = 0.98) in the analysis of mean data (n = 55), with recovery half-times in the range 32-39 s on the four recording sites. Analysis of individual recordings, for which r(2) values in general were lower, revealed further details. Median values of the half-times in general agreed well with the half-times obtained from the analysis of mean data. Recovery half-time and median frequency slope during contraction were not correlated; this is in agreement with what may be expected from an exponential time dependence. Non-significant negative slopes occurred on apparently randomly selected sites, possibly indicating varying muscle coordination in the seated test method. The reliability of the half-time was not sufficient to allow for follow-up measurements on individuals, due to the fluctuations of the recovery data in recordings from individuals.

Influence of interelectrode distance and force level on the spectral parameters of surface electromyographic recordings from the lumbar muscles

In order to study the influence of interelectrode distance and force level on the electromyographic (EMG) spectral parameters and on their reliability, bipolar surface EMG measurements were performed on the lumbar muscles of 15 subjects. Two test contractions (45 s) at 40% of maximal voluntary contraction (MVC) were performed, one with 2 cm interelectrode distance and the other with 4 cm, followed by two contractions at 80% MVC with the same change in interelectrode distance. Increasing the interelectrode distance from 2 to 4 cm caused a significant mean decrease (about 8%) in the initial median frequency. It is shown that this shift is of an order of magnitude that may be expected from the bipolar electrode filter factor, and we further conclude that the observed individual variations in the shift are likely to be connected to fluctuations in the shape of the power spectrum and to variations in conduction velocity. No significant change was found for the median frequency slope when changing the interelectrode distance. Increasing the force (from 40 to 80% MVC) also caused a significant mean decrease (about 10%) in the initial median frequency. The median frequency slope became significantly more negative by more than 200%. We conclude, however, that torque fluctuations during the fatigue contractions should have had only minor influence on the standard error of measurement of the initial median frequency and of the median frequency slope.

Electromyography and ratings of lumbar muscle fatigue using a four-level staircase protocol

Objective. The purpose of the study was to explore whether a relationship existed between subject's experience and objective measurements of back-muscle fatigue in healthy subjects. This may be used as reference material later. Design. Muscle fatigue in the lower back was measured in healthy subjects using a staircase protocol. Background. Muscle fatigue measurement in the lower back is of importance when assessing patients with low-back pain. The subject's experience of muscle fatigue needs to be explored further. Methods. Twenty healthy subjects with informed consent participated. Borg CR-10 scale responses and electromyography measurements were obtained from low-back muscles of the subjects following a staircase protocol with sustained contractions at 20%, 40%, 60%, and 80% of maximum voluntary contraction. Results. A strong correlation was found between Borg ratings and force ( r=0.83) and between slope and force ( r=−0.72) but not between Borg ratings and slopes which was at best 0.43 (| r|⩽0.43). Conclusions. Although subjective ratings and slopes showed a low correlation; increases in force with a concomitant increase in fatigue was reflected both by the subjective ratings and the slope. One cannot use subjective ratings for prediction of the slope for this type of protocol. However, a more complete description of muscle fatigue needs both the slope and the Borg ratings. Relevance In clinical testing, both subjective and objective measures of muscle fatigue must be considered since they measure different aspects of muscle fatigue. The median frequency slope probably reflects “physical discomfort” but the Borg ratings add a factor of “lack of energy”.

Electromyographic Power Spectral Analysis of the Paraspinal Muscles: Reliability study

Summary Background and Purpose Despite previous studies on the reliability of median frequency (MF) and median frequency slope (MFS) as specific surface electromyography (SEMG) measures of back muscle function, there is still no general consensus regarding the results. A reliability study was carried out to determine the test-retest reliability (within and between-days) of SEMG power spectral analysis of paraspinal muscle activity in healthy subjects. Methods Fifteen volunteer subjects with no history of low back pain were tested. The unsupported trunk holding position was used to measure initial MF and MFS three times. Two measurements were taken on the same day with five minutes interval and the third was taken one week later. Results For initial MF, the high ICC values (0.82-0.92) and high level of agreement between measurements for multifidus and iliocostalis indicated high within and between-days reliability. For MFS, the ICC values (0.65-0.82) and high (for multifidus) to moderate (for iliocostalis) level of agreement between measurements demonstrated high within-day reliability for multifidus and moderate for iliocostalis. Between-days reliability of MFS was found to be good for multifidus and moderately good for iliocostalis with ICC values of 0.60-0.85 and high to moderate level of agreement. Conclusion In this study the SEMG power spectral analysis was shown to be a reliable method of measuring back muscle activity in an unsupported trunk holding test, when measurements are taken in healthy subjects by the same investigator.