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Journal of Back and Musculoskeletal Rehabilitation 24 (2011) 17–22 DOI 10.3233/BMR-2011-0270 IOS Press

Trunk muscle fatigue in subjects with a history of low back pain and a group of healthy controls measured by similarity index Saeed Talebiana, Majid Hosseinib,∗, Hossein Bagheria, Gholam Reza Olyaeia and Asghar Reazasoltanib a

Physical Therapy Department, Rehabilitation Faculty, Tehran University of Medical Sciences and Health Services, Tehran, Iran b Physical Therapy Department, Rehabilitation Faculty, Shahid Beheshti University of Medical Sciences (M.C), Tehran, Iran

Abstract. Background: Trunk muscle fatigue has high relevance in human performance. Most authors agree that the use of the median frequency is preferred as a fatigue indicator. Most work has to date, been done on dynamic fatigue measurements, using the similarity index (SI). Objective: Repeated trunk flexion-extensions were measured using the B200 Isostation. Muscle activity was recorded by surface electromyography (EMG) in order to evaluate fatigue of trunk flexor and extensor muscles. Twenty male university students participated in this pilot study, including 6 apparently healthy subjects with a history of low back pain (LBP), and 14 healthy matched controls. All participants were instructed to perform repeated trunk flexion-extension against 50% of their back extensors maximal volutary contractions (MVCs) resistance until they could no longer perform the task. The SI was calculated from quantitative analysis of EMG data recording during dynamic trunk flexion extension task. Median frequency was also measured before and after fatiguing contractions. Results: The results of this study revealed a significant decrease in the SI and median frequency measurements following fatigue in both groups (p < 0.05). Conclusion: The results of this study indicated that in both groups changes in SI measurements following fatigue are in agreement with the changes in median frequency measurements. Keywords: Similarity Index (SI), muscle fatigue, surface electromyography (EMG), B200 isostation

1. Introduction Low back pain is (LBP) one the most common types of musculoskeletal pain [4–7,36]. The decrease in back muscle performance following a first episode of LBP has been suggested to be an important factor in recurrent LBP [7–12,20]. Many investigators have reported alterations in motor control in low back pain patients [33,37]. Following fatigue, muscles are activat-

∗ Address for correspondence: Majid Hosseini MSc, PT., Physical Therapy Department, Faculty of Rehabilitation, Shahid Beheshti University (M.C), Damavand Avenue, across from Bo-Ali Hospital, 161693111 Tehran, Iran. Tel.: +98 21 775 617 22; Fax: +98 21 775 614 06. E-mail: [email protected].

ed by motor control in an inappropriate function such as improper muscle contractions and relaxations [5,6]. Fatigue assessment of trunk muscles is important because many researchers has indicated that patients with LBP developed a deconditioning syndrome that affects the back muscles [7,10,15,17,28]. Muscle fatigue may be considered with two components: peripheral and central fatigue [35]. 1. Peripheral fatigue includes any process occurring at or distal to the neuromuscular junction, or within the muscle itself [13]. 2. Central fatigue shows the failure of the nervous system to drive the muscle maximally. Fatigue is accompanied by changes in muscle electrical activity [3,12].

ISSN 1053-8127/11/$27.50  2011 – IOS Press and the authors. All rights reserved

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S. Talebian et al. / Trunk muscle fatigue in subjects with a history of low back pain and a group of healthy controls

Surface electromyography (EMG) has been used extensively to study muscle fatigue. EMG is able to detect muscle fatigue in the early beginning of a muscle effort although in absence of mechanical manifestations of fatigue [29,32]. To study muscle fatigue, spectral analysis has been used widely by many authors [12, 18,19]. Also Champagne et al. [8] used median power frequency changes to study back and hip extensor fatigue [8]. A decrease in the frequency content of the EMG signal is usually caused by muscle fatigue [36]. Median frequency divides the spectrum into two halves of equal power [4]. However, in most cases the analysis of EMG data has been limited to the examination of signal amplitude, and muscle timing [22]. It is rather difficult to measure voluntary motor control objectively [23]. In order to solve the problem of processing and analyzing of signals in a multichannel EMG recording, SI has been proposed for evaluation of voluntary motor output [25]. According to Lee et al. [23]. The SI is an objective analysis method to evaluate central nerve system’s (CNS) motor control of voluntary movement [23]. It is a quantitative analysis of EMG data derived from voluntary motor control and provides a quantitative measure of CNS output to muscles. EMG during voluntary movement task performance helps to know the way the way that CNS begins and arranges muscle contractions. It has been suggested that the SI should be applicable to the study of any voluntary movement that is carefully instructed, performed and repeated. SI has been used in different studies such as incomplete spinal cord injuries and shoulder dysfunction [23–27]. Research studies indicates a higher SI value suggesting better motor control and vice versa [24,27]. However, to our best knowledge, muscle fatigue has not yet been examined by the SI. The goal of this study was to investigate trunk muscle fatigue in apparently healthy subjects with a history of low back pain, and healthy control subjects by using the SI.

2. Methods 2.1. Participants A group of 20 healthy students from the Tehran University of Medical Sciences volunteers, without actual back complaints, was formed following a routine check-up in the Physiotherapy Department Centre. The trial subjects had not participated in previous re-

Table 1 The mean±S.D. and range of the subjects characteristics (n = 20) Variables Age (year) BMI (Kg/m2 )

Group I (n = 6) 26.6 ± 4.0 24.0 ± 3.5

Group II (n = 14) 25.2 ± 3.0 23.0 ± 3.0

Group I: apparently healthy subjects with the history of the LBP. Group II: healthy subjects as control group.

search and received detailed verbal and written explanations about the experimental course and procedure and signed Informed Consent forms. The inclusion criteria were: physically active men between the ages of 20 and 30 years, having a BMI les than 28 Kg/cm2 and no actual low back pain. Exclusion criteria were: current back pain and/or neuropathy (sciatica), medication, undergoing psychological treatment, and the existence of any obvious disease. The trial subjects were questioned about their history of back pain and range of motion level. History of back pain includes nonspecific low back pain (LBP) complaints which were present for no longer than 3 days, but had not recurred for at least 1 year [11,30,31]. Six apparently healthy participants with a history of LBP were recognized as “Group I”. Fourteen healthy subjects with no “history of LBP” were recognized as “Group II”. There were no significant differences between the groups regarding their age and body mass indexes (BMI) parameters. (Table 1). Both groups were not involved in an exercise training program and all subjects were right-handed. The study was approved by the Ethical Committee of Tehran University of Medical Sciences.

2.1.1. Electromyography Trunk muscle activities were recorded using an eightchannel portable EMG data logger (Type NO.P3X8, DataLog, Biometrics Ltd, Cwmfelinfach, Gwent, UK). The EMG signals were recorded with three preamplified bipolar active electrodes (Type NO.SX-230, Biometrics Ltd, Cwmfelinfach, Gwent, UK) with a fixed center to center inter electrode distance of 20 mm, with a 10 mm recording diameter, built-in differential amplifier with a gain of 1000, input impedance of 1015 Ohms, a common-mode rejection ratio of 110 dB at 60 Hz, bandwidth of 25–450 Hz, and sampling frequency of 1 KHz. Channel sensitivity was 3 microvolts. Reference electrode was placed on the right wrist. Signals were digitally recorded by the data loggers on 256 MB flash memory.

S. Talebian et al. / Trunk muscle fatigue in subjects with a history of low back pain and a group of healthy controls

2.1.2. Isostation dynamometer Trunk muscle strength was measured using B200 Isostation, (Isotechnologies Inc., Hillsborough, N.C., USA). Subjects stood in a standing position, with the lumbosacral junction aligned with flexion/extension axis of the B200 Isostation machine. The subject was firmly restrained according to the instructions recommended by the manufacturer. 2.2. Procedures All subjects participated in a pre-test measurement to become familiar with the trunk dynamometer apparatus and the test procedures. In the main study (after five days), a short warm up was carried out and the EMG electrodes were placed on trunk muscles. Then the maximum voluntary contractions (MVC) of back extensor muscles were determined by Isostation trunk dynalmometer. Participants were instructed to perform repeated trunk flexion-extension in Isostation trunk dynamometer against a resistance equal to 50% of their MVCs until they were not able to do the next repetitions. During the test procedures, EMG signals were recorded from trunk muscles before and after fatigue. The SI and median frequency measurements were derived from the collected data. After a general physical examination, the trial subject was requested to stand calm in an erect position.Then the pre-amplified bipolar active electrodes (Type NO.SX-230, Biometrics Ltd, Cwmfelinfach, Gwent, UK) were attached to the subjects’ trunk flexor and extensor muscles. Prior to electrode placement, the overlying skin on the muscles was carefully cleaned by using light skin abrasion and cleansing with alcohol. Electrodes were linked to the EMG apparatus through cables. The electrodes were firmly fixed to the skin to prevent electrode displacement during the test procedures. EMG activities were measured from the paravertebral muscles (Multifidus, Longissimus) and the External abdominal oblique muscle on the right side. The electrodes were positioned parallel to muscle fiber direction. The Multifidus electrode was placed at the level of iliac crest (L5 ) 3 cm from the midline of the back. Electrodes were aligned with the line connecting L1 to the posterior superior iliac crest [31]. The Longissimus electrode was placed 3cm lateral to the vertebral column at the level of L1 [21]. The External oblique abdominis electrode was placed just below the rib cage and along a line connecting the most inferior point of the costal margin and the contralateral pubic tubercule [31]. A reference electrode was fixed on the

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right wrist. After warming up and electrode placement, each subject was asked to stand in the Isostation trunk dynamometer, in such a way that L5 /S1 was at the level of the axis of trunk flexion-extension motion. The subject was fixed by straps at the level of his ankles, knees, hips, iliac crests, and shoulders. The subjects were asked to remain relax and motionless. The first ten seconds were initially allowed to pass with no contraction to ensure a stable EMG baseline [18]. Then the subject performed three MVCof back extensor muscles while the Isostation trunk dynamometer was mechanically locked in sagittal plane. Each isometric contraction lasted for five seconds, with one minute interval. The maximum value was considered as the subject MVC. When MVC value of back extensor muscles was determined, Isostation trunk dynamometer was placed in unlocked position (to permit dynamic trunk flexion-extension) and the resistance was adjusted at 50% of subject MVC. The subject exerted his force during dynamic trunk flexion-extension against a pad in front of his chest (for flexion) and a pad in the back against upper thoracic vertebrae, at the level of T4 (for extension). The subject was instructed to perform repeated trunk flexion-extension against 50% of his MVC until he was not able to do the next three repetitions or feel intolerable discomfort [14]. During the MVCs and repeated contractions, all subjects were encouraged verbally in order to do their best [34]. EMG signals were recorded from the beginning to the end of the test procedure while the subject performed dynamic trunk flexion-extension. Among the EMG signals, three EMG signals were selected (before fatigue) from the first five repeated trunk flexion-extension movements and also, three EMG signals were selected among the last five repeated movements (after fatigue). Beginning and ending of each EMG signal was determined [2]. Ensemble EMG data were produced by averaging to generate a vector (response vector: RV) with 3 response elements, one from each muscle being examined, presented in units of for each of the scalar elements [23]. Equation 1. Vector normalization: Ri represents the average of the EMG during the test procedure in the EMG channel (e.g channel 1, right Multifidus muscle, etc.). R1 = right Multifidus, R2 = right Longissimus, R3 = right Exernal oblique. The normalization of the response vector (RV) [Eqs (1) and (2)] in this way, allows to consider the relative activity in each muscle

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S. Talebian et al. / Trunk muscle fatigue in subjects with a history of low back pain and a group of healthy controls

quantitatively during the test procedures. Response vectors were used to produce prototype response vector (PRV). Equation 2. The SI is computed as the inner product between the two vectors where lRVl and lPRVl are the magnitudes of vectors RV and PRV, and RVi represents each element of vector RV [23]. Equation 2.

2.3. Statistical analysis Descriptive statistical analysis (mean, standard deviation and range) were calculated for the SI and median frequencies pre and post fatigue frequency for both groups. All statistics were computed using SPSS software program version 16.5 (SPSS Inc. Headquarters, 233 s, Wacker Drive, Chicago, Illinois 60606, USA). Paired t-test was used to compare median frequency and SI measurements between pre and post fatigue in both groups. Significance level was set at p < 0.05.

3. Results The SI and median frequency values decreased significantly in all subjects in both groups compared with the pre-fatigue measurements (p < 0.05). Table 1 shows the characteristics of the subjects participated in this study. As Table 2 shows, there were significant reductions in all trunk muscles’ median frequency measurements in both groups (p < 0.05). In healthy subjects, Multifidus muscles showed greater reduction (17%) compared to Langissimus and External oblique muscles (7%). While in subjects with the history of low back pain, Longissimus muscles indicated maximal reduction (15.74%) compared to Multifidus and External oblique muscles (11%). The result revealed that SI was also shifted toward the lower values in all subjects following fatigue (p < 0.05) (Table 3). The reduction in the percentage of SI in subjects with the history of low back pain and healthy subjects were almost the same (9.2% and 7.9% respectively).

4. Discussion Several investigators have reported differences in neuromuscular control in patients with mechanical low back pain [33]. In spite of diversity in methods, many authors believe that the motor control is a key factor in the aetiology of low back pain [9]. Following fatigue postural strategies changes both in healthy subjects and subjects with a history of low back pain. For example, Allison et al. [1] reported alterations in anticipatory postural adjustments in normal subjects [1]. Also Hart et al. [16] reported that following paraspinal muscle fatigue, participants with low back pain exhibited different strategies from healthy participants [16]. Low back pain people increased muscle activation to provide trunk stability, which may increase the risk of fatigue in these muscles during prolonged tasks. Therefore, persons with a history of low back pain often experience excessive fatigue in the muscles that support the trunk and pelvic belt. The results of the present study indicated a decrease in SI measurements after fatigue in both groups. However, statistical analysis showed no differences between SI measurements in both groups pre and post fatigue. Fatigued subjects appeared to expend more energy and used asymmetrical muscles recruitment patterns during the test procedures. Following fatigue, subjects use accessory muscles or the same muscle at different level of activity, which may affect the SI. Some limited evidence suggests that in muscle groups with more than one tendon insertion central nervous system has more freedom to generate the same joint torque through different muscle activation patterns. The back extensors have many muscle slips with different moment arms. Neuromuscular activation patterns can be altered within a muscle or between synergistic muscles in such a way that an increase in the activation of one muscle group is associated with deactivation of another muscle group for a short period of time. During the test procedure, load sharing occurs between these synergistic muscles. It is possible that variability of load sharing increases with muscle fatigue. It was hypothesized that load sharing between synergistic muscles acts as a muscle strategy to delay muscle fatigue. When muscles become fatigue, synergistic muscles needed to perform the motor task. However, synergists that produce considerable EMG signal amplitudes will change the EMG distribution and SI [23]. Piper was the first who noticed frequency reduction in EMG following a sustained contraction [7]. Also the results of this study showed a decrease in median

S. Talebian et al. / Trunk muscle fatigue in subjects with a history of low back pain and a group of healthy controls

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Table 2 Median frequency measurements (Med Freq ± S.D.) and range of all muscles measured before and after induced fatigue in both groups Group I (n = 6) Med Freq before fatigue Med freq after fatigue P value Mean ± S.D. Range Mean ± S.D. Range Multi-fidus 93.66 ± 15.06 (71–113) 77.83 ± 18.42 (44–98) 0.037 Longissimus 73.83 ± 11.85 (57–87) 68.66 ± 10.05 (55–81) 0.035 External oblique 52.83 ± 8.84 (45–69) 49 ± 7.48 (14–63) 0.042 Muscles

Group II (n = 14) Med freq before fatigue Med freq after fatigue P-value Mean ± S.D. Range Mean ± S.D. Range 101.45 ± 27.38 (52–148.81) 90.29 ± 27.92 (32–133) 0.033 77.70 ± 20.46 (39–102) 65.47 ± 19.34 (23–93) 0.034 58.05 ± 10.75 (41.78–68) 51.16 ± 7.92 (40–67) 0.044

Group I: apparently healthy subjects with the history of the LBP. Group II: healthy subjects as control group. P < 0.05 was considered significant. Table 3 Similarity Index measurements (SI ± S.D.) and range of all subjects measured before and after induced fatigue in both groups Group I (N = 6) SI before fatigue SI after fatigue Mean ± S.D. Range Mean ± S.D. Range 0.54 ± 0.12 (0.42–0.73) 0.49 ± 0.12 (0.33–0.57)

P value 0.043

Group II (N = 14) SI before fatigue SI after fatigue Mean ± S.D. Range Mean ± S.D. Range 0.63 ± 0.15 (0.38–0.78) 0.58 ± 0.170 (0.34–0.78)

P value 0.038

Group I: apparently healthy subjects with the history of the LBP. Group II: healthy subjects as control group. P < 0.05 was considered significant.

frequency measurements after fatigue in both groups which is in agreement with previous studies [12,18, 19]. Median frequency is the frequency that divides the spectrum into two halves of equal power [20]. There are two sub systems of paraspinal muscles, the globalmobilizing system and the local stabilizing system. The Multifidus muscles are assigned to the local system and stabilize the joints. In contrast, the Longissimus muscles are assigned to the global system and force exertion. In healthy sujects Multifidus muscles showed greater reductioin in median frequency. Multifidus muscle is a local stabilizer which imits excessive motion across individual motion segments. In subjects with a history of low back pain, Longissimus muscles showed greater reduction in median frequncy. One advantage of this study is the use of dynamic contraction. In most EMG-based back muscle fatigue assessments a sustained static effort at high force level is used. However, tasks actually performed in daily living are usually intermittent contractions at a low to moderate force level [20]. Isometric contractions are not usual in most daily activities. Therefore, in this study submaximal (50% of MVC) dynamic contractions were used to fatigue trunk muscles. In addition, abdominal muscles were included in the test procedures. The main finding of this preliminary study was that in both groups changes in SI measurements following fatigue was in agreement with the changes in median frequency measurements. In other words, both the SI and median frequency reduced significantly after fatigue. Therefore, it would be possible to consider SI as an alternative method to study muscle fatigue

which may help to open a new insight in fatigue studies with regard to motor control. However, in this study SI was evaluated only in male subjects, further studies with a greater population are required to look at the differences between genders. Researchers may also interested in comparing different athlets with different training backgrounds by the SI. For example power lifting which requires muscle strengths with swimming which requires coordination.

Acknowledgments The authors would like to acknowledge the contribution made by the medical staff at the rehabilitation faculty of Tehran University of Medical Sciences.

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