The relationship between flexibility and EMG activity

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Journal of Electromyography and Kinesiology 19 (2009) 746–753 www.elsevier.com/locate/jelekin

The relationship between flexibility and EMG activity pattern of the erector spinae muscles during trunk flexion–extension Fahime Hashemirad a,b,c,*, Saeed Talebian a, Boshra Hatef c, Amir H Kahlaee d a

Physical Therapy Department, Rehabilitation Faculty, Tehran University of Medical Sciences and Health Services, Tehran, Iran b Saba Spine Specific Physical Therapy Clinic, University of Social Welfare and Rehabilitation Sciences, Tehran, Iran c ‘Sports Medicine Research Center, Tehran University, Tehran, Iran d Physical Therapy Department, Medical School, Tarbiat Modares University, Tehran, Iran Received 23 August 2007; received in revised form 17 February 2008; accepted 20 February 2008

Abstract Background: Movements in the lumbar spine, including flexion and extension are governed by a complex neuromuscular system involving both active and passive units. Several biomechanical and clinical studies have shown the myoelectric activity reduction of the lumbar extensor muscles (flexion–relaxation phenomenon) during lumbar flexion from the upright standing posture. The relationship between flexibility and EMG activity pattern of the erector spinae during dynamic trunk flexion–extension task has not yet been completely discovered. Objective: The purpose of this study was to investigate the relationship between general and lumbar spine flexibility and EMG activity pattern of the erector spinae during the trunk flexion–extension task. Methods: Thirty healthy female college students were recruited in this study. General and lumbar spine flexibilities were measured by toe-touch and modified schober tests, respectively. During trunk flexion–extension, the surface electromyography (EMG) from the lumbar erector spinae muscles as well as flexion angles of the trunk, hip, lumbar spine and lumbar curvature were simultaneously recorded using a digital camera. The angle at which muscle activity diminished during flexion and initiated during extension was determined and subjected to linear regression analysis to detect the relationship between flexibility and EMG activity pattern of the erector spinae during trunk flexion–extension. Results: During flexion, the erector spinae muscles in individuals with higher toe-touch scores were relaxed in larger trunk and hip angles and reactivated earlier during extension according to these angles (P < 0.001) while in individuals with higher modified schober scores this muscle group was relaxed later and reactivated sooner in accordance with lumbar angle and curvature (P < 0.05). Toe-touch test were significantly correlated with trunk and hip angles while modified schober test showed a significant correlation with lumbar angle and curvature variables. Conclusion: The findings of this study indicate that flexibility plays an important role in trunk muscular recruitment pattern and the strategy of the CNS to provide stability. The results reinforce the possible role of flexibility alterations as a contributing factor to the motor control impairments. This study also shows that flexibility changes behavior is not unique among different regions of the body. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Flexibility; Erector spinae muscles; Flexion–relaxation

1. Introduction

* Corresponding author. Address: Saba Spine Specific Physical Therapy Clinic, University of Social Welfare and Rehabilitation Sciences, No. 25, 4th Street, Fatemi Avenue, Tehran, Iran. Tel.: +98 21 88965652. E-mail address: [email protected] (F. Hashemirad).

1050-6411/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2008.02.004

Back muscle function electromyography (EMG) tests are commonly used in the assessment of physical performance capacity improvement of patients with lumbar complaints (Ritvanena et al., 2007). In healthy subjects, low back muscles are electrically silent during upright standing

F. Hashemirad et al. / Journal of Electromyography and Kinesiology 19 (2009) 746–753

and in full trunk flexion. During trunk flexion with straight knees, the surface EMG activity initially increases during the initial phase of the flexion and then decreases with increments of flexion angle and relaxation of the muscles occurs in the outmost range of flexion; this phenomenon is referred to as the flexion–relaxation phenomenon (FRP) (Floyd and Silver, 1951; Ahern et al., 1988; Andresson et al., 1996; Kippers and Parker, 1984; McGill and Kippers, 1994; Paquet et al., 1994; Schultz et al., 1985; Triano and Schultz, 1987). During extension, EMG activity pattern is reversed until the erect posture is attained (Ritvanena et al., 2007). Several studies have shown that tension in the stretching passive tissues (dorso-lumbar fascia and posterior ligaments) is sufficient to support the gravitational load of the trunk in deep flexion (Allen, 1948; Floyd and Silver, 1955; Golding, 1952). Indeed, FRP is indicative of increased load sharing on passive structures, which allows the paraspinal muscles to decrease their activity (Colloca and Hinrichs, 2005). The electrical signal reduction found in healthy subjects during lumbar spine flexion has been interpreted as the relief of the extensor musculature from their moment supporting role by the passive tissues, particularly the posterior spinal ligaments (McGill and Kippers, 1994). Physical activities involving full trunk flexion are common among activities of daily living, occupational demands and sports. Thus, knowledge of the biomechanics and clinical implications of trunk flexion is of great clinical importance (Colloca and Hinrichs, 2005). The spinal stabilizing system is conceptualized by Panjabi (1992) to consist of three subsystems: (a) passive (vertebral bones, intervertebral discs, ligaments and fascia), (b) active (muscles and tendons) and (c) neural control unit. In the normal state, the three subsystems work together to provide the needed mechanical stability. The various components of the spinal column provide information about the static and dynamic mechanical status of the spine. The neural control unit estimates the stability demands and generates appropriate muscular recruitment pattern to match the needs of the corresponding status (Panjabi, 1992). Therefore movements in the lumbar spine, including flexion and extension are governed by a complex neuromuscular system involving both active and passive units (Kaigle et al., 1998). Investigation of load sharing pattern between the erector spinae musculature and passive lumbar spine tissues may shed light on the nature of the FRP from both a biomechanical and clinical point of view. It seems that EMG activity pattern of the erector spinae may have been influenced by flexibility; therefore different flexibilities may elicit different neuromuscular responses which is manifested by the different load sharing strategies and reflexive muscular responses of afferents from the viscoelastic tissues. Shin and his collogues in 2004 studied the effects of combined trunk and knee flexion angles, and individual’s flexibility on FRP during static trunk flexion tasks both with and without external load, but, to our knowledge, the investigation of the activity pattern of the erector spinae during a

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dynamic task has not been included in any of the previous studies on the effect of these factors on FRP. The purpose of this study was to investigate the relationship between flexibility and EMG activity pattern of the erector spinae during the dynamic trunk flexion–extension task. It was hypothesized that different flexibilities will elicit different neuromuscular responses defined as an altered period of electrical silence of the erector spinae muscles during the flexion and extension task. 2. Materials and methods 2.1. Subjects Thirty healthy female college students were recruited from Tehran university of Medical Science and Health Services and participated voluntarily. All participants were free from chronic and current back problems and signed written informed consent after being introduced to the nature of the study. The anthropometric data and flexibility scores of the subjects including the results of the toe-touch and modified schober tests are presented in Table 1. 2.2. Instrumentation Surface EMG data were collected using a 4-channel electromyography device (Medelec, Promiere model). The EMG signals were detected by pregelled Ag–AgCl electrode pairs applied at the L3-4 level over the left erector spinae musculature (about 4 cm lateral from midline). Center to center electrode distance was 2.5 cm; electrodes were longitudinally oriented along the fibers of the muscle. A reference electrode was taped on the left wrist. The EMG signals were amplified by 1000 with a frequency band pass of 20–500 Hz, Gain 100lv/Div., 80 dB signal to noise ratio and CMRR of 90 dB. Maximum acceptable skin impedance level was set at 5 kX. Sampling rate of recording was 1000 Hz and the data were digitized and stored by a 12-bit A/D board. Angular variables were estimated by a digital camera (JVC – GZ-MG50AS) placed 1 m away from the subject at waist level with a direct view of the subject’s right side in the sagittal plane. The camera collected kinematics data at the rate of 25 frames per second. The markers used to measure the segments angles were attached to the subjects as follows: three circular markers were attached to the right greater trochanter, lateral midline along the iliac crest and the lower palpable edge of the rib cage (Solomonow et al., 2003a). Three spherical markers were adhered to T12, L3 and S2 spinous processes. Fig. 1 is a schematic representation of a subject instrumented with the markers. The lateral markers set-up

Table 1 Anthropometric measures and flexibility tests results of the subjects (N = 30) Variables

Mean (SD)

Range

Age (year) Height (m) Weight (kg) BMI (kg/m2) TTT (cm) MST (cm)

22.3 (3.3) 1.6 (0.05) 55.9 (6.07) 21.2 (2.3) 3.2 (11.2) 7.7 (1.2)

18, 28 1.53, 1.74 48, 69 17, 27 26, +19 5.5, 10

TTT: toe-touch test; MST: modified schober test.

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Since people with different flexibilities are supposed to have different ranges of motion of trunk flexion and extension, no predetermined time was chosen for the task duration and the subjects were just ask to move not too fast and not too slow to attain a natural dynamic state which could mean different durations for different individuals. The only dictated time was the duration of holding the full flexion position (static phase of the task) as 4 s. Later, the analysis of the subject’s motion revealed that all flexion and extension phases were limited within the range 3–5 s with the total time of 10–14 s for the whole task. After introducing the task to the subjects and making sure of the accuracy of the maneuver, subjects performed two trials with a 30–50 s interval, one of which was chosen depending on signal quality for data analysis. 2.4. Data analysis

Fig. 1. Schematic representation of a subject performing the forward bending task and the measured angles where a, b and c are the trunk, hip and lumbar angles, respectively, and h is the lumbar curvature.

allowed measurement of the trunk, hip and lumbar flexion angles and the spinal markers made the measurement of the lumbar curvature possible. Video and EMG data were synchronized by an electrical circuit which triggered them at the same time. 2.3. Protocol After attaching the markers to the skin, toe-touch and modified schober tests were performed to measure the general (Kraus and Hirschland, 1954; Nicholas, 1975; Brodie et al., 1982) and lumbar spine flexibility (Shober, 1937; Macrae and Wright, 1969; Fitzgerald et al., 1983), respectively. To test the natural state of these flexibilities, subjects performed the tests without any prior stretching or warm-up (Kippers and Parker, 1987). To perform the toe-touch test, participants stood on a 20-centimeter high platform and maximally bent forward towards the toes with the knees kept extended. Then the vertical distance between the tip of the middle fingers and the floor was recorded. Not reaching the toe levels was assessed as a positive score and reaching beyond this level yielded a negative one. The modified schober test score was defined as the difference between the distance from T12 to S2 spinous processes in the erect and full trunk flexion postures. The forward bending maneuver was performed while sitting on a stool with the feet wide apart to allow hanging of the arms and ensuring maximal forward bending range. The skin was cleaned with alcohol prep pads before EMG electrodes attachment. The electrodes and skin markers were placed as described above and the signal was checked prior to test trials to make sure of proper marker detection and lack of EMG signal noises. The subjects stood just behind a horizontally drawn line on the ground barefoot with their feet pelvis-width apart, their wrists hooked together in the front of their body, and their knees kept straight and bent forward as far as possible at a natural speed to provide a natural dynamic state (Sihvonen, 1997).

The recorded EMG data were full-wave rectified and smoothed with the time constant of 50 ms to yield linear envelops. The EMG values were normalized using the peak EMG magnitude during the task. A threshold level of 5% of this magnitude was used to determine the onset and the end of the flexion– relaxation period. The onset of the flexion–relaxation phenomenon (EMG-Off) was defined as the point at which the magnitude of EMG signal got less than the threshold level and the end point of the phenomenon (EMG-On) during the extension phase was defined as the point at which EMG signals amplitude exceeded the threshold level (Olson et al., 2004) (Fig. 2). The video data were analyzed using Ulead video studio software (version 7) to match the frame of the video with the corresponding EMG signals (frames of EMG-Off and EMG-On). Measurement of the angles of interest in each specific frame was done with Auto CAD software (2006). The trunk, hip and lumbar angles were measured by lateral markers. Trunk angle was defined as the angle between the vertical line crossing the ilium marker and the line connecting the rib and ilium markers. The hip angle was defined as the angle between the vertical line crossing the ilium marker and the line connecting the greater trochanter and ilium markers while the angle of lumbar flexion was defined as the difference between the two previous ones (trunk angle–hip angle) (Solomonow et al., 2003a) (Fig. 1). The lumbar curvature was calculated using Youda’s formula (h = 4 arctg2H/L), where H is the depth of the curve at its midpoint, L is the length of the line connecting the end points of the curve and h is the angle at the center of the circle formed by the radii to points of curve at T12 and S2 spinous processes (Youdas et al., 1995, 1996) (Fig. 1). The dependent variables included trunk, hip and lumbar angles and the lumbar curvature at the frames corresponding to

Fig. 2. Typical recording of EMG activity during flexion–extension. Raw EMG and the linear envelope were used to estimate EMG-Off and EMGOn points. The extension phase used for normalization of the EMG linear envelope is also provided.


the upright posture, EMG-Off, end range of flexion and EMGOn. Pearson’s correlation coefficient and t-tests of the differences between the means of the first and second trials were used to test the reliability of the measurements. The relationship between flexibility tests results, anthropometrics and measured angles was determined by Pearson correlation while multiple linear regression analysis (stepwise model) was used to detect the linear association between flexibility tests results and dependent variables at the points of the EMG-Off and EMG-On. The alpha level was set at 0.05.

3. Results The correlation coefficients for inter-trial reliability tested in six of the subjects, ranged from 0.83 to 0.99. No significant differences existed between the means of the two trials, so no averaging was used and just one of the trials, depending on signal quality, was chosen. Correlation coefficient between the flexibility tests results and anthropometric measures are shown in Table 2. None of the anthropometric measures correlated significantly with flexibility tests results excepting the weight and BMI which were at the border line of significance with modified schober test (r = 0.40, P < 0.05 and r = 0.42, P < 0.05, respectively). There was no significant correlation between toe-touch and modified schober tests results (r = 0.26, P = 0.1). Table 3 shows the means and standard deviations of angular variables at the erect and full flexion postures and correlation coefficients of flexibility test results with these variables. No significant correlation was found between these variables and flexibility test results at the erect posture. The correlation between trunk and hip angles at full flexion with toe-touch test was found to be significant (r = 0.79, P < 0.001 and r = 0.75, P < 0.001, respectively), but no significant correlation was found between these variables and modified schober test which was significantly correlated with lumbar flexion and curvature (r = 0.47, P < 0.01and r = 0.45, P < 0.05, respectively). Individuals with higher general flexibility had greater trunk and hip angles at full flexion while higher lumbar spine flexibility provided greater lumbar angle and curvature at this point.

Table 2 Correlation coefficients between flexibility tests results and anthropometric measurements Variables

TTT

MST

Age Height Weight BMI TTT MST

0.03 0.21 0.08 0.21 – 0.26

0.11 0.07 0.40* 0.42* 0.26 –

TTT: toe-touch test; MST: modified schober test. * P < 0.05.

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Table 3 Correlation coefficient between flexibility tests results and angular measurements in the erect and full flexion postures Measurement

Position

Mean (SD)

Pearson correlation TTT

MST

Trunk angle

Erect Full flexion

12.2 (7.1) 120.5 (17.3)

0.08 0.79***

0.1 0.17

Hip angle

Erect Full flexion

8.6 (4.6) 6.3 (17.8)

0.17 0.75***

0.12 0.05

Lumbar angle

Erect Full flexion

3.6 (6.7) 54.1 (8.5)

0.03 0.03

0.19 0.47**

Lumbar curvature

Erect Full flexion

61.03 (13.2) 91.9 (14.8)

0.26 0.03

0.22 0.45*

TTT: toe-touch test; MST: modified schober test. * P < 0.05. ** P < 0.01. *** P < 0.001.

The relationship between the toe-touch and modified schober tests results with variable angles at EMG-Off and EMG-On is demonstrated in Tables 4 and 5. Toe-touch test results were linearly related to trunk and hip angles at EMG-Off (r2 = 0.61, P < 0.001 and r2 = 0.55, P < 0.001) and EMG-On (r2 = 0.35, P < 0.001 and r2 = 0.40, P < 0.001). No linear relationship between toe-touch test and lumbar angle and curvature was noted (Table 4) (Figs. 3 and 4). Modified schober test results were related to lumbar angle and curvature at EMG-Off (r2 = 0.20, P < 0.05 and Table 4 The result of linear regression analysis of toe-touch test and EMG activity pattern at the points of EMG-Off and EMG-On Unstandardized coefficients B (standard error)

P-value

R2

Model

Constant TTT

115.8 (2.02) 1.16 (0.17)

0.001