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Work 51 (2015) 793–797 DOI 10.3233/WOR-141837 IOS Press

Trunk extensor muscle fatigue influences trunk muscle activities Tahere Seyed Hoseinpoora,∗, Sedighe Kahrizia and Bahram Mobinib a

b

Department of Physiotherapy, Faculty of Medical Sciences, Tarbiat Modares University, Tehran, Iran Department of Orthopedics, Tehran University of Medical Sciences, Tehran, Iran

Received 10 July 2013 Accepted 22 October 2013

Abstract. BACKGROUND: Trunk muscles fatigue is one of the risk factors in workplaces and daily activities. Loads would be redistributed among active and passive tissues in a non-optimal manner in fatigue conditions. Therefore, a single tissue might be overloaded with minimal loads and as a result the risk of injury would increase. OBJECTIVE: The goal of this paper was to assess the electromyographic response of trunk extensor and abdominal muscles after trunk extensor muscles fatigue induced by cyclic lifting task. METHODS: This was an experimental study that twenty healthy women participated. For assessing automatic response of trunk extensor and abdominal muscles before and after the fatigue task, electromyographic activities of 6 muscles: thorasic erector spine (TES), lumbar erector spine (LES), lumbar multifidus (LMF), transverse abdominis/ internal oblique (TrA/IO), rectus abdominis (RA) and external oblique (EO) were recorded in standing position with no load and symmetric axial loads equal to 25% of their body weights. RESULTS: Statistical analysis showed that all the abdominal muscles activity decreased with axial loads after performing fatigue task but trunk extensor activity remained constant. CONCLUSIONS: Results of the current study indicated that muscle recruitment strategies changed with muscle fatigue and load bearing, therefore risks of tissue injury may increase in fatigue conditions. Keywords: Electromyography, load, abdominal muscles

1. Introduction Muscle fatigue is one of the work-related complaints leading to absence from work and work incapability [1]. In fatigue condition the muscle is no longer able to sustain the required force or power output [2]. Fatigue can arise as a result of peripheral changes in the muscles and/or as a result of changes in the central nervous system (CNS) when motorneuron’s drive is not performed adequately [3]. Trunk cyclic flexion ∗ Corresponding author: Tahere Seyed Hoseinpoor, Department of Physiotherapy, Faculty of Medical Sciences, Tarbiat Modares University, Jalal Ale Ahmad Highway, Tehran, Iran. Tel.: +98 218 2884511 (ext: 3824 lab); Fax: +98 218 8006544; E-mail: Tahere. [email protected]

and extension have been associated with high mechanical stresses on the lumbar spine that expose individuals to a greater risk of low back problems [4,5]. In fatigue condition, different strategies in motor recruitment were chosen to maintain a given level of effort [6]. It has been suggested that during dynamic tasks, the central nervous system utilizes a complex strategy of recruiting muscles for sharing the load to decrease the occurrence of muscle injury by providing some recovery periods for the fatigued muscles [7]. Brereton and McGill [8] suggested that muscle fatigue may increase the risk of musculoskeletal injury by altering the muscle activation patterns and kinematics and they indicated that musculoskeletal injuries may also occur in consequence of random motor control errors which may induce by muscle fatigue.

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T.S. Hoseinpoor et al. / Trunk extensor muscle fatigue influences trunk muscle activities

Previous studies revealed that trunk muscle fatigue changes the anticipatory postural adjustment (APA) in healthy subjects [9]. Strange et al. [10] indicated that after muscle fatigue, dynamic postural stability is maintained by early APA onsets in some of the fatigue muscles during rapid unilateral arm-raising maneuvers. Other studies such as Grondin et al. [11] demonstrated that spinal column stiffness increases when spinal muscles become fatigue and trunk muscles baseline activities increase before a sudden loading in this situation. Many other previous studies investigated the effect of trunk muscle fatigue on postural strategies and muscle anticipatory response; however, none of them examine the activity of trunk muscles in axial loading conditions. While sustained neutral standing or sitting posture is one of the common body positions in workplace and daily activities, the stability of the spine in this position has been the topic of many studies. The present study aimed to assess the electromyography responses of the trunk extensor and abdominal muscles in standing posture after back extensor muscle fatigue.

2. Materials and methods 2.1. Subject The study was designed as an experimental study that twenty right handed healthy women with no history of low back pain participated. The mean (± sd) age, height and weight of group respectively were 26.3 (± 1.86) yrs, 161.59 (± 6.18) cm, 58.65 (± 7.66) kg. Before starting the test, all participants completed a medical health questionnaire. None of the participants reported having dysfunction of upper or lower extremity such as joint or muscle pain, any history of acute or chronic back pain in their life, surgery of lower extremity and trunk, lifting jobs or having professional sports activities, neurologic, systemic and heart diseases. All subjects were tested for scoliosis, kyphosis and lordosis of the spinal column. Exclusion criteria for the participants included not intended to continue the experiment in the middle of test and having pain more than 3 in evaluating day according to visual analog scale (VAS). 2.2. Apparatus Eight channels EMG surface recording (Biometrics LS900) were used for recording trunk extensor and abdominal muscles activity. After skin preparation,

bipolar Ag-AgCl, disposable surface electrodes were placed over the following muscles similarly to those suggested by Hodges (1996) [12]: TES (5 cm lateral to T9 spinous process), LES (3 cm lateral to L3 spinous process), LMF (3 cm lateral to L5 spinous process), TrA/IO (1 cm medial to anterior superior iliac spine), RA (1 cm above and 2 cm lateral to the umbilicus) and EO (15 cm lateral to the umbilicus on a 45 degree superior angle). A ground electrode was placed over the right styloid process of ulna. EMG sampling frequency was 1000 HZ with 20–450 HZ band width and 50 HZ notch filter and sensitivity was 500 µs. Total root mean square (RMS) of the recorded signals was obtained by Data link software. It should be mentioned that electromyographic activities of these muscles were obtained from the dominant (right) side of all the participants. Maximum voluntary contractions (MVCs) were collected from the trunk extensor and abdominal muscles. The subjects were asked to gradually ramp up the force until absolute maximum was reached, and then to hold for 2–3 s. Approximately 30 s of rest was given between MVC contractions. For the TES, LES and LMF MVCs, subjects were positioned in prone on a padded table with their hands beside their trunk. Two straps were used to fix the subject’s legs and the researcher held their shoulders in place to secure them while isometric MVCs of the trunk extensor were being performed. For the right TrA/IO, EO and RA muscle MVCs, subjects laid supine on the padded table, with their knees extended. Subjects were then asked to bend at the waist and to elevate their upper torso (to about 45 degree with the table) by flexing their abdominal muscles. Isometric MVC trials were performed in symmetrical flexion as well as twisting and flexing efforts to the right and left and flexing without twisting. For each direction, two straps fixed the subject’s legs and the researcher provided as much resistance at the shoulder(s) as was necessary to keep the efforts isometric. 2.3. Procedure Subjects were asked to stand in a comfortable standing posture with their hands hanging at the sides, electromyographic activities of the trunk extensor and abdominal muscles were recorded in two conditions in relax standing position. One of the conditions was when the participants just put on a vest (with 4 symmetric pockets in back and front) and another, when loads equal to 25% of subject’s body weight were put

T.S. Hoseinpoor et al. / Trunk extensor muscle fatigue influences trunk muscle activities

Fig. 1. Comfortable upright standing with a vest with four symmetric pockets.

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Fig. 2. Dead-lift exercise used to induce fatigue.

in the vest’s pocket symmetrically (sequence of two conditions were randomly selected by the subject before and after the fatigue task) (Fig. 1). EMG signals were recorded for 10 seconds in every condition. 2.4. Fatiguing task For fatiguing trunk extensor muscles, a dead-lift task was performed based on Strange et al. [10]. All participants performed dead-lift repetitions to exhaustion (based on borg scale) with the effort equal to 50% of their maximum voluntary efforts. The maximum isometric force exerted by the leg and back musculature was measured using an isometric leg and back dynamometer before starting the fatigue task (Fig. 2).

Fig. 3. TrA/IO muscle activity (RMS) in the trials (V = Standing and putting on a vest, VW = Standing and putting on a vest with the load equal to 25% body weight on the vest pockets, VF = Standing and putting on a vest after fatigue task, VWF = Standing and putting on a vest with the load equal to 25% body weight on the vest pockets after fatigue task).

3. Results 2.5. Data analysis Total root mean square of the EMG signals was used for analysis. All trial signals were normalized to the muscle MVC. SPSS statistical package (version 16) was used for statistical analysis. Kolmogrov-Smirniv test was utilized for assessing the normality of distribution of the data and a repeated measure ANOVA was used with the statistical significance set at P < 0.05.

According to the results of this study, symmetric axial loads (equal to 25% body weight) and trunk extensor muscles fatigue had no significant effect on the muscle activity (RMS) of trunk extensor and abdominal muscles. When the participants bore axial load after fatigue task (load × fatigue), activity of TrA/IO (Fig. 3), RA (Fig. 4) and EO (Fig. 5) decreased significantly (P < 0/05), but trunk extensor (TES, LES and LMF) activity did not change meaningfully (Fig. 6).

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T.S. Hoseinpoor et al. / Trunk extensor muscle fatigue influences trunk muscle activities

Fig. 4. RA muscle activity (RMS) in the trials (V = Standing and putting on a vest, VW = Standing and putting on a vest with the load equal to 25% body weight on the vest pockets, VF = Standing and putting on a vest after fatigue task, VWF = Standing and putting on a vest with the load equal to 25% body weight on the vest pockets after fatigue task).

Fig. 5. EO muscle activity (RMS) in the trials (V = Standing and putting on a vest, VW = Standing and putting on a vest with the load equal to 25% body weight on the vest pockets, VF = Standing and putting on a vest after fatigue task, VWF = Standing and putting on a vest with the load equal to 25% body weight on the vest pockets after fatigue task).

4. Discussions It has been proposed that spinal stability can be achieved by co-contracting the muscles surrounding the lumbar spine, particularly the oblique abdominals, transversus abdominis, eredor spinae, and multifidus [13]. Coordinated activation of the trunk extensor and abdominal muscles is necessary for stability of the lumbar spine. It has been revealed that relatively low levels of superficial muscle EMG activity was enough to maintain erect posture with or without loads of up to 223 N carried in each hand [14]. Results of the current study were in line with previous studies. The axial loads lesser than 170 N were used in the present study and no significant changes in EMG activities of the muscles were represented.

Fig. 6. Trunk extensor muscles (TES, LES, LMF) activity (RMS) in the trials (V = Standing and putting on a vest, VW = Standing and putting on a vest with the load equal to 25% body weight on the vest pockets, VF = Standing and putting on a vest after fatigue task, VWF = Standing and putting on a vest with the load equal to 25% body weight on the vest pockets after fatigue task).

Results of the this study revealed that abdominal muscles activity decreased after the trunk extensor muscle fatigue, when the subject bore axial symmetrical load equal to 25% their body weight. Figure 4 illustrates that in a fatigue condition, when the load was placed in the vest pockets, TES and LES activities increased but it was not significant. Neuromuscular control in spinal stability may change with muscular fatigue [15] and as previous studies mentioned force producing capacity of muscle and neuromuscular system decreases with fatigue [2], therefore fatigue can reduce sensitivity of muscular recruitment system and consequently spinal stability may diminish [15]. Hence, neuromuscular errors will arise with muscular fatigue and these can cause abnormal intervertebral movements which induce tissue injury [8]. In the present study, the results revealed that bearing loads equal to 25% of body weight, in a fatigue condition, can significantly reduce abdominal muscles activities; this might be related to the decreased sensitivity of muscular recruitment system and existing errors in relation to fatigue condition according to the previous mentioned suggestions [2,15]. As we know, human spinal column may encounter more demanding conditions than axial loading (which was studied in the present study) in daily activities such as asymmetric loadings and having movements with rotation and flexion of the spine. Therefore more attention should be placed about fatigue phenomenon in work and daily activities to lessen the risk of musculoskeletal injuries. As Fig. 4 illustrates, trunk extensor muscles activity were not significantly changed but little increase in LES and TES activity were seen. It seems that when

T.S. Hoseinpoor et al. / Trunk extensor muscle fatigue influences trunk muscle activities

trunk extensor muscles became fatigue, neuromuscular strategy changed [16] and CNS chose the safest strategy with the least cost to maintain spinal stability. EMG activities of the abdominal muscles were decreased to reduce the probable perturbation on the spinal column (which might be induced by increasing abdominal muscles activity) and then the system tried to maintain spinal stability by increasing the activation of theTES and LES muscles. Other researches can design to study the effect of muscular fatigue and axial load on the postural strategies to show that the body sways change in this situation or not. Many researchers have studied the effects of muscular fatigue on their agonist and antagonist activities. Sparto et al indicated that activity of internal oblique abdominis (IO) increase in the back extensor muscle fatigue task [17]. Increasing in bilateral IO and right erector spine muscles before sudden load was mentioned by Granata et al. [18] and Grondin et al. [11] saw that back extensor muscle fatigue caused an increase in baseline abdominal muscles activity (IO and EO) and raised TES and LES activities before sudden loading. They suggested that muscular coordination reduces with fatigue and motor strategy will change [11]. Despite the differences, results of the present study confirm this suggestion that muscular strategies change by muscular fatigue such as Sparto [17], Granata [18] and Grondin et al. [11] but in a different manner according to the task being evaluated.

5. Conclusions According to the results of the present study when trunk extensor muscles became fatigue, abdominal muscles activity reduced in the situation of greater challenge, this indicated that muscle recruitment strategy changed with muscle fatigue and load bearing. It should be mentioned that muscular system capacity for stability decreased in fatigue condition, therefore risks of tissue injury may increase.

Acknowledgments The authors wish to acknowledge the financial support and equipments provided by Tarbiat Modares University. Thanks are also extended to all subjects who volunteered to participate in this study.

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Trunk extensor muscle fatigue influences trunk muscle activities.

Trunk muscles fatigue is one of the risk factors in workplaces and daily activities. Loads would be redistributed among active and passive tissues in ...
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