The Effect of Fatigue on the Postural Balance of Young Women Using Electro- myography of Lower Extremity Muscles

* Corresponding Author: Sadegh Naserkhaki, PhD. Address: Department of Medical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. Tel: +98 (937) 6636103 E-mail: snkhaki@gmail.com 1. Department of Medical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. 2. Department of Medical Engineering, Abhar Branch, Islamic Azad University, Abhar, Iran. Afsaneh Ghodsi1 , *Sadegh Naserkhaki1 , Ali Tanbakoosaz2


Introduction
he ability to maintain body posture is essential not only during daily activities such as walking, but also for the proper performance of complex sports skills. Its measurement in laboratory and clinical settings is used as a tool to assess the stability and neuromuscular control in healthy and injured individuals [1]. Maintaining balance is T the result of a dynamic combination of internal and external forces and environmental factors [2,3]. Balance or postural control body is the consciousness of the body on the status of its different parts in relation to each other and in relation to space, with the help of information received from the visual and vestibular systems, proprioception of the joints and skin, basal ganglia and cerebellum [4]. It has three types of static, semi-dynamic and dynamic [5]. Static balance refers to the body's ability to maintain stability in static mode or in a standing position [6,7].
Studies have shown that the balance impairment, injury and reduced performance occurs at the time of fatigue [8,9]. Fatigue is one of the most important factors in reducing muscle strength and stability. Several studies have shown that fatigue has negative impact on postural control [10][11][12][13][14]. Fatigue is a common phenomenon in sports and daily activities [2,15]. It can affect the stability of the body by affecting environmental factors and central system mechanisms [16]. Fatigue refers to a condition that the efficiency of the body to perform a particular activity is reduced and the body is unable to continue working [17]. The muscle fatigue is the inability of muscle to respond to the stimuli it receives. As a result, the muscle is unable to respond to signals transmitted from the nervous system [18].

Methods
Participants were 10 young women (mean age, 26.4±3.01 years; mean weight, 65.5±5.75 kg; mean height, 165±2.61; BMI, 24.08±2.08 kg/m 2 ) with no history of lower limb injury. To evaluate static balance, the stork test was used. The postural control was determined through calculating foot plantar Center of Pressure (COP) on AMTI force plates (GCO model). Force plate results were calculated at 1000 Hz sampling rate by Nexus software. Then, the mean displacement in the medial/lateral and anterior/posterior directions was calculated to obtain the impaired balance point. To assess fatigue, electrical activity was recorded with a biometric Electromyography (EMG) device in seven muscles active in the lower extremities.
At first, the subject lifted her left leg from the ground according to the stork test and raised it to her knees and stood with one leg on the right foot on which the electrodes were installed. Synchronization between force plate data and EMG results were done by a synchronizer. The test continued until the person was able to stand on one leg. The test was terminated when the subject expressed her exhaustion or was unable to maintain her balance and tended to put the other foot on the ground. All these steps and the installation of the electrodes were done again on the left foot of the subjects. The results of the EMG with a sampling frequency of 1000 Hz were transferred to the Biometrix device and then analyzed in MATLAB software. The raw signal was normalized to Maximum Voluntary Contraction (MVC).
The amplitude of the EMG signal was represented by calculating the overall Root Mean Square (RMS). The raw signal from the EMG device was divided into two groups of before fatigue and after fatigue, based on the stages of muscle fatigue. Then, the last 30 seconds of each signal were used due to the importance and imbalance of the subject, and analyzed under three 10-second time intervals. At each stage, three parameters of muscle power (P), median frequency (F) and RMS were obtained from raw EMG signals and were considered as fatigue indicators for each muscle. Finally, the normality of the data was assessed in SPSS v.16 using Shapiro-Wilk and Kolmogorov-Smirnov tests. The significance level of the parameters was then tested by Mann-Whitney U test.

Results
It was observed that the mean COP displacement increased after fatigue and imbalance. There was a significant difference between the measurements before and after fatigue in the P and RMS parameters (P<0.05), but not in F parameter. Hence, it can be said that fatigue has a significant effect on muscle strength and RMS, but does significantly affect the median EMG frequency of muscle. Moreover, fatigue in-

Discussion
The results showed that there was a negative relationship between fatigue of the lower extremity muscles and balance impairment. This means that by increasing the level of fatigue in the muscles, balance impairment increases. Studies that reported the results opposite of the results of this study were very rare. Almost all of them confirmed the negative effect of muscle fatigue on balance control; the only difference was related to the severity of fatigue in different muscles or the different direction of balance control. This may be due to interfering factors such as the amount of rest the day before the test, the nature of the activities that subjects experience during the day, whether the subjects are athletes or not, the subjects' type of sports activities, activity and readiness level of the subjects, or the difference in the protocol of fatigue induction. Since fatigue causes balance impairment and increases the risk of injury, this issue requires considerable attention.

Compliance with ethical guidelines
All ethical principles were considered in this article.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or notfor-profit sectors.

Authors' contributions
All authors contributed in preparing this article.

Conflicts of interest
The authors declared no conflict of interest.