Assessment of local muscle fatigue by NIRS - development and evaluation of muscle suit -
© Muramatsu and Kobayashi; licensee Springer. 2014
Received: 15 January 2014
Accepted: 27 October 2014
Published: 18 November 2014
Although the machinery automation technology and robotics have made significant progress in making production and distribution easy and efficient, there is still a need for manual work such as nursing and other manufacturing work. Those work could possibly cause the work-related disorders. In order to reduce the risk of work-related injury or illness, we have been developing a compact and lightweight wearable robot “Muscle Suit” to provide direct and physical motion supports for assisting human motion. The development of the Muscle Suit becomes possible through the use of the McKibben artificial muscle. This type of artificial muscle has the advantages of being compact, lightweight and reliable. The wearable Muscle Suit has high versatility and conveyance, it supports the user physically with the capability of loading more weights. The evaluation of effectiveness is a key point in the development of the Muscle Suit and it could be estimated by evaluating the muscle fatigue. The evaluation of muscle fatigue is required not only for muscle suit but also for rehabilitation, muscle training and human physical support. Although the frequency transition of electromyogram (EMG) method is generally used for fatigue assessment, it has certain drawbacks depending on conditions and environment. For instance, it is impossible to apply it in the case of isotonic contraction. There are no apparatus or methods which measure muscle fatigue noninvasively and simply. In this study, we used near-infrared spectroscopy (NIRS) to measure the serum oxygenated hemoglobin and deoxygenated hemoglobin concentration of wearer’s muscle indirectly, the new term ΔHbt has been defined and the degree of fatigue has also been evaluated. And then, we had the evaluation experiment of muscle suit using blood oxygenation and the new term ΔHbt to estimate the effects of muscle suit.
The European Agency for Safety and Health at Work reports that Musculoskeletal Diseases account for 30% to 46% of all work-related sick leave . Based on USA’s National Institute for Occupational Safety and Health, 67% of nursing care personnel and 84% of automobile factory workers have back pain as work-related disorders . For workers, there are income losses when they are out of work due to work-related disorders. Furthermore, for organizations and factories, the industrial accident compensation is also an unexpected rise in production costs., Thus, we come to a conclusion that work-related disorder is a major and/or serious societal issue.
In the process of development, the evaluation method of muscle suit becomes as important as the development itself. Evaluation of muscle fatigue is required not only for muscle suit but also for rehabilitation, muscle training and human physical support, estimation of muscle fatigue is required. In order to prevent the economic losses such as the compensation of the work-related injuries, the fatigue evaluation for workers is also desired. Although the frequency transition of electromyogram (EMG) method is generally used for fatigue assessment, it has certain drawbacks depending on conditions and environment. For instances, it is impossible to apply in the case of isotonic contraction. Thus, there are no apparatus or methods which measure muscle fatigue noninvasively and simply .
We therefore propose a method of muscle fatigue assessment using near-infrared spectroscopy (NIRS) . NIRS is a noninvasive optical measurement method of blood oxygenation based on changes in the near-infrared absorption characteristics of hemoglobin (Hb) in vivo due to the state of oxygen bonding. In general, NIRS is used for visualization of brain activity, and several studies have been done on the measurement of blood oxygenation by NIRS -, which indicates the possibility of assessing muscle fatigue using NIRS. However, there is no research so far explaining the relation between blood oxygenation and muscle fatigue directly.
In this study, values of oxygenated and deoxygenated hemoglobin obtained by NIRS are employed for muscle fatigue estimation. Under the specific operation, we acquire these values and investigate the relation between muscle fatigue and those values. Then we define the index which seems to represent muscle fatigue using values of oxygenated and deoxygenated hemoglobin. In the paper written by us , we discussed the comparison of ΔHbt of non-state fatigue and exhaustion state in the biceps brachii, and in the other paper , we discussed the comparison of ΔHbt of exhaustion state of low load and exhaustion state of high load in the biceps brachii. And then using the defined index, we have an experiment of the muscle suit, and we discuss the comparison of the change in blood oxygenation with and without muscle suit in the erector spinae.
In this paper, muscle suit is introduced in chapter 2, NIRS is explained in chapter 3, index for indicating muscle fatigue is described in chapter 4, muscle fatigue assessment and the experiment of muscle suit is discussed in chapter 5.
Configuration and performance of muscle suit
McKibben artifical muscle
Muscle suit specification and effects
Near-infrared spectroscopy (NIRS)
NIRS is a noninvasive method for measuring oxygenation concentration in the blood using near-infrared ray. It was commercialized in 1996  by Hitachi Co. Ltd. basically for showing brain activity and has been used for brain science.
where I in is the light input to the target muscle, I out is the transmitted light pass through the body, D [mm] is the transit distance, ε [1/(mM·mm)] is the extinction coefficientand C [mM] (M: mol) indicates concentration we would like to have.
where ΔI out is the change in transmitted light pass through the body, ΔC the change in concentration, D the average optical path lengthand ΔS the change in the scattering effect. Equation (2) is called the Modified Lambert-Beer law.
In general, oxygenated hemoglobin gives oxygen to each part of the body including the muscles and then changes into deoxygenated hemoglobin. The deoxygenated hemoglobin obtains oxygen from the lungs and then changes into oxygenated hemoglobin. Note that the total amount of oxygenated hemoglobin + deoxygenated hemoglobin in the whole body should be the constant because the blood flow is constant. If the muscle is used, the oxygen is consumed there, and more oxygen is demanded. As a result, oxygenated hemoglobin is provided to the oxygen-consumed area, the ratio of oxygenated hemoglobin increases and deoxygenated hemoglobin decreases. Thus we may say that by observing oxygenated hemoglobin and deoxygenated one, muscle fatigue can be estimated.
Index to estimate muscle fatigue by NIRS
By analyzing the relation between ΔC oxy · D and ΔC doxy · D, we propose the index which seems to represent the muscle fatigue. Following experiments are given approval of International University of Health and Welfare.
Effect of various conditions
There are individual differences exist between subjects, such as gender, age, physique, affects blood oxygenation . The straining during operation is called as the Valsalva maneuver and physical exercise before the operation also influences blood oxygenation .
We then employ subjects who are the same gender, age, physique, and also ask them to rest 10 minutes before the operation and not to strain during the operation in order to avoid uncertainly factors for blood oxygenation.
Definition of ΔHbt
Results and discussion
Review and evaluation of muscle suit in actual operation by NIRS
Experimental set up
The operation of moving patients is performed by two people in cooperation. One is responsible for carrying the lower side of body and the other is in charge of the upper side. The upper side of body accounts for 70% of whole body weight, as a result, the subject in charge of the upper body uses the muscle suit. In this experiment, the height of bed is set to 750 mm, which is an average value of beds using in the actual home-visit bathing service. The distance between bed and bath is set to 900 mm. Furthermore, the height of bath is set to 500 mm as the bath used is the standard type of everyday bathing service. The weight of the user of bathing service is set to 60 kg. Subjects perform the operation carrying the patient from bed to bath, up and down, in 12.5 seconds, similarly, from bath to bed in 12.5 seconds as well. The total 25 seconds operation is considered as one cycle, subjects perform 5 cycles of operation in a row as the prior investigation suggests after 5 times the fatigue reaches the peak value. We then compare blood oxygenation in case of using muscle suit and not using muscle suit.
The results of an experiment
We have proposed a new method for evaluating local muscle fatigue using NIRS. We focus on the change in oxygenated and deoxygenated hemoglobin under the operation and define ΔHbt as the differences between them. In the experiment, as for the muscle of biceps brachii, we find that ΔHbt is constant in case of non-fatigue. Whereas in case of exhaustion, it increases according to elapsed time and shifts upward following with cycles.
We conducted an investigation of blood oxygenation during carrying operation between bed and bath in home-visit bathing service. It can therefore be concluded that there is the difference in blood oxygenation by occupying muscle suit or not. In addition, not only the measurement of blood oxygenation after operation but also during the operation could possibly be evaluated in real time to assess the muscle fatigue. The evaluation method of NIRS could be used in further research of assessing the effects of using muscle suit.
Investigation of local muscle fatigue assessments by NIRS will continue, and adaptation to various scenes require assessment of muscle fatigue such as medical treatment, welfare, the work place and evaluation of muscle suit will be considered.
Written informed consent was obtained from the patient for the publication of this report and any accompanying images.
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