Influence of Exercise-induced Local Muscle Fatigue ...

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Hospital, Shandong University, Jinan, China. Abstract—The thumb and index finger are important digits to performed dexterous and complex manipulation of ...
Influence of Exercise-induced Local Muscle Fatigue on the Thumb and Index Finger Forces during Precision Pinch Wenjing Hu1, Ke Li1, * 1. Laboratory of Motor Control and Rehabilitation, Department of Biomedical Engineering, School of Control Science and Engineering, Shandong University, Jinan, China * Corresponding author. Email: [email protected]

Na Wei2,3, Shouwei Yue4, Cuiping Yin4 2. Department of Geriatrics, Qilu Hospital, Shandong University, Jinan, China 3. Suzhou Institute of Shandong University 4. Department of Physical Medicine and Rehabilitation, Qilu Hospital, Shandong University, Jinan, China

Abstract—The thumb and index finger are important digits to performed dexterous and complex manipulation of objects. However, the local muscle fatigue will induce the capacity of force production and impair the precision motor control of the thumb and index finger. This study focused on the exerciseinduced local muscle fatigue influencing the COP (center of pressure) area, the SD and the PA (projection angle) of both the thumb and index finger and the CA (coordination angle) of two force vectors. Sixteen right-handed female subjects participated in this study. They performed the experiments including MVC, precision pinch and fatigue generation. The results show that the local muscle fatigue led an obvious increase in the COP area regardless of hands and digits, but only a great numerical increase in the SD of grip force in both digits of the left hand. Furthermore, for the CA of two force vectors, the statistical analysis showed a greater value in post-fatigue than that in prefatigue, there was no difference between both hands. No consistent significant difference of the PA was observed in both digits of both hands. However, the PA of the thumb force vector was bigger than that of index finger. The findings shed light on the exercise-induced local muscle fatigue had a significant effect on the force vector and the stability of force amplitude and provided a understanding of the force vectors, even could applied in a manual labor fatigue detection, and we also got that the local muscle fatigue has an obvious periphery mechanism.

fatigue in the upper or lower limbs using sEMG and force magnitude analyzed in both frequency domain and time domain. They had found much knowledge about the effects of muscle fatigue in sEMG, such as the increase of sEMG amplitude with the force output decreasing in the maximal or submaximal contraction experiments for duration, the MF decrease and the coherence changes and so on. In the present study, the quantity of force vectors was included to assess the influences of muscle fatigue and explore the neuromuscular control function.

Keywords—local muscle fatigue; digit force vector; precision pinch

II. MATERIALS AND METHODS

I. INTRODUCTION Exercise-induced local muscle fatigue leads reduction in the muscle’s capability to generate force and has been studied extensively during submaximal (20 % ~ 50 %) isometric contractions. Under these conditions, fatigue leads to an increase in force fluctuations and a decrease in the maximal or submaximal force production, as well as the discharge rate of muscle’s activity, additionally it could change the coherence of multiple muscles [1, 2]. The stability of grasped object is not only rely on the magnitude of force, but also the wellcoordinated digit force vectors to generate the fitting external torque. When we hold an object stable in the air with only two digits, it needs well-coordinated of the two digits force. In previous studies, subjects were asked to hold the lifted object stable or keep the force closed to the target force as much as possible, and they would try best to meet the motion command. Many scientists had studied the effects of muscle

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Our study focused on the COP of two digits and the force vectors of two digits, investigating the directional relationship between two force vectors. The force exerted by the fingertip on the interface of object is a 3D vectors consisted of x-, y(tangential force) and z- directions (normal force). They play an important role especially in dexterous manipulation to maintain the stability and prevent the object slipping. Usually, the coordinated angle is calculated to determine whether wellcoordinated or not. We hypothesized that the SD and the COP area will bigger with the reduced capacity to sustain the stability of output force and the angle (CA and PA) will decrease result from the weaker coordinated force vectors caused by the exercise-induced muscle fatigue.

A. Subjects Sixteen right-handed female subjects volunteers (Age: 21.94 f 1.98 years; Height: 162.94 f 3.68 cm; Weight: 53.63 f 6.95 Kg) participated in the study. Their handedness was assessed with Edinburgh handedness inventory. The objects had no history of musculoskeletal or neurological disorders in their hands or upper limbs and had normal or corrected-to-normal vision. All the participants were gave informed consent to participate in the experiment prior to being tested. The experiment procedures were approved by the Shandong University and were in accordance with the Declaration of Helsinki. B. Experimental Apparatus and Data Recording Normal and tangential forces of digits were measured by two six - components force/torque transducers (Nano 17, ATI Industrial Automation, Inc., Apex, NC, frequency sampling = 1000 Hz) mounted on the apparatus. The interface and the data

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collection were implemented using a custom LabView program (National Instrument, Austin, TX). C. Experimental Task All subjects were seated in the front of the table with the distance of 10 cm and the device was placed on the table at a distance of 30 cm from the table edge aligned with the subject’s shoulder of action side (right or left) to allow subjects to grip the device using a natural hand posture and speed.

(A)

(B)

x

Subjects performed four isometric force production tasks with the both hands: MVC and pre-fatigue precision pinch, a submaximal fatiguing contraction, post-fatigue precision pinch. For all tasks, subjects generated isometric normal forces simultaneously with the thumb and index finger on a grip apparatus, and in the fatiguing experiment, the average normal forces of both digits was presented as a tank and the target was a line in the center of the screen, subjects should keep the exerted force nearly close to the target line. MVC: We asked subjects to reach, grasp, lift, hold and replace object using the thumb and index finger with the uninvolved three fingers flexed (curled against the palm of the hand). Subjects used their maximum force to grip object with verbal encouragement three times. There was 5 minutes rest interval for relaxation between trials to minimize the effects of muscle fatigue.

Force vector

PA

y (C)

Fig. 1. Experimental set-up and definition of coordination angle. (A, B) The experiment setup and performances. (C) Definition of the coordinate system and angular parameters.

The PA of each force vector was defined the angle of the force vector with respect to the x-y shear plane and was performed as the following formula:

Precision Pinch: Subjects used natural force and speed to pinch the object in the same gesture of MVC for 40 s, three times before fatigue, and another three times immediately after three fatigue experiments. Fatigue experiment: Subjects used submaximal voluntary force to generate local muscle fatigue lasting for nearly 300 s with the same hand gesture. D. Data Processing Data from MVC experiment was used to determine the target line in fatigue experiment. Only precise pinch experiment data was processed in MATLAB. For reducing the error, the first 20 s and the last 20 s of the data were removed. The COP area and the SD of both digits forces were calculated to evaluate the COP change range and the force variation, also as a scale to assess the stability of digit force control. The force directions of the thumb and index finger during the two conditions (pre- and post- fatigue) were assessed using the following metrics: 1) the coordination angle (CA) between the force vectors of two digits; 2) the projection angle (PA) of each force vector with respect to the x-y plane (Fig. 1). The CA was defined as the angle formed by the two digits vectors and calculated as follows[3]: CA = cos

−1

FT ⋅ F I

(1)

FT ⋅ F I

where the FT and FI are the 3D force vectors of the thumb and the index finger, respectively, expressed in the common coordinate system.

z

CA

PA = tan −1

Fz 2 x

F + Fy2

(2)

E. Statistical Analysis Statistical analyses were performed using SPSS 23.0. Firstly, data were assessed for normality using the Kolmogorove-Smirnov test (K-S test). Then, two-way repeated measures ANOVA was performed to determine the effects of fatigue conditions (pre- and post- fatigue) and hands (right and left) on the CA. And for the SD and the COP area of grip force and the PA, three-way repeated measures ANOVA were used to evaluate the effects of hands, digits (the thumb and the index finger) and fatigue conditions. Comparisons of interest exhibiting statistically significant differences were further analyzed using the pairwise t-test. A p-value of less than 0.05 was considered statistically significant. III. RESULTS The stability of the thumb and index finger forces were calculated and evaluated by the SD and three-way repeated measures ANOVA was performed to determine the main effect and interaction effect. Only the significant difference occurred in the left hand (Fig. 2). The pairwise t-test was observed a significantly difference for the thumb and index finger, t = 2.588, p = 0.021 and t = -2.672, p = 0.017, respectively. The COP area was assessed by the three-way repeated measures ANOVA focus on the main effect fatigue conditions (F1,15 = 9.086, p = 0.009) and other effects or interaction effects had no significant difference (Fig. 3), that indicated there was an obvious difference between the pre- and postfatigue in any condition.

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Fig. 2. The average SD of the thumb and index finger grip force of all the subjects. * showed the significant difference between pre- and post- fatigue conditions. 3UHIDWLJXH

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Fig. 4. Coordination angles of the two force vectors at different fatigue conditions (i.e. pre-fatigue and post-fatigue) of both hands. * is represented that a significant difference between pre- and post-fatigue conditions in both hands.

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Fig. 3. Center of pressure area of two digits in both hands during the two different fatigue conditions (p < 0.05).

The coordination angle of two digits force vectors was assessed using two-way repeated measures ANOVA. The result (Fig. 4) showed a significant difference effect across fatigue condition (F1,15 = 6.729, p = 0.02), but failed to have a significant difference in both hands (F1,15 = 0.503, p = 0.489) and hand * condition interaction effect (F1,15 = 0.271, p = 0.610). The mean absolute values of the PA for all subjects were presented in Fig. 5. The three-way repeated measures ANOVA showed the main effects of the fatigue condition (F1,15 = 5.107, p = 0.039) and digit (F1,15 = 26.148, p < 0.001), and the pairwise t-test showed the PA of the thumb was significantly bigger than that of index finger, except for index finger of right hand in post-fatigue (t = 1.817, p = 0.089). In the point of fatigue condition, only the left index finger was effected by local muscle fatigue (t = 2.157, p = 0.048) and the right index finger failed to show the same obvious tendency (t = 1.833, p = 0.087). IV. DISCUSSION Local muscle fatigue is common for our hands manipulating objects or completing several kinds of tasks in our daily life, the purpose of our present study was to determine the influence of exercise-induced local muscle fatigue on force control during the precision pinch. The study investigated the force amplitudes and directions applied by the

Fig. 5. Force projection angles for the thumb and index finger in both hands between pre- and post-fatigue.

thumb and index finger during precision pinch on an externally stabilized object. From the point of the SD and COP of the thumb and index finger forces, a greater COP area of post-fatigue finger forces than that of pre-fatigue in both digits of both hands, but only a significant difference was observed in the SD of the left hand. These findings showed local muscle fatigue led an increase of force variability and COP area. That is thought to be related to the recruitment of additional motor units and contributions from peripheral afferent feedback. Force production resulted from the muscle actions innervated by related nerves excited by the responses of the mechanoreceptors and afferent signals arising from the sensors associated with fingertip pulp. In the condition of the fatigue, the sensitivity of the skin receptors was not as good as normal condition. The SD and COP area was represented the variability of force magnitude and force location, respectively. The result showed both hands had a weaker control of force location but only the left hand in force magnitude which indicated that the dominant right hand has a better ability in force control during the fatigue situation [4]. For the visual feedback of the target line, the stability of force production was little better than the digit placement, especially the right hand, maybe combination between the visuomotor and

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somatic motor system in the dominant hand was stronger than non-dominant hand resulting from the functional practice of daily life. The coordinated angle between the force vectors showed a great decrease after fatigue for both hands and there was no significant difference between the right hand and the left hand. The changes in CA for the two digits was indicative of reorientation of force vectors far from the straight angle (180 r) and the both hands had the same change without significant difference. Because of thumb-finger opposition, the human can complete complex and dexterous tasks in daily life or manual work, so how the coordinate control of both digits to generate an alignment angle by orient their force vectors in precision pinch is important. Li et al. had found that when the digit force had increased and stayed above 2N, the directional coordination between the force vectors was far from alignment (>30rdeviation)[5]. The opposite alignment angle provided a reference to the force orientation and coordination, which means only normal force was exerted to apply in the experimental task. And the difference between the actual CA and the reference provides a quantitative assessment of hand function. The nonalignment angle was indicative of the fatigue of the hand muscles, and the component of the resultant forcegrip force was reduced. The individual digits did not apply forces perpendicular to the shear plane, and even had significantly decreased resulted from the muscle fatigue especially for the left hand. Furthermore, a difference between the thumb and index finger occurred, which showed the digit specificity for the unique anatomical composition, biomechanical function and neuromuscular control.

another way to go to further explore the contralateral or global effect. ACKNOWLEDGMENT This research was supported by National Natural Science Foundation of China (31200744), Key Research & Development Programs of Shandong Province (2015GSF118127), China Postdoctoral Science Foundation (2014M560558, 2015T80723), Postdoctoral Innovation Foundation of Shandong Province (201401012), Young Scholars Program of Shandong University, Natural Science Foundation of Jiangsu Province (BK20170398). The authors thank all volunteers for their participation in the experiment. REFERENCES [1]

[2]

[3] [4] [5]

In conclusion, exercise-induced local muscle fatigue had several significant changes on the muscle output i.e. force stability and directions. From the recent study, we got the muscle fatigue has the periphery mechanism, that is local muscle fatigue has an ipsilateral effect, but if the central mechanism has a deeper influence on motion, we should have

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R. M. Enoka and J. Duchateau, "Muscle fatigue: what, why and how it influences muscle function," Journal of Physiology-London, vol. 586, pp. 11-23, Jan 2008. S. Kattla and M. M. Lowery, "Fatigue related changes in electromyographic coherence between synergistic hand muscles," Experimental Brain Research, vol. 202, p. 89, 2010. T. L. Marquardt and Z. M. Li, "Quantifying Digit Force Vector Coordination during Precision Pinch," J Mech Med Biol, vol. 13, p. 1350047, Apr 02 2013. H. H. B. E.-H. A. Gordon, "Asymmetric control of bilateral isometric finger forces," Experimental Brain Research, vol. 105, pp. 304-311, 1995. K. Li, R. Nataraj, T. L. Marquardt, and Z. M. Li, "Directional coordination of thumb and finger forces during precision pinch," PLoS One, vol. 8, p. e79400, 2013.