The quantitative recruitment of motor units depends on specific demands, including movement velocity. DNA microarrays differ according to sex, and these sex-related differences affect the fiber type composition.
OBJECTIVE:
The aim of this study is to demonstrate inter-sex differences in the isokinetic parameters, isokinetic phases, and muscle activities and to investigate the correlation between muscle activity and isokinetic data.
METHODS:
A total of 41 healthy adults (20 male, 21 female) performed concentric knee extension at angular velocities of 60/s, 180/s, and 240/s. The outcome measures consisted of the isokinetic peak moment (PM), normalized PM (PM/BW), total work, and power, alongside the acceleration, iso-speed and deceleration, sub-phases. Muscle activity was recorded from the rectus femoris, vastus lateralis, and vastus medialis using surface EMG.
RESULTS:
There were significant two-factor main effect and interaction between sex and angular velocity on the power of knee extension and isokinetic phase ( 0.05). As the velocity increased, the increase in power of males was greater than that of females. In contrast, with the increase in velocity, PM, PM/BW, and total work decreased, but no significant interaction was observed between velocity and sex. At high velocity, males showed higher acceleration ability than females.
CONCLUSION:
The sex-dependent responses to velocity were more affected by differences in total movement time than force production. Fast isokinetic exercise should consider the acceleration ability rather than the ability to produce force.
The physical parameters associated with the task performance include speed of movement, resistance, distance moved, and the number of repetitions [1]. Isotonic exercise is the most widely applied method of strength training. However, the maximum resistance applied in isotonic exercise is the load at the weakest point in the entire range of motion [2]. As a result, the applied exercise intensity is significantly less than the maximum capacity [1]. Isokinetic exercise is a method for fixing the maximum velocity, which in theory produces the maximum force throughout the active range of motion [1, 3]. An intuitively positive effect of higher speed is the increase in power, which is the amount of work done per unit of time [4, 5, 6, 7]. The higher velocity for the constant range of motion results in a decrease in total movement time. In contrast, due to a decrease in peak moment (PM) with increasing velocity, a lower work value was reported [4, 5, 6, 7, 8, 9, 10, 11, 12]. For the reason for moment reduction, two explanations have been proposed [12]. To explain the decrease in muscle activity with increasing velocity, these hypotheses were adopted. The first explanation is either facilitator or inhibitor effect on the motor neuron pool because the proprioceptive feedback is velocity dependent. However, this explanation has contradictions that can explain both the increase and decrease in moment or/and muscle activity. An alternative explanation is that the qualitative recruitment of motor units with different functional properties is determined by specific demands. Conversely, there was a result of the neural inhibitory mechanism as an explanation for the increase in muscle activity with increasing velocity [6, 9, 13]. Thus, this neural mechanism suppresses the force during maximum contraction to protect the joint and surrounding tissues from injury.
Previous studies on muscle activity have inconsistent reports of increasing [4, 6, 9], decreasing [2, 5, 12], and no change [4, 5, 6] in the patterns of electromyographic (EMG) amplitude with increasing speed. The recruitment of more motor units and a higher firing rate results in increased EMG amplitude [14]. Using Significance Analysis of Microarrays program, Welle et al. [15] reported that there were more than 3,000 sex-related differences between male and female skeletal muscles. These sex-related differences affected the fiber type composition [16]. The distribution ratio of slow oxidative fibers was high in females as a result of biopsy for the vastus lateralis (VL) muscle. The percentages of the fast glycolytic fiber were high in male VL muscle. Also, during the straight leg raise, it was reported that females had higher initial firing rates at recruitment than males [17]. However, it had different mechanisms for force production based on movement velocity [18, 19, 20]. Therefore, it is necessary to study the response of muscle activity according to the velocity of the isokinetic movement through a comparison between males and females with different muscle fibers’ composition.
This study aimed 1. to compare the change in the isokinetic parameters: (PM, normalized PM (PM/BW), total work, and power, isokinetic phases: acceleration (Acc), load range (LR), and deceleration (Dec), and the EMG activity of the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM)) based on sex and velocity 2. to investigate the correlation between muscle activity and isokinetic data.
Methods
Participants
A total of 41 healthy young adults, 20 males (age 23.85 2.13 years, weight 72.45 9.21 kg, height 174.2 4.46 cm) and 21 females (age 22.43 1.83, weight 56.90 13.46, height 160.62 5.43 cm), were included in this study. They were free from the pain that could influence the movements performed in this study. The exclusion criteria included previous musculoskeletal injury and surgery, neurological disorder, or cardiovascular disease. Written informed consent was obtained from each participant before testing after explaining the experimental purpose and protocol in accordance with the principles of the Declaration of Helsinki. This study was approved by the Institutional Review Board of Sunmoon University (SM-201910-059-1).
An isokinetic dynamometer (HUMAC NORM, CSMI Medical Solutions, Stoughton, MA) was used to collect the isokinetic data of knee extensors of the dominant leg. The dominant leg was defined as the leg used by the participant to kick the ball. Participants were seated in a chair, with the hip and knee joints at 90 flexion. The trunk and thigh were stabilized using straps. The axis of the dynamometer was aligned with the knee joint (lateral femoral epicondyle), and the lever arm pad was placed approximately 3 cm above the medial malleolus. The total range of motion was allowed from 90 knee flexion to full extension. Participants performed 3 to 5 submaximal warm-up trials before testing at each velocity. After 3 min of rest, all participants performed 3 repetitions of concentric maximal contraction for knee extension at randomly ordered velocities of 60/s, 180/s, and 240/s.
Isokinetic measurement
An isokinetic test of the knee extension was performed to acquire the data of the isokinetic parameters (PM, PM/BW, total work, and power) and isokinetic phases (Acc, LR, and Dec). The PM was normalized for BW (Nm/kg). The PM/BW is used to compensate for individual differences related to weight [21]. The isokinetic concentric contraction involves three phases including Acc, constant velocity, and Dec [7, 22]. The Acc phase represents reaching a preset velocity and is performed without resistance. Constant velocity, known as the LR, is the phase in which the mechanical speed and the individual’s movement velocity correspond. The Dec phase is the phase in which the movement stops as the velocity decreases after LR. The ratio of time for each phase to total movement time (% total movement time) was calculated and used for analysis.
Differences in isokinetic parameters according to sex (male and female) and angular velocity (60/s, 180/s, and 240/s) during isokinetic exercise
Angular velocity (/s)
value
Isokinetic parameters
60
180
240
Sex
Angular velocity
Interaction
PM (Nm)
Male
140.31 41.88
88.35 27.59
74.65 20.99
0.001
0.001
0.775
Female
92.29 29.43
47.05 15.71
35.25 12.54
PM/BW (Nm/kg)
Male
1.96 0.61
1.23 0.39
1.04 0.31
0.001
0.001
0.874
Female
1.62 0.37
0.83 0.26
0.63 0.23
Total work (J)
Male
309.20 103.93
212.10 82.19
173.65 61.02
0.001
0.001
0.836
Female
225.10 77.14
113.43 43.57
74.60 34.25
Power (Watt)
Male
69.40 20.86
110.90 48.45
106.25 44.84
0.001
0.001
0.032
Female
50.10 16.28
62.90 23.93
53.80 21.52
Values are presented as mean standard deviation. PM: peak moment, BW: body weight. Significant differences are indicated in bold ( 0.05).
Differences in isokinetic parameters of knee extensors according to sex (male and female) and angular velocity (60/s, 180/s, and 240/s). (A) Peak moment (Nm). (B) Peak moment per body weight (Nm/kg). (C) Total work (J). (D) Power (Watt). Solid and dashed lines represent males and females, respectively. Main effect of angular velocity ( 0.05). Main effect of sex ( 0.05). Interaction effects between sex and angular velocity ( 0.05).
Electromyography
The muscle activity in RF, VL, and VM during isokinetic contractions was measured using wireless surface EMG system (Zerowire EMG, Aurion, Italy). Based on the Surface Electromyography for the Non-Invasive Assessment of Muscles guidelines, electrodes were determined [2, 23]. The EMG data for each participant were normalized using the maximal voluntary isometric contraction (MVIC) test. An isokinetic dynamometer was used for the MVIC test, and the participants’ knee joint and hip joint were maintained at 60 and 90 flexion, respectively [24, 25, 26]. In 3 MVIC tests, a 5 s EMG signal was measured for the knee extension motion. The average values for the middle 3 s of each muscle were calculated. Participants were given a 1 min break between each test.
Comparison of quadriceps muscle activity, traditional isokinetic parameter, and isokinetic period during isokinetic exercise between males and females
Angular
velocity
Variables
Male
Female
value
60/s
Muscle activity (% MVIC)
RF
68.99 14.12
80.34 13.72
0.013
VL
68.93 16.95
78.51 15.13
0.063
VM
69.99 15.44
76.78 14.55
0.155
Traditional isokinetic parameter
PM (Nm)
140.31 41.88
92.29 29.43
0.001
PM/BW (Nm/kg)
1.96 0.61
1.62 0.37
0.042
Total work (J)
309.20 103.93
225.10 77.14
0.006
Power (Watt)
69.40 20.86
50.10 16.28
0.002
Isokinetic period (% total movement time)
Acc
14.91 1.13
15.47 0.87
0.084
LR
74.33 2.18
73.73 1.81
0.341
Dec
10.76 1.64
10.80 1.77
0.932
180/s
Muscle activity (% MVIC)
RF
63.63 18.12
62.49 15.66
0.831
VL
70.88 19.22
71.81 17.61
0.873
VM
71.97 17.37
71.01 20.21
0.871
Traditional isokinetic parameter
PM (Nm)
88.35 27.59
47.05 15.71
0.001
PM/BW (Nm/kg)
1.23 0.39
0.83 0.26
0.001
Total work (J)
212.10 82.19
113.43 43.57
0.001
Power (Watt)
110.90 48.45
62.90 23.93
0.001
Isokinetic period (% total movement time)
Acc
47.65 3.72
50.91 3.99
0.010
LR
23.87 4.21
22.24 3.86
0.202
Dec
28.48 4.93
26.86 5.07
0.307
240/s
Muscle activity (% MVIC)
RF
61.82 21.46
51.95 16.08
0.108
VL
71.18 18.44
61.92 14.57
0.086
VM
71.93 16.81
62.44 17.53
0.089
Traditional isokinetic parameter
PM (Nm)
74.65 20.99
35.25 12.54
0.001
PM/BW (Nm/kg)
1.04 0.31
0.63 0.23
0.001
Total work (J)
173.65 61.02
74.60 34.25
0.001
Power (Watt)
106.25 44.84
53.80 21.52
0.001
Isokinetic period (% total movement time)
Acc
49.85 4.91
57.15 4.93
0.001
LR
14.01 4.22
11.43 3.27
0.038
Dec
36.15 5.43
31.41 4.86
0.006
Values are presented as mean standard deviation. MVIC: maximal voluntary isometric contraction, RF: rectus femoris, VL: vastus lateralis, VM: vastus medialis, PM: peak moment, BW: body weight, Acc: acceleration, LR: load range, Dec: deceleration. Significant differences are indicated in bold ( 0.05).
Video recording was used to identify the beginning and completion of the isokinetic movement. Each muscle’s EMG signal was analyzed from 90 knee flexion to a full extension during isokinetic exercise. The sampling frequency for the EMG was 1000 Hz. MyoRESEARCH software (XP Master, version 1.07.1, Noraxon, Scottsdale, AZ, USA) was used to analyze raw data from EMG. The signal was filtered using a bandpass filter between 20 and 450 Hz. Using the root mean square (RMS) with a 10 ms window, the filtered signal was full wave rectified and smoothed. Subsequently, all EMG amplitude values obtained during the isokinetic test were normalized to the corresponding muscle’s MVIC (MVIC %).
Statistical analysis
The isokinetic parameters, isokinetic phases, and EMG values were averaged over three trials and used for analysis. The effect of sex and angular velocity on isokinetic parameters, isokinetic phases, and muscle activity was assessed using a two-way analysis of variance (ANOVA) using SPSS software (SPSS 22.0, SPSS Inc., Chicago, IL, USA). When a significant two-factor interaction or the main effect was observed, Tukey’s HSD was conducted as a post hoc test. The variables between sex were compared using an independent -test (Student -test). The associations between the mean EMG value of quadriceps in microvolts (RMS) and isokinetic measures during isokinetic exercise were determined using Pearson’s product-moment correlation. The level of statistical significance was set at a value 0.05.
Results
Differences in isokinetic periods according to sex (male and female) and angular velocity (60/s, 180/s, and 240/s) during isokinetic exercise
Isokinetic period
Angular velocity (/s)
value
(% total movement time)
60
180
240
Sex
Angular velocity
Interaction
Acc
Male
14.91 1.13
47.65 3.72
49.85 4.91
0.001
0.001
0.001
Female
15.47 0.87
50.91 3.99
57.15 4.93
LR
Male
74.33 2.18
23.87 4.21
14.01 4.22
0.010
0.001
0.428
Female
73.73 1.81
22.24 3.86
11.43 3.27
Dec
Male
10.76 1.64
28.48 4.93
36.15 5.43
0.007
0.001
0.041
Female
10.80 1.77
26.86 5.07
31.41 4.86
Values are presented as mean standard deviation. Acc: acceleration, LR: load range, Dec: deceleration. Significant differences are indicated in bold ( 0.05).
Differences in isokinetic phase of knee extensors according to sex (male and female) and angular velocity (60/s, 180/s, and 240/s). The ratio of time for each phase to total isokinetic movement time (% total movement time) was calculated. Solid and dashed lines represent males and females, respectively. Main effect of angular velocity ( 0.05). Main effect of sex ( 0.05). Interaction effects between sex and angular velocity ( 0.05).
A significant interaction between sex and angular velocity was observed on the power of knee extension ( 3.532; 0.032) (Table 1). However, no significant interaction effects were noted for PM ( 0.255), PM/BW ( 0.134), and total work ( 0.179) ( 0.05). The main effect of sex was observed on PM, PM/BW, total work, and power ( 0.001). Similarly, a significant main effect of angular velocity was found on PM, PM/BW, total work, and power ( 0.001; power was 0.001). Post hoc analysis revealed that the PM, PM/BW, and total work at 60/s were significantly higher than those at 180/s and 240/s ( 0.001) (Fig. 1). PM/BW and total work at 180/s were significantly greater than those at 240/s, respectively ( 0.042 and 0.032, respectively) (Fig. 1). The power at 60/s was significantly lower than at 180/s and 240/s ( 0.001 and 0.022, respectively) (Fig. 1). For the three angular velocity conditions, the isokinetic parameters (PM, PM/BW, total work, and power) were higher in males than females ( 0.05) (Table 2).
Significant interaction effects were found between sex and angular velocity on Acc and Dec ( 0.05) (Table 3). Furthermore, the main effect of sex was observed on Acc, LR, and Dec ( 0.05). Also, the main effect of angular velocity was observed on Acc, LR, and Dec ( 0.05). According to post hoc analysis, Acc and Dec at 240/s were greater than those at 60/s and 180/s, respectively ( 0.05) (Fig. 2). Acc and Dec at 180/s were also significantly higher than those at 60/s ( 0.05) (Fig. 2). In contrast, at 240/s, LR was significantly lower than that at 180/s and 60/s, and at 180/s, it was also lower than that at 60/s ( 0.05) (Fig. 2). Acc was significantly longer in females than males at 180/s and 240/s ( 0.05) (Table 2). On the other hand, at 240o/s, LR and Dec were significantly shorter in females than males ( 0.05).
Differences in quadriceps muscle activity according to sex (male and female) and angular velocity (60/s, 180/s, and 240/s) during isokinetic exercise
Angular velocity (/s)
value
Muscle activity (MVIC%)
60
180
240
Sex
Angular velocity
Interaction
RF
Male
68.99 14.12
63.63 18.12
61.82 21.46
0.952
0.001
0.013
Female
80.34 13.72
62.49 15.66
51.95 16.08
VL
Male
68.93 16.95
70.88 19.22
71.18 18.44
0.971
0.123
0.035
Female
78.51 15.13
71.81 17.61
61.92 14.57
VM
Male
69.99 15.44
71.97 17.37
71.93 16.81
0.562
0.164
0.065
Female
76.78 14.55
71.01 20.21
62.44 17.53
Values are presented as mean standard deviation. MVIC: maximal voluntary isometric contraction, RF: rectus femoris, VL: vastus lateralis, VM: vastus medialis Significant differences are indicated in bold ( 0.05).
Correlation between quadriceps muscle activity (V) and isokinetic parameters and periods during isokinetic exercise
Muscle activity ()
RF
VL
VM
Isokinetic exercise at 60/s
Isokinetic parameter
PM (Nm)
0.210
0.188
0.383
0.014
0.383
0.014
PM/BW (Nm/kg)
0.278
0.079
0.506
0.001
0.520
0.001
Total work (J)
0.097
0.546
0.316
0.044
0.343
0.028
Power (Watt)
0.123
0.443
0.281
0.075
0.351
0.025
Isokinetic period (% total movement time)
Acc
0.341
0.029
0.214
0.179
0.244
0.124
LR
0.107
0.505
0.073
0.650
0.051
0.753
Dec
0.081
0.613
0.044
0.785
0.089
0.579
Isokinetic exercise at 180/s
Isokinetic parameter
PM (Nm)
0.444
0.004
0.544
0.001
0.535
0.001
PM/BW (Nm/kg)
0.465
0.002
0.647
0.001
0.626
0.001
Total work (J)
0.357
0.022
0.450
0.003
0.481
0.001
Power (Watt)
0.251
0.113
0.314
0.046
0.361
0.020
Isokinetic period (% total movement time)
Acc
0.429
0.005
0.399
0.010
0.387
0.012
LR
0.267
0.092
0.17
0.289
0.140
0.382
Dec
0.139
0.385
0.193
0.227
0.208
0.193
Isokinetic exercise at 240/s
Isokinetic parameter
PM (Nm)
0.502
0.001
0.580
0.001
0.459
0.003
PM/BW (Nm/kg)
0.491
0.001
0.620
0.001
0.482
0.002
Total work (J)
0.512
0.001
0.566
0.001
0.468
0.002
Power (Watt)
0.395
0.012
0.383
0.015
0.376
0.017
Isokinetic period (% total movement time)
Acc
0.409
0.009
0.407
0.009
0.402
0.010
LR
0.443
0.004
0.381
0.015
0.260
0.105
Dec
0.133
0.412
0.175
0.280
0.253
0.115
Values are presented as mean standard deviation. PM: peak moment, BW: body weight, Acc: acceleration, LR: load range, Dec: deceleration, RF: rectus femoris, VL: vastus lateralis, VM: vastus medialis. Significant differences (moderate correlation) are indicated in bold ( 0.05).
The two-way ANOVA results for the difference in quadriceps muscle activity according to sex and angular velocity are shown in Table 4. These results show a significant interaction between sex and angular velocity on RF and VL muscle activity ( 0.05) (Table 4). For the VM muscle activity, no significant interaction and no main effect of sex and angular velocity were observed ( 0.05). A significant main effect of the angular velocity was found on RF muscle activity ( 0.05). However, there was no main effect of sex on RF and VL as well as the main effect of angular velocity on VL ( 0.05). However, no significant differences in VL and VM muscle activity were observed between the sexes ( 0.05).
There was a significant correlation (moderate) between RF muscle activity and PM and total work in an isokinetic exercise at 240/s and a significant correlation (moderate) between VL muscle activity and PM, PM/BW, and total work at 240/s (Table 5). Furthermore, there was a significant correlation (moderate) between VL muscle activity and PM and PM/BW at 180/s and PM/BW at 60/s and a significant correlation (moderate) between VM muscle activity and PM and PM/BW at 180/s and PM/BW at 60/s.
Discussion
This study aimed primarily to identify the considerations for effective isokinetic exercise through a comparison between the isokinetic parameters, isokinetic phases, and muscle activities according to velocity and sex. In this study, the power increased with the increase in velocity. The power variation of males was larger than that of females. In contrast, the PM, PM/BW, and total work decreased with the increase in velocity, but no significant interaction was observed between velocity and sex. We suggest that sex-dependent responses to velocity were more influenced by differences in total movement time than force production. Furthermore, there is additional evidence to support this purpose from the results of this study. At a higher velocity, the percentage of Acc was increased. Particularly, the change in Acc showed a significant interaction between velocity and sex. Therefore, the sex difference in the change of velocity might be due to the different production of explosive movements.
In this study, the PM, PM/BW, and total work significantly decreased at higher speeds and the power increased. Previous studies reported a decrease in moment with the increase in movement velocity [4, 6, 7, 8, 11, 12, 27]. The increase in power was accompanied by an increase in the maximum speed within the feasible range [4, 5, 6, 7]. The tendency of power to increase with increasing velocity was statistically greater in males than females. Therefore, the higher power might be due to a decrease in the execution time of the movement because the moment was reduced at higher speeds. This trend was found in the statistical results for the isokinetic phases. The Acc and Dec phases occupied a greater proportion at a higher speed for females than males. Compared to the isokinetic phase between males and females, no significant difference was observed in sex for the LR and Dec at 60/s and 180/s. However, the LR of females was significantly reduced compared to males at 240/s. In previous studies, males reached their maximum moment faster than females [22, 27, 28]. The authors attributed a shorter LR to the lower neuromotor efficiency in females [22]. The underlying cause of sex differences has been speculated to affect the fiber type composition. Males have a higher proportion of FG fibers than females, which are effective at higher speeds [16].
The order of recruitment of motor neurons was related to their size or threshold, which was defined as the Henneman’s size principle [18]. The recruitment of motor units by this principle was observed in the movement of a smooth and gradual increase in force [19, 20]. The recruitment order of motor units is not only determined by the excitability of motor neurons but also affected by the contraction time, force, and fatigue resistance. An increase in the force produced by the muscle is indicated to appear with an increase in the signal intensity and the activated area at the primary sensorimotor cortex [29]. Isokinetic exercise requires the generation of maximum force during the entire exercise time [1, 3]. In theory, during isokinetic exercise, all types of motor units would be recruited almost simultaneously, regardless of sex and velocity. However, with increasing velocity, the results reported increasing [4, 6, 9], decreasing [2, 5, 12], and no change [4, 5, 6] in muscle activity. Purkayastha et al. [2] reported that the conflicting evidence of muscle activity according to the velocity was due to differences in target muscles and/or the range of velocities tested. Therefore, it seems that none of the muscle activities showed significant interaction with the two main effects (sex and velocity).
This study investigated the correlation between quadriceps muscle activity and isokinetic parameters and phases. As a result, there was a moderate positive correlation between the muscle activity of VL and VM and PM/BW at 60/s and 180/s. At 240/s, there was a moderate relationship between moment production capacity and muscle activity of VL and RF. The increase in EMG amplitude has a high linear correlation with the increasing force [14]. The results of this study showed that participants with high muscle activity had a high moment production capacity at all velocities. Therefore, we consider that there is a correlation between muscle activity and moment generation, regardless of sex and velocity during isokinetic exercise. Furthermore, previous conflicting evidence might be due to differences in experimental conditions, as well as differences in response to velocity among individuals.
There were some limitations to this study, which should be taken into consideration when interpreting the results. First, the EMG amplitude normalization might contaminate the quantitative comparison between males and females. Second, the muscle activity of RF for females was statistically significantly higher than that of males at 60/s. The muscle activity of the antagonist influenced the muscle activity of the agonist through volume conduction [12]. In this study, the highest moment was measured at 60/s. Females might have a smaller quadriceps muscle volume than males. Contamination by antagonists was considered to be greater at the highest moment production.
Conclusion
This study provides come potential insights for effective isokinetic exercise. It is necessary to check the increase in power rather than the decrease in moment that accompanies the higher velocity to achieve effective isokinetic exercise. The higher power is closely related to the explosive momentary Acc ability. The moment production capacity at a relatively slow or moderate velocity is highly concerned with muscle activity of VM and VL regardless of sex and velocity. At a relatively high velocity, muscle activity of the RF is required. Further studies are needed to investigate the change of momentary Acc capacity according to the application of repetitive fast isokinetic exercise.
Author contributions
CONCEPTION: Jiheon Hong and Jeongwoo Jeon.
PERFORMANCE OF WORK: Jiheon Hong, Jeongwoo Jeon, Dongyeop Lee and Jaeho Yu.
INTERPRETATION OR ANALYSIS OF DATA: Jiheon Hong, Jeongwoo Jeong, Jinseop Kim and Seong-Gil Kim.
PREPARATION OF THE MANUSCRIPT: Jiheon Hong and Jeongwoo Jeong.
REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Dongyeop Lee, Jaeho Yu, Jinseop Kim and Seong-Gil Kim.
SUPERVISION: Jiheon Hong.
Ethical considerations
Sunmoon University Ethics Committee approved this study on 03/19/2020 (SM-201910-059-1). All experiments were undertaken with the understanding and written informed consent of each participant before the assessment.
Funding
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2020R1C1C1012483).
Footnotes
Acknowledgments
The authors have no acknowledgments.
Conflict of interest
The authors have no conflict of interest to report.
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