Abstract
The limit of dynamic endurance during repetitive contractions has been referred to as the point of muscle fatigue, which can be measured by mechanical and electrophysiological parameters combined with subjective estimates of load tolerance for revealing the human real-world capacity required to work continuously. In this study, an isotonic muscular endurance (IME) testing protocol under a psychophysiological fatigue criterion was developed for measuring the retentive capacity of the power output of lower limb muscles. Additionally, to guide the development of electrophysiological evaluation methods, linear and non-linear techniques for creating surface electromyography (sEMG) models were compared in terms of their ability to estimate muscle fatigue. Forty healthy college-aged males performed three trials of an isometric peak torque test and one trial of an IME test for the plantar flexors and knee and hip extensors. Meanwhile, sEMG activity was recorded from the medial gastrocnemius, lateral gastrocnemius, vastus medialis, rectus femoris, vastus lateralis, gluteus maximus, and biceps femoris of the right leg muscles. Linear techniques (amplitude-based parameters, spectral parameters, and instantaneous frequency parameters) and non-linear techniques (a multi-layer perception neural network) were used to predict the time-dependent power output during dynamic contractions. Two mechanical manifestations of muscle fatigue were observed in the IME tests, including power output reduction between the beginning and end of the test and time-dependent progressive power loss. Compared with linear mapping (linear regression) alone or a combination of sEMG variables, non-linear mapping of power loss during dynamic contractions showed significantly higher signal-to-noise ratios and correlation coefficients between the actual and estimated power output. Muscular endurance required in real-world activities can be measured by considering the amount of work produced or the activity duration via the recommended IME testing protocol under a psychophysiological termination criterion. Non-linear mapping techniques provide more powerful mapping of power loss compared with linear mapping in the IME testing protocol.
Highlights
The limit of dynamic endurance during repetitive contractions has been referred to as the point of muscle fatigue, which can be measured by mechanical and electrophysiological parameters combined with subjective estimates of load tolerance for revealing the human real-world capacity required to work continuously
The study was approved by the medical ethics committee of Jilin University and was performed in accordance with relevant guidelines and regulations of the institutional review board after each subject had given written informed consent, and all procedures performed in studies involving human participants were in accordance with the Declaration of Helsinki
The values of coefficient of variation (CV) were all less than 10%, and they were 5.13 ± 3.10%, 5.11 ± 2.63%, and 4.46 ± 3.21% for the plantar flexors and knee and hip extensors, respectively
Summary
The limit of dynamic endurance during repetitive contractions has been referred to as the point of muscle fatigue, which can be measured by mechanical and electrophysiological parameters combined with subjective estimates of load tolerance for revealing the human real-world capacity required to work continuously. CWT Continuous wavelet transform EV Estimated value Fihlrx High and low frequency ranges Finsmk Dimitrov’s spectral fatigue index FT Fourier transform GM Gluteus maximus HT Hilbert transform IMDF Instantaneous median frequency IME Isotonic muscular endurance IMNF Instantaneous mean frequency IPT Isometric peak torque LG Lateral gastrocnemius MAV Mean absolute value MDF Median frequency MG Medial gastrocnemius MLPNN Multilayer perceptron neural network MNF Mean frequency MVC Maximum voluntary isometric contraction r Pearson’s correlation coefficient R2 Coefficient of determination RF Rectus femoris RMS Root mean square sEMG Surface electromyography SNR Signal-to-noise ratio SSC Slope sign changes VL Vastus lateralis VM Vastus medialis WL Wavelength ZC Zero crossings Most human activities such as sports, industrial tasks, and daily living activities require dynamic endurance because most muscles are required to work continuously to accomplish these a ctivities[1,2,3]. In many previous studies, subjects could not independently determine their load tolerance in dynamic fatiguing exercises, which involved fixed sets, repetitions, and rest times with the individual maximum load (e.g., repetition maximum, RM)[15,30,31,32,33,39], so the actual muscle fatigue experienced by the subjects in those exercises may have exceeded their voluntary limits
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