| The nervous system dynamically regulates the activation of the skeletal muscles for human movements.For instance,muscle function needs to vary in time with the request of movement phases during dynamic limb movement,whereas,for sustained constant-force contraction task,muscle contraction force requires to reach a desired target force level and maintain it.In order to realize these movements,the recruitment strategies of motor units(MUs)within a given skeletal muscle was regulated by the nervous system according to the time-varying requirement of the specific tasks.Thus,the dynamic recruitment mechanism of MU population in skeletal muscle is very complex,which involves not only the interaction of different types of MUs,but also the differential activation among different neuromuscular functional compartments in one muscle.Therefore,the nervous system should utilize an efficient approach to regulate different types of MUs within individual muscle.It has been found in the previous studies that the group activity of MU in muscle is regulated,such as well-ordered recruitment of MU,left-shifting spectrum during fatigue,dynamical changes of the spectrum distribution during exercise,spatial reorganization.However,existing studies do not show how the nervous system coordinates and controls MUs group activities.Especially,it still is not clear whether this coordinated organization can be affected by neuromuscular physiological or pathological states.Therefore,it is important to explore the coordination regulation of MUs within individual skeletal muscle,which can be helpful to understand the regulation mechanism in neuromuscular system,and provide a basis for clinical diagnosis and rehabilitation.The surface electromyography(sEMG)consists of the superposition of the motor unit action potential trains(MUAPT)in the skeletal muscle superimposed in time and space,which can reflect the information about the dynamic control of MU recruitment strategy by the central nervous system(CNS).There are varied ways of recording sEMG signal to fulfill different research requires.For example,multi-channel sEMG can be simultaneously recorded from multiple muscles by multiple electrode pairs,and the overall muscle activity of individual muscle can be recorded by the electrode array.Thus,sEMG signal has been an effective tool for studying the dynamic regulatory mechanism of neuromuscular system.Considering the existing problems,this PhD thesis aims to explore the coordinate organization of MUs within individual skeletal muscle during movements,and analyze changes of the coordinate organization under abnormal physiological and pathological states.The research was supported by the funding of the National Natural Science Foundation of China(NO: 31470953).The main contents of this study are as follows:1.A novel sEMG oscillation synergy analysis was proposed,which can be used to evaluate the MU recruitment strategies.This can reflect the dynamic regulation process of myoelectric activity by the nervous system.It has been reported that high-frequency components and low-frequency components of sEMG signals correspond to the recruitment of fast MUs and slow MUs,respectively.Based on this relations,multi-variant empirical mode decomposition(MEMD)is used to extract scale-aligned intrinsic oscillation modes from high frequency to low frequency in multi-channel sEMG signals.It was different from the conventional method of directly extracting features from sEMG oscillation mode,this study proposed a new hypothesis that the nervous system might adopt an efficient synergistic recruitment strategy for MUs in skeletal muscle.The essence of synergy theory is a kind of control strategy that uses the specific pattern of low-dimensional synergistic elements to dominate a multi-element motor system aiming to simplify the control and improve the efficiency.Next,non-negative matrix factorization(NMF)was employed to extract the sEMG oscillation synergy and corresponding recruitment coefficient curves from sEMG oscillations.The synergistic organization of MUs can be characterized by sEMG oscillation synergies,and its dynamic adjustment process of CNS to oscillation synergy can be indicated by the recruitment coefficient curves.This method was applied onto the multi-channel sEMG signals from the biceps brachii(BB)and the triceps brachii(TB)during infant crawling.The results showed that there were high-,medium-and low-frequency oscillation synergies with stable structure for bilateral BB and TB.It can be observed from the time-varying recruitment coefficient curves that three kinds of synergy elements were dynamically adjusted according to the changes of muscle functions during crawling,which is exactly corresponding to recruitment and adjustment activities of different types of MUs.During swing phase,muscles contract/relax alternately,therefore the slow MUs are mainly recruited/de-recruited.At this moment,the recruitment of low-and medium-frequency synergies reached a peak.During stance phase,fast MUs are recruited to realize muscles contract quickly and respond to loading,leading to produce the high-frequency synergies.These results demonstrated that sEMG activities of the muscles can be represented by synergistic recruitment of sEMG frequency components,and verified the effectiveness of the sEMG oscillation synergy analysis in knowing the dynamic regulation process of different types of MUs groups.2.The impact of neuromuscular injury in infants with cerebral palsy(CP)on the dynamic activation characteristics of muscles during crawling were explored by using the sEMG oscillation synergy analysis proposed the above.It was found that the synergistic structure of sEMG oscillations extracted from CP infants was highly similar to that of typical developing infants,while neuromuscular injury of CP leads to the defects of the nervous system to dynamically regulate MU group activity.For swing phase,due to the decreased drive ability of MUs with low recruitment threshold in CP,the activation of the low-and medium-frequency synergy was insufficient during concentric contractions;For backward-swing phase and braking phase,due to declined activity of descending inhibitory system in CP,the low-and medium-frequency synergies were de-recruited insufficiently during eccentric contracions.These results not only verified the relationship between sEMG oscillation synergy and corresponding type of MUs,but also revealed the abnormal recruitment of oscillation synergy for CP during crawling by recruitment curves.3.In order to explore the regulation of forearm multitendoned muscles for progressive fatigue,this PhD thesis selected extensor digitorum(ED)as the object muscle,and designed two tasks with index finger and middle finger extension,respectively.The task finger was extended to perform a sustained isometric contraction(90s)at 15%,30%,and 45% MVC.To analyze the dynamic variation of the ED compartments,root mean square value(RMS),median frequency(MDF),fuzzy approximate entropy(FuzzyEn)and their normalized slope were employed to evaluate the activation intensity,fatigue,complexity and their change trends,respectively.The activation intensity(RMS)of compartments indicated that index finger extension needed the aid of the middle-finger compartment,while the middle finger extension required the assist of the index-finger and ring-and-little-finger compartments.The main contributors showed fatigue trends(slope of normalized MDF was negative)and decreased complexity(slope of normalizd FuzzyEn was negative),and faster decrease in complexity at the higher force level.Furthermore,the MU recruitment of ED compartments was analyzed by sEMG oscillation synergy analysis.The results show that the high-frequency and low-frequency oscillation synergies with stable structure were extracted from ED compartments under different conditions.It was found that the slopes of recruitment curves were positive for low-frequency synergy in each compartment under different conditions,while the slope of recruitment curves for high-frequency synergy in task compartments decreased with increasing force.These results indicated that the decreased complexity of compartments was due to increased recruitment of the low-frequency synergy,while the decreased recruitment of the high-frequency synergy accelerated this downward trend at higher force levels.In addition,based on the coefficient of variation(CV)of recruitment curves,it was found that the volatility of the high-frequency synergy was smaller than that of the low-frequency synergy,which suggested that the control strategies were different between the two types of MUs.Since the high-threshold MUs determine force output,the high-frequency synergy recruitment remained stable;and because low-threshold MUs achieve the fine adjustment of force,the low-frequency oscillation synergy recruitment was flexibly adjusted.The slope of CV further indicated that the volatility increased for the high-frequency synergy but decreased for the low-frequency synergy over time,which demonstrated that the ability of the nervous system to regulate two synergies was both affected by fatigue.Therefore,the recruitment curves of the sEMG oscillation synergy revealed that the fatigue-induced decrease in sEMG activity complexity was related to changes in MU recruitment strategy and the decreased ability of the nervous system regulating MU recruitment.4.Based on the non-uniform variation of spatial distribution of muscles,a new regional analysis of subtracted topographic map(STM)was proposed to reflect the dynamic activation of MUs in different spatial regions.A 32-channel array electrode was used to record the sEMG spatial activity of ED during sustained constant-force isometric contractions.Firstly,the RMS topographic map(TM)of 9 phases for ED was drawn.Pixel-wise subtraction method was performed as subtracting the initial TM from the other TMs to extract STM.STMs exhibited non-uniform changes in the ED spatial distribution,which can be divided into hot,warm and cold regions corresponding to higher,medium and lower degree of change.The relative normalized area of three regions demonstrated different trends.Moreover,the relative normalized area reached the plateau period after a longer dynamic adjustment period(rising/falling)at lower force,while the plateau period was longer at higher force,but the dynamic adjustment period was shortened or even disappeared.These results demonstrated that the dynamic regulation of ED space reorganization played an important role during sustained isometric contraction.The reduced dynamical regulation ability of spatial reorganization may induce muscle fatigue.In summary,this PhD thesis studied the dynamic regulation mechanism of the nervous system on MU synergistic recruitment in skeletal muscle.A novel sEMG oscillation synergy analysis was proposed and performed on infants crawling and sustained constant-force isometric contractions,which revealed the impact of neuromuscular injury and fatigue on MU recruitment.In addition,based on non-uniform sEMG spatial distribution in individual skeletal muscle,a new regional analysis was proposed to assess the ability of dynamic regulation and anti-fatigue for the neuromuscular system during sustained constant-force isometric contraction. |
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