Study On Muscle Fatigue Characteristics Of The Muscle-Tendon Units

Muscle fatigue analysis has important applications in the fields of rehabilitation medicine,biomechanics and kinematics.In the field of rehabilitation medicine,it can be used as an auxiliary diagnostic method for neuromuscular system-related diseases.In the field of biomechanics,it can optimize the musculoskeletal model.And in the field of kinematics,it can help athletes improve their training efficiency and avoid muscle injuries.Surface electromyography(SEMG)is a mainstream tool in analyzing muscle fatigue due to its non-invasive and objective characters.The movement of the human body is the result of the coordination of the musculoskeletal system,which under the control of the central nervous system(CNS).According to human anatomy,a skeletal muscle is made up from muscle tendon units(MTUs).Relevant studies revealed that CNS took MTUs as the basic control units.However,recent studies about muscle fatigue have took each skeletal muscle as a whole unit and ignored the functional difference across MTUs.Regarding the issue mentioned above,the activities and fatiguing characteristics at the MTU level of the deltoid during different contraction tasks were analyzed and discussed.In addition,the control mechanism of the CNS was discussed.The main contents and achievements of this study were summarized as follows:(1)The scheme in muscle fatigue analysis based on SEMG was proposed,which consisted of four parts including data acquisition,signal preprocessing,feature extraction and fatigue description.Specifically,the performances of different parameters including root mean square(RMS),mean power frequency(MPF),the first coefficient of AR model(ARC1),sample entropy(SE)and Higuchi fractal dimension(HFD),in quantification of fatigue in terms of "sensitivity" and "robustness" were proposed.Then,the optimized SEMG parameter was selected for the analysis of the fatiguing characteristics of MTUs.(2)The activities and fatiguing characteristics of the three deltoid heads(corresponding to three MTUs)during isometric contraction tasks under different loads were explored.Three static isometric contraction tasks were designed with different portion of loads to the three deltoid heads.The activities of the three heads were analyzed by the amplitude of the SEMG signals.Results revealed that the three heads showed different activation mode in three tasks but it's hard to purely activate any single head.The performances of different SEMG parameters in quantification of fatigue were analyzed,and results showed that HFD was the most suitable parameter for fatigue analysis.Further HFD-based fatigue analysis revealed that the fatiguing characteristics of MTUs were strongly correlated with its activation levels.The heads which maintained in high activation level were more prone to fatigue.The difference in fatiguing rate between the head to which the primary load was exerted and the others become obvious with the load increase.(3)The activities and fatiguing characteristics of the three deltoid heads during dynamic periodic contraction tasks under different loads were explored.Three dynamic periodic contraction tasks were designed to activate the three deltoid heads relatively independently.Due to the non-stationary property of SEMG,short time Fourier transform(STFT)and time-varying AR model(TVAR)were applied to optimize the calculation of MPF and ARC1.Then,corresponding data processing methods were used for these signals.Results also confirmed that HFD parameter was the most suitable parameter for MTU fatigue analysis during dynamic periodic contraction tasks.The three deltoid heads showed different activation modes and fatiguing rates.The difference in fatiguing rates among heads also increased with the increase in load.However,the fatiguing characteristics of the MTUs did not show a correlation with its activation levels compared with that during static isometric contraction tasks.(4)The activities and fatiguing characteristics of MTUs during these two types of contractions were discussed.Furthermore,the possible control strategies of CNS were explored.We speculated that the control strategies of the CNS can be seen to be mixed with independence and coordination.The CNS might control the contractions of the muscle by taking the MTUs as functional units,although certain degree of co-activation and coordination between MTUs might exist when the muscle is forced to accomplish a task.