Mechanomyographical signals during prolonged intermittent muscular contractions at low levels under various duty cycles / 体力科学

To obtain one of the possible factors on the abrupt increment of mechanomyographic (MMG) signal during prolonged isometric constant contraction (PICC; <20%MVC), the present study focused on the iMMG changes from m. vastus medialis (VM) and/or lateralis (VL) during the intermittent isometric constant contractions (IICC) at various condition of the duty cycle (or relaxation/contraction ratio: R-C ratio) and the target tension. Target tension set at 5, 10 and 15 %MVC and the duty cycle set at 16s-ON/4s-OFF, 56s-ON/4s-OFF and 116s-ON/4s-OFF. Simultaneous recording of myoelectrical signal (MES) was made on VM and/or VL. 1) IMMG increased weakly and monotonously with the cumulative load which is accumulated the acreage of tension and time through the IICC. Also, iMES sustained or increased weakly but the increment ratio was lower than iMES under PICC condition. 2) The increment ratio of iMMG (or gradient of linear regression formula with the cumulative load: G-iMMG) from both muscles increased depend on the increment ratio of the cumulative load (or gradient of linear regression formula with the time) in the stored data under the IICC conditions. But there could not be seen the distinct relation in that of iMES. 3) The G-iMMG decreased with R-C ratio. But G-iMES did not show distinct relationship with R-C ratio among subjects and between muscles. Linear incremental tendency of iMMG would originate from the inserted muscle relaxation during IICC. Therefore, the abrupt increase of iMMG during PICC suggests to be caused from the continuous muscle contraction itself.

RELATIONSHIP BETWEEN INTER-SPIKE INTERVAL AND AMPLITUDE OF MECHANOMYOGRAPHIC SIGNAL FROM SINGLE MOTOR UNITS DURING VOLUNTARY MUSCLE CONTRACTION / 体力科学

To investigate factors that increase the mechanomyographic (MMG) signal during voluntary prolonged muscle contraction at relatively low tension levels, the MMG signal from the motor unit (MU mechanical signal: MUMS) was analyzed. In the present study, the author focused on the interval dependency of MUMS amplitude as one of the factors to increase MMG. From the m. vastus medialis or m. vastus lateralis, MUAP and myoelectrical signal (MES) were recorded by Ag/AgCl disc electrode and MMG and MUMS were respectively recorded by condenser microphone during various types of muscle contractions; brief isometric constant contraction (BICC), prolonged isometric constant contraction (PICC) and prolonged isometric contraction under constant MU firing interval (PIC-CFI).The amplitude of positive phase in MUMS (MS-V_positive) increased proportionally with the firing interval of the MU in BICC. The firing interval of MU showed an initial elongation, followed by shortening during PICC. During PICC and PIC-CFI, MS-V_positive sustained in an initial part of the contraction and then abruptly increased. There was no meaningful relationship between the firing interval and MS-V_positive during PICC and PIC-CFI. However, the increment ratio of the MS-V_positive in PIC-CFI was smaller than that in PICC.The present results on MS-V_positive suggested that the increment of MMG during PICC did not depend on the firing interval change in the activated MUs. Based on findings with different increment ratios in MS-V_positive between PICC and PIC-CFI, it is suggested that the hysteresis of muscle contraction is one of the factors causing an increase in MUMS and MMG.

MOTOR UNIT MECHANOMYOGRAPHIC SIGNAL AND SYNCHRONIZED ACTIVITY IN MOTOR UNITS DURING VOLUNTARY MUSCLE CONTRACTIONS / 体力科学

The aim of the present study was to investigate, by analysis of motor unit action potential (MUAP) and motor unit mechanomyogram (MUMS) wave-forms, whether the synchronized activity of motor units (MUs) is a factor in increasing the integrated value of a mechanomyogram during muscle contraction at relatively low tension levels. MUAP and MUMS of m. vastus medialis were recorded by Ag/AgCl disc electrode ( 5mmφ) and condenser microphone ( 10mmφ), respectively, during muscle contractions, brief isometric constant contractions (BICC) and prolonged isometric constant contraction (PICC) at the target torques from just above the decruitment threshold torque of the objective MU to 20% of maximal voluntary contraction (MVC). The degree of synchronization of MUs, defined from the amplitude of late positive deflection (VLPD), could be seen in MUAP wave-forms.The amplitude of the positive phase in MUMS (MS-V_positive) had no relationship with the increase of V_LPD in BICC condition. During PICC, MS-V_positive and V_LPD increased with time. Applying linear regression analysis on the relation between V_LPD and MS-V_positive, except for data at 20%MVC, there was significant correlation. However, the scale of the time increments, between V_LPD and MS-V_positive, were different comparing exponential and logarithmic figures, respectively. Therefore, in the present experiment, the meaningful relationship between the two parameters could not be introduced. It is necessary to further investigate the relationship between the two parameters including the firing frequency of MU, intramuscular pressure and extent of recording area of both sensors.

Efeitos de dois parâmetros antropométricos no comportamento do sinal mecanomiográfico em testes de força muscular / Effects of two anthropometric parameters in the behavior of the mechanomyographic signal on muscle force tests

Apesar da mecanomiografia (MMG) ser uma técnica com crescente destaque em investigações relativas ao fenômeno da contração muscular, poucos trabalhos se dedicaram a entender os possíveis efeitos de variáveis antropométricas no sinal de MMG. Portanto, o objetivo do estudo foi avaliar os efeitos da dobra cutânea e do perímetro do braço no comportamento do sinal de MMG nos domínios do tempo e da freqüência em diferentes níveis de contração isométrica. Participaram do estudo 21 indivíduos do sexo masculino (24,9 ± 7,8 anos) e 21 do feminino (20,7 ± 2,5 anos), todos destros. O protocolo experimental constou de um teste de contração voluntária máxima (CVM) que permitiu determinar cinco cargas percentuais administradas durante os testes de força (20 por cento, 40 por cento, 60 por cento, 80 por cento e 100 por cento da CVM). Um acelerômetro biaxial foi colocado sobre o ventre muscular do bíceps braquial direito. O sinal de MMG foi analisado nos domínios do tempo, mediante o comportamento da amplitude do sinal, por meio do cálculo da raiz média quadrática (valor RMS), e da freqüência, através da freqüência mediana (FM) calculada a partir do seu espectro de potência. Estes parâmetros foram extraídos a partir dos sinais gerados na direção X, perpendicular às fibras musculares. Foram também medidos os parâmetros antropométricos dobra cutânea biciptal (BD_DC_B) e perímetros de braço relaxado (BD_PR) e contraído (BD_PC) no terço médio do braço direito. Ambos os grupos apresentaram um comportamento decrescente da FM com o nível de contração. O valor RMS apresentou comportamento crescente com a carga para ambos os grupos. Houve diferenças estatisticamente significativas (p < 0,05) entre as variáveis antropométricas quando comparados os grupos. Sugere-se que o valor RMS, ao contrário da FM, que poderá sofrer atenuações por parte dos tecidos presentes na interface entre o músculo e o transdutor, seja mais consistente na discriminação da força muscular.

In spite of being a useful method on muscle contraction analysis, few studies have been dedicated to understand how some anthropometrical parameters can interfere in the mechanomyographic (MMG) signal properties. Therefore, the aim of this study was to evaluate the effects of skinfold thickness and arm circumference in the MMG signal behavior in the temporal and frequency domains at different levels of isometric contraction. Twenty-one males (24.9 ± 7.8 years) and 21 females (20.7 ± 2.5 years), all right-handed, participated in this study. Prior to the MMG signals collection, a maximum voluntary contraction test (MVC) was performed to further determine the five levels (20 percent, 40 percent, 60 percent, 80 percent and 100 percent of CVM) used during the tests. The two groups performed isometric contractions in the five different levels while MMG signal was collected from the right biceps brachii muscle by a biaxial accelerometer. The root mean square value (RMS), and the median frequency (MF) calculated from the MMG power spectrum were extracted from the MMG signals in X direction, which meant perpendicular to the muscle fibres direction. The bicipital skinfold thickness (BD_DC_B), and the circumference of the right arm, with and without contraction of elbow muscles flexors, have also been measured. The MF presented a decreasing behaviour with the increasing level of contraction; the RMS value followed an increasing pattern, though. There were significant statistical differences (p < 0.05) for the anthropometric variables between groups. It is suggested that the RMS value, differently from the MF, which can be disrupted by the tissues between the muscle and the transducer, should be better correlated with muscle force level.