CHANGES IN CHARACTERISTICS OF RECRUITMENT MODES IN HUMAN MOTOR UNITS AT VOLUNTARY MUSCULAR RELAXATION / 体力科学

ABSTRACT

It had been proposed by Kurata that relative threshold value F<SUB>th</SUB> of single motor units depends on the gradient G (kg/sec) of the tension increase of muscle in such a manner as<BR>F<SUB>th</SUB> (≡T.G) =ρG<SUP>λ</SUP><BR>Here T<SUB>1</SUB> is the time interval from the onset of EMG to the moment of recruitment of a motor unit and ρ is a proportional constant. The motor unit with a <I>positive/negative</I> value of λ is characterized to be <I>static/phasic</I>.<BR>Morimoto and Hasegawa reported that the above relation also holds for the relaxation period, provided that Ftn and G are replaced by the tension at the moment of silence and the absolute value of G, respectively. In this case, the motor unit with a <I>positive/ negative</I> value of λ is characterized to be <I>phasic/static</I>.<BR>In order to investigate the characteristics of motor units during relaxation period under the influence of pre-relaxation states, such as the increase of tension up to 4kg at the gradient of 0.5kg/sec and the state sustained at 4.0kg for 30sec. Single motor units of <I>m, vastus medialis</I> were studied by the same method as that of Morimoto and Hasegawa.<BR>The following results were obtained;<BR>1) Both at contraction and relaxation periods, the above relation was confirmed.<BR>2) The motor unit with characteristics of recruitment modes was influenced by the pre-relaxation state, but its tendency was not definite. These results suggest that <I>phasic/static</I> characteristics of motor units are not stationary but fluctuate depending on the pre-relaxation states.