Variation of the length of skeletal muscle fibers and sarcomere lengths were investigated in extensor digitorum longus (EDL) and soleus (SOL) muscles in mice (ICR) during postnatal growth in the range from 2 to 12 weeks after the birth. EDL and SOL were fixed in 2.5% gultaraldehyde contained Lock's solution at right angle position of articulatio genus and talocruralis. Single muscle fibers were dissected and lengths of fibers were measured under the dissecting microscope. For measuring sarcomere lengthes, laser diffraction technique was used. Following results were obtained; (1) the length of muscles and muscle fibers increased rapidly up to 8 and 5 weeks in EDL and SOL respectively; (2) relative muscle and fiber lengths at 12 weeks against 2 weeks were 1.72 and 1.30 in EDL, 1.60 and 1.32 in SOL; (3) sarcomere lengths at both ends of the fiber showed no significant change during growth; (4) on the other hand, sarcomere lengths at mid point showed maximum values at 8 and 5 weeks in EDL and SOL respectively. These observations suggest that the sarcomere are stretched especially at mid point when lengths of muscle fibers increase rapidly. It would be possible to consider that this changes of the sarcomere length during postnatal growth affect the contractile properties of muscles.

The pressure exerted by the index finger tip was continuously recorded while the human subject was trying to maintain the pressure at the level of a given pressure (this pressure is named ‘target pressure’, 0.5-500g) with optical information for ten minutes, and the results obtained under various conditions were analysed.
1) The errors tended to be minimal in the range between 40 and 50g target pressure, and increased approximately in proportion to the target pressure above 50g.
2) When the subject relaxed his index finger, the errors decreased with decreasing target pressure in the range of less than 50g.
3) The errors were increased by excluding optical information. In target pressure less than 50g, the increment of the errors due to a decrease in the skin pressure intensity was further increased by exclusion of optical information.
4) In small target pressures, the errors tended to decrease transiently in evening, while in large target pressures the errors showed a gradual increase from morning toward evening.
5) The errors showed generally minimal values 2 or 3 minutes after the beginning of the measurement, and thereafter increased more remarkably in large target pressures than small ones.

In this study, the relationships between the contractile properties of human motor units and their discharge patterns, characteristics of recruitment and others were examined. Action potentials of single human motor units in m, extensor digiterumwere detected by use of an inserted electrode. Tension curves of single motor units were recorded under the conditions that tension in finger extension was gradually increased, and that the subject relaxed his muscle as soon as he observed a sweep on a Braun tube triggered by the action potential of a single motor unit. About 100 twitch tension curves were summed up to obtain the average tension curve of a single motor unit. The τ-S diagrams were obtained from the mean discharge intervals (τ) of motor units at various sustained tensions and their standard deviations (S) . The λ values were also obtained from the relation F_th (-T₁G) =ρG^λ.
It was observed that the contraction time of the twitch tension curve of single human motor unit in a muscle was nearly proportional to τs (the τ values at a constant S value) . We presume that the regularity of discharging of motor unit affects the contraction speed in human muscle, because the motor unit with more regular discharging has longer contraction time of twitch.

1) The reaction time of the avoidance movement of jumping off toward a given place, which was regarded as an indicator of the quickness of movement of the group of muscles of the lower limbs, was measured and compared with the quickness of these of the upper limbs.
2) The reaction time of movement of placing one's hand from one to the other of the two pushing-plates, by pushing which the electric circuits were closed, was measured. The reaction time of the commencement of the movement was similar to the simple reaction time, and its correlation with age was not high. The time taken to place one's hand on the other plate tended to be a little longer with subject of middle and high ages.
3) The dispersion of the values of reaction time of both the commencement and the termination of movement when measured repeatedly was similar to that of the simple reaction time, but the dispersion of the difference between these two kinds of reaction times, that is, that of the time taken to move one's hand was smaller than that of the simple reaction time.
4) Concerning one-step jumping aside to the right hand side, the reaction time of the commencement of movement under the condition that the legs were moved one by one was longer than the simple reaction time, and was also longer than that of the simultaneous movement of the both legs. The reaction time of both the commencement and the termination of movement was longer with higher ages, and was longer with women than with men. This relation was more evident in the successive movement of each legs than in the simultaneous one. The frequency distribution curve obtaind by repeating the measurement of the reaction time of jumping off showed a nearly normal distribution curve. The correlation between the reaction time of hand movement and that of jumping off was not so high.
5) The scattering of the landing position of one-step jumping aside to the right hand side did not show any correlation with the values of the time taken to move one's legs.There was, however, a high correlation (r=+0.96) between the scattering of the landing position and that of the reaction time.
6) There was no large difference between the reaction time of one-step jumping aside to the right hand side and that to the left hand side. With respect to forward jumping, the period of time during which the pressure at the first pushing-plate remained equal to the body weight was longer when the both legs were not simultaneously moved. There was no large difference between forward and backward jumping.
7) The variation of the reaction time of two-steps jumping with respect to age was similar to that of one-step jumping.
8) The results mentioned above indicate that one-step jumping aside is to be adopted as an indicator of the quickness of movement of the group of muscles of the lower limbs, and that it is practically useful to measure the time taken for one's one leg to tread on the second pushing-plate.

1) The time course of nearly isometrical tension development of human voluntary muscular contraction was analyzed for the purpose of finding some desirable indicators of the capability of human body to exert maximal muscle force as fast as possible.
2) The isometric tension curves of maximal exertion of hand gripping, arm and indexfinger bending recorded, the linear relation between P_max-P, which is a difference between its maximal tension and each value of tension in the time course, and log t, which is the logarithm of time corresponding to each value of tension, was found in each of three portions of a whole curve of time course of tension, the three portions being a range from P₀ (log t₀) to P₁ (log t₁), from P1 (log t₁) to P₂ (log t₂) and from P₂ (log t₂) to P_max (log t_max.) respectively.
3) The range from P₁ (log t₁) to P₂ (log t₂) is a phase in which the tension increases most rapidly from an initial small tension to a large tension and is, therefore, regarded as a most characteristic one with respect to the time course of muscle force development exerted voluntarily as fast as possible.
4) The angles of intersection of the line connecting P₁ (t₁) with P₂ (t₂) or P₁ (log t₁) with P₂ (log t₂), with the abscissa in the tension-time or tension-log t curve, which are denoted as or respectively, were found to be able to be adopted as indicators of the capability of human body to exert maximal muscular force for a short period of time.
5) The tension curves were found to be nearly the same each other when they were recorded three or four times in the interval of 30 seconds with one and the same subject. If the difference is small between the first and the second record of the time course of a given subject, either of them may be adopted, but if not so, one have to only record the third curve and adopt either of the two resembling each other among the three records.
6) Concerning the subjects of the same age, the difference between P_max and P₂ was small and the tan was large with a subject whose P was large. The tan ψ or tanθ was as a rule smaller with subjects of middle and high ages than with younger ones.
7) The Pmax decreased with lapse of time when the maximal effort of contraction whose duration was one second was repeated in the interval of three seconds for two minutes. Under this condition the tan ψ increased in some cases, but did not in other cases. Also there were some cases in which t₁ remained constant in spite of a decrease in the P_max with subjects of middle and high ages, P₁ of them decreased with decreasing t₁. In short the change of the characteristics of the tension curves due to fatigue showed some individual variation.
8) The decrease in the P_max during a continuation of maximal effort of contraction was more remarkable than in the case of repeated contraction mentioned above, but the changes of the values of indicators were nearly the same in both cases.
9) The P_max of hand gripping and index-finger bending decreased to some extent when examined immediately after the effort of contraction had been continued under the condition that the blood stream in the fore-arm was stopped by applying the maximal blood pressure by a manchette of sphygnomanometer. The change of the values of the indicators under this condition was nearly the same as under the condition without the application of pressure, but under the former condition there were some cases in which a portion of the tension curve corresponding to P₁ (log t₁) deviated from the intersection of the lines, or some cases in which the records of one