Motor time (MT) of the rectus femoris was measured for seven normal subjects during rapid extension of the knee joint at 60° flexion in the sitting position with and without external loads on the leg. MT prolonged linearly with external load from 0 to 4 kg. Tangent of this linearity, the rate of tension development (RTD), was significantly different from one another of the subjects and was not related to MT without external load. RTD is a relevant index to assess an efficiency of rapid force

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... eration of a muscle, motor time ; external load Motor time (MT) is defined as a latency from onset of EMG activities of a prime mover muscle to initiation of an actual movement in reaction time experiments. MT represents the time necessary for the muscle to develop the tension required for the initiation of rapid movement (Weiss 1965) . MT has been regarded as an index of rapid force generation independent of the strength of maximal isometric voluntary contraction (MYC) ). However, recent study suggests that MT is influenced by three independent factors, namely, the load including the weight of the body segment to be moved, the rate of tension development (RTD), and the tension-lag time (TLT) which is defined as a latency of tension measurable externally from onset of EMG activities. TLT is known to have little individual difference among normal subjects (Nakamura et al. 1984) . MT prolongs in proportion to the increase in the weight to be moved Irie et al. 1983 ). RTD is obtained by calculating 'increment in weight loaded/increment in MT' in their studies. RTD should be interpreted as an efficiency of rapid force generation in an individual. The present study attempts to explore the influence of load and RTD on MT, measuring MTs of the rectus femoris for rapid knee extension under loading of different weights. Seven normal men aged from 26 to 48 years participated in the experiment. By means of the water-immersion method, the mid-point of the left leg and foot was defined as the point corresponding to one half of the volume of them. The weight of the left leg and foot was roughly estimated as 7 per cent of the total body weight (Matsui 1956). The subject sat on a chair with his knees at 60° flexion and with his heels supported by a horizontal bar. He was asked to extend his left knee as fast as possible in response to a tone stimulus. The details of measurement were already reported ). In short, MT was measured as a time elapsed from onset of EMG activities of the rectus femoris to off-signal of a switch attached to the left heel. Five or more trials without external load were performed with intervals more than 10 sec. Then external loads of 1, 2, 3 and 4 kg were attached to the mid-point of the left leg by belting a band with the lead, and the same trials were performed