It is necessary now to discuss the particular ability of the muscles to produce movement rapidly. This neuromuscular process manifests itself in essentially two forms: reactive ability and quickness. Although Verkhoshansky (1996) distinguishes between quickness, which describes a rapid movement involving little resistance or muscular effort, and velocity, which he relates to movements requiring considerable energy output against large resistance, this sort of distinction is avoided, because velocity has a highly specific and well-established meaning in biomechanics. It is entirely unambiguous to apply the term velocity to the actual movement of the limb, body or implement concerned without confusing it with events between stimulus and final response of the body.
Rather, quickness may be referred to the ability of the central nervous system to contract, relax or control muscle function without involvement of any preliminary stretch. Its primary role is to produce high-speed movement which do not encounter large external resistance or require great strength, power or energy consumption. It is measured as the time interval or reaction time be¬tween stimulus and response (or initiation of movement). This time must be distinguished from the movement time, which is the interval from the end of the reaction phase to the end of the movement (i.e. from beginning to end of movement). It is important to note that no correlation between reaction time and movement time has been demonstrated (Harbin et al, 1989). The reaction time consists of two stages: a latency phase between receiving of the stimulus and the appearance of electrical activity in the relevant muscles, and a response phase between the appearance of the EMG signal and the motor action. There are two electrical transmission lags associated with the reaction time (excluding any central processing time):
the time taken for the sensory input to reach the central nervous system
the time taken for a motor impulse to travel from the central nervous system to the muscles.
Average reaction times for simple tasks are (Harbin et al, 1989):
0.142 second for auditory stimuli
0.155 second for tactile stimuli
0.194 second for visual stimuli.
Since the early research of Helmholtz, it has been known that reaction times are long and variable, and that nerve conduction velocities do not account for the length and variability of times. Work by Hanes and Schall (1996) examined changes in firing rates in monkeys deciding to initiate eye movements and found that the variability in reaction time is due to the variability in how rapidly the neuronal rate of firing reaches a fixed threshold, at which point movement is initiated. In another experiment, they found that movement was not initiated until the firing rate actually cross the threshold level.
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Article Added on Wednesday, March 24, 2010
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