| The purpose of this study was to compare motor preparation profiles between young and old adults. Twenty old adults, mean age of 68.5 years, and 20 young adults, mean age of 23.1 years, were tested. The subjects were tested on four days during a one-week period. On each day the subject performed 84 right plantar flexions in a simple reaction time paradigm. Day one was a practice session. On days two, three and four, motor preparation was assessed during the foreperiod, the interval between the presentation of the warning stimulus and the presentation of the response stimulus, and during the response period, the interval between the presentation of the response stimulus and the onset of the voluntary movement, by eliciting one of the following reflexes: unilateral right Achilles tendon reflex, unilateral right tibial H-reflex, or simultaneous bilateral tibial H-reflexes. There was an age-related decrease in the amplitude of the electrically evoked H-reflex without a concomitant change in the amplitude of the Achilles tendon reflex. It was postulated that the loss of functional motoneurons with age contributed to the decrease in the electrically evoked reflex response, while the increase in muscle-tendon stiffness with age was the compensatory mechanism by which the mechanically evoked reflex response was maintained with age. When the H-reflex was used to assess motor preparation during the response period, the onset of reflex facilitation was delayed and the duration of reflex facilitation was prolonged in the old adults. There was no age-related difference in the percent of H-reflex facilitation. When the Achilles tendon reflex was used to assess motor preparation during the response period, there was a prolonged duration of reflex facilitation with age and the percent of reflex facilitation was greater in the young adults than in the old adults. These age-related differences in reflex facilitation during the response period suggest a reduced sensitivity of the motoneuron pool to temporal and spatial summation effects with age. During the foreperiod, the responding motoneuron pool was depressed as compared to non-responding motoneuron pool in the young adults only suggesting an age-related decrement in the general facilitation effect of motor preparation. |
