Muscle activation (activation time) and the beginning of movement (motor reaction time) can be changed depending on the complexity of the task. The objectives of this study were to compare the time for activation of the paraspinal and the vastus lateralis muscles, and the motor reaction time during the execution of the tasks sit-to-stand (STS) and sit-to-walk (STW), which includes the execution of the subsequent task of gait initiation. Twelve healthy young subjects participated in the study. They performed two tasks (STS and STW), five times each, randomly, separated by two minutes of rest. The kinematics of the movement were recorded using a digital electrogoniometer attached to the hip joint and muscle activation using surface electromyography in both muscles. The average of the five repetitions was calculated for each task. The beginning of the task was signaled by a luminous device, which was also used to identify the initial point for calculating the activation time and motor reaction time. Both muscles showed a longer latency for the activation time and motor reaction time during the STW task when compared with STS. Based on these results, it can be concluded that both the postural (paraspinal) and prime mover muscles (vastus lateralis) undergo change in the motor programming during the execution of the STS task when a subsequent task (gait initiation) is included. Motor programming is dependent on task complexity, where a more complex task (STW) will result in delays of movement programming and execution.
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