Motor planning perturbation: muscle activation and reaction time

Abstract

Reaction time (RT) is the time interval between the appearance of a stimulus and initiation of a motor response. Within RT, two processes occur, selection of motor goals and motor planning. An unresolved question is whether perturbation to the motor planning component of RT slows the response and alters the voluntary activation of muscle. The purpose of this study was to determine how the modulation of muscle activity during an RT response changes with motor plan perturbation. Twenty-four young adults (20.5 ±1.1 yr, 13 women) performed 15 trials of an isometric RT task with ankle dorsiflexion using a sinusoidal anticipatory strategy (10-20% maximum voluntary contraction). We compared the processing part of the RT and modulation of muscle activity from 10 to 60 Hz of the tibialis anterior (primary agonist) when the stimulus appeared at the trough or at the peak of the sinusoidal task. We found that RT ( P = 0.003) was longer when the stimulus occurred at the peak compared with the trough. During the time of the reaction, the electromyography (EMG) power from 10 to 35 Hz was less at the peak than the trough ( P = 0.019), whereas the EMG power from 35 to 60 Hz was similar between the peak and trough ( P = 0.92). These results suggest that perturbation to motor planning lengthens the processing part of RT and alters the voluntary activation of the muscle by decreasing the relative amount of power from 10 to 35 Hz. NEW & NOTEWORTHY We aimed to determine whether perturbation to motor planning would alter the speed and muscle activity of the response. We compared trials when a stimulus appeared at the peak or trough of an oscillatory reaction time task. When the stimulus occurred at the trough, participants responded faster, with greater force, and less EMG power from 10-35 Hz. We provide evidence that motor planning perturbation slows the response and alters the voluntary activity of the muscle.

Keywords: electromyography; motor neuron pool modulation; reaction time.

Fig. 1.

A: schematic drawing of the experimental set up and arrangement of the left foot for 1 participant. The left foot was placed and rested on a customized foot device with an adjustable footplate and secured by a strap over the metatarsals. Participants performed a sinusoidal reaction time task with ankle dorsiflexion. B: representation for 1 participant of the reaction time task at 3 different time points. Participants were presented with unanticipated visual stimulus in the middle of the screen while performing a sinusoidal isometric force-tracking task. This stimulus appeared with illumination of a green background. Participants were asked to respond to the stimulus as quickly as possible by dorsiflexing the ankle with adequate force to show the reaction clearly. The duration of the task was 37 s. Left: anticipatory strategy (type of force control before the stimulus). Middle: moment when the stimulus first occurs. Right: reaction of the participant.

Fig. 2.

Example of data collection and analysis of a trial in the trough (left) and peak (right) condition. Force (A) and electromyography (EMG) (B) data were collected from 1 participant for the trough condition and another participant for the peak condition with anticipation, stimulus, and response identified. C: wavelet analysis from frequencies 10–35 Hz corresponding to the same length of time as force and EMG data. D: representation of motor planning hypothesis in which the trough shows compatibility in the intention and reaction, whereas, in the peak, there is incompatibility. RT, reaction time.

Fig. 3.

A: reaction time (RT) (premotor time) was faster when the stimulus occurred at the trough than at the peak. B: during the response, the force produced was greater when the stimulus occurred at the peak than at the trough. C: electromyography (EMG) power from 10 to 35 Hz, on the other hand, was less when the stimulus occurred at the peak than at the trough. *Significant differences between the stimulus occurring at the trough and the peak (n = 24). MVC, maximum voluntary contraction.

Fig. 4.

On the y-axis is the change in force produced during the response as a percentage of pre-maximum voluntary contraction (MVC) between peak and trough. On the x-axis is the change in electromyography (EMG) power from 10 to 35 Hz as a percentage of the total power from 10 to 100 Hz between peak and trough. An increase in force produced during the response is significantly associated with a decrease in EMG power from 10 to 35 Hz (n = 24).