Effect of conventional transcranial direct current stimulation devices and electrode sizes on motor cortical excitability of the quadriceps muscle.

BACKGROUND: There is a growing concern among the scientific community that the effects of transcranial direct current stimulation (tDCS) are highly variable across studies. The use of different tDCS devices and electrode sizes may contribute to this variability; however, this issue has not been verified experimentally. OBJECTIVE: To evaluate the effects of tDCS device and electrode size on quadriceps motor cortical excitability. METHODS: The effect of tDCS device and electrode size on quadriceps motor cortical excitability was quantified across a range of TMS intensities using a novel evoked torque approach that has been previously shown to be highly reliable. In experiment 1, anodal tDCS-induced excitability changes were measured in twenty individuals using two devices (Empi and Soterix) on two separate days. In experiment 2, anodal tDCS-induced excitability changes were measured in thirty individuals divided into three groups based on the electrode size. A novel Bayesian approach was used in addition to the classical hypothesis testing during data analyses. RESULTS: There were no significant main or interaction effects, indicating that cortical excitability did not differ between different tDCS devices or electrode sizes. The lack of pre-post time effect in both experiments indicated that cortical excitability was minimally affected by anodal tDCS. Bayesian analyses indicated that the null model was more favored than the main or the interaction effects model. CONCLUSIONS: Motor cortical excitability was not altered by anodal tDCS and did not differ by devices or electrode sizes used in the study. Future studies should examine if behavioral outcomes are different based on tDCS device or electrode size.

Evaluation of motor cortical excitability using evoked torque responses: A new tool with high reliability.

BACKGROUND: Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) are typically recorded via surface electromyography (EMG). However, another suitable alternative may be recording torque output associated with MEPs, especially when studying multiheaded muscles (e.g. quadriceps) for which EMG may not be ideal. METHODS: We recorded the motor evoked torque elicited by TMS along with conventional EMG-based MEPs (MEPEMG) over a range of TMS intensities (100-140 % of active motor threshold [AMT]) from twenty healthy young adults on two different days. MEPs were normalized using different normalization procedures (raw, normalized to maximum voluntary isometric contraction [MVIC], and peak MEP). Additionally, motor evoked torque was normalized to TMS-evoked peripheral resting twitch torque. Intraclass correlation coefficients (ICCs) were determined for each of these variables to compute reliability. RESULTS: Motor evoked torque showed good to excellent reliability (ICC: 0.65-0.90) at TMS intensities ≥ 110 % AMT, except when normalized by peak MEP. The reliability of raw MEPEMG and MVIC normalized MEPEMG was fair to excellent only at ≥ 130 % AMT (ICC: 0.42-0.82) and at ≥ 120 % AMT (ICC: 0.41-0.83), respectively. The reliability of both MEPEMG and motor evoked torque generally increased with increasing TMS intensities, with motor evoked torque normalized to the resting twitch torque yielding the best ICC scores. COMPARISON WITH EXISTING METHODS: When compared with conventional MEPEMG, motor evoked torque offers superior and reliable estimates of corticospinal excitability, particularly when normalized to resting twitch torque. CONCLUSIONS: TMS-induced motor evoked torque can reliably be used to measure corticospinal excitability in the quadriceps muscles.

Learning new gait patterns is enhanced by specificity of training rather than progression of task difficulty.

The use of motor learning strategies may enhance rehabilitation outcomes of individuals with neurological injuries (e.g., stroke or cerebral palsy). A common strategy to facilitate learning of challenging tasks is to use sequential progression - i.e., initially reduce task difficulty and slowly increase task difficulty until the desired difficulty level is reached. However, the evidence related to the use of such sequential progressions to improve learning is mixed for functional skill learning tasks, especially considering situations where practice duration is limited. Here, we studied the benefits of sequential progression using a functional motor learning task that has been previously used in gait rehabilitation. Three groups of participants (N = 43) learned a novel motor task during treadmill walking using different learning strategies. Participants in the specific group (n = 21) practiced only the criterion task (i.e., matching a target template that was scaled-up by 30%) throughout the training. Participants in the sequential group (n = 11) gradually progressed to the criterion task (from 3% to 30% in increments of 3%), whereas participants in the random group (n = 11) started at 3% and progressed in random increments (involving both increases and decreases in task difficulty) to the criterion task. At the end of training, kinematic tracking performance on the criterion task was evaluated in all participants both with and without visual feedback. Results indicated that the tracking error was significantly lower in the specific group, and no differences were observed between the sequential and the random progression groups. The findings indicate that the amount of practice in the criterion task is more critical than the difficulty and variations of task practice when learning new gait patterns during treadmill walking.