This study examined transient effects of transcranial direct current stimulation (tDCS) on bilateral force control in lower extremities. We recruited 14 healthy young adults and administered bilateral ankle-dorsiflexion force control tasks at 10% of maximal voluntary contraction. Participants were able to use real-time visual information on a targeted force level and forces produced by both feet. While performing bilateral force control, we provided active- and sham-tDCS in a random order. Bilateral tDCS protocol used for this study included anodal and cathodal stimulation targeting left and right leg areas of the primary motor cortex between hemispheres. Bilateral force control capabilities were estimated by calculating force accuracy, variability and regularity. In addition, we determined whether force control patterns differed between feet across active- and sham-tDCS conditions. The findings revealed that force accuracy and variability were significantly improved after applying active-tDCS protocol as compared with those for sham-tDCS condition. However, no differences in force control between feet were observed. These findings suggest that bilateral tDCS protocols may be a viable option for improving motor functions of lower limbs.

Background: Aging can cause degenerative changes in motor and cognition-related brain areas, presumably by interfering with gait performance in healthy aging populations. We aimed to assess the effects of transcranial direct current stimulation (tDCS) on single- and dual-task walking performances in healthy older adults using meta-analytic approaches. Methods: Eleven studies were qualified based on the inclusion criteria: (a) healthy older adults, (b) treatment = tDCS protocols, (c) control = sham stimulation, (d) gait performance outcomes, and (e) randomized controlled trials using parallel or crossover designs. Effect sizes were estimated using standardized mean difference (SMD) to examine gait performances between active tDCS and sham stimulation. A separate random-effect meta-analysis was performed to determine the effects of tDCS protocols on gait performance during single- and dual-task walking tasks. Results: During single-task walking, the random-effects meta-analysis showed improvements in stride time variability (SMD = 0.203; p = 0.005) and functional mobility (SMD = 0.595; p < 0.001). Moreover, single-task walking performances were improved when the tDCS protocols targeted the primary motor cortex (SMD = 0.424; p = 0.005) and used off-line stimulation (SMD = 0.168; p = 0.008). During dual-task walking, tDCS improved gait speed (SMD = 0.177; p = 0.025) and dual-task cost for gait speed (SMD = 0.548; p < 0.001). Dual-task walking performances were advanced when the tDCS protocols targeted the dorsolateral prefrontal cortex (SMD = 0.231; p = 0.029) and multiple areas including prefrontal cortex (SMD = 0.382; p = 0.001), and applied off-line stimulation (SMD = 0.249; p < 0.001). Conclusions: These findings indicate that the tDCS protocols may be a promising tool to support mobility and reduce gait-related challenges in the healthy aging population.

Older adults normally showed impaired fine motor control capabilities, as indicated by increased force error and variability during submaximal force control tasks. However, whether aging additionally disrupts temporal smoothness of the force trajectory remains unclear beyond the magnitude of force fluctuation. This study investigated age-related changes in bimanual isometric force smoothness using YANK analysis (i.e., time-derivative of force; dF/dt). Fifteen younger adults (mean ± SD of age = 25.4 ± 2.5 years) and fifteen older adults (mean ± SD of age = 64.9 ± 6.5 years) performed bimanual hand-grip isometric force control at 10 % of maximum voluntary contraction across the vision and no-vision conditions. Force accuracy (relative root-mean-square error; rRMSE), variability (coefficient of variation; CV), smoothness (YANK), and interlimb force coordination (correlation coefficient) were quantified. Receiver operating characteristic (ROC) curve and Pearson’s correlation analyses were conducted to evaluate the discriminative ability of each variable and potential relationships between YANK and conventional force control variables. Older adults showed significantly higher YANK values than younger adults in both vision and no-vision conditions, indicating reduced force smoothness. Greater rRMSE, CV, and correlation coefficient values for older adults were observed only in the vision condition. The ROC curve analysis revealed that YANK effectively discriminated between the younger and older adults in both vision and no-vision conditions. For older adults, increased YANK was significantly correlated with greater rRMSE and CV in the vision condition and with greater CV in the no-vision condition, whereas no significant correlations appeared in younger adults. These findings suggest the YANK approach can capture age-related fine motor control deficits that conventional variability measures may overlook.