Systematic Review of the Impact of Transcranial Direct Current Stimulation on the Neuromechanical Management of Foot and Ankle Physical Performance in Healthy Adults

The effects of combined transcranial direct Current stimulation with physiotherapy for physical function in subjects with knee osteoarthritis: a systematic review and meta-analysis

BACKGROUND

Transcranial Direct Current Stimulation (tDCS) emerges as a promising therapeutic intervention for knee osteoarthritis (KOA), yet its impact on physical function remains insufficiently explored.

OBJECTIVES

To evaluate the relative effects of tDCS for physical function in patients with KOA.

METHODS

Pubmed, Web of Science, Scopus and Cochrane Database were explored as of August 2023 to identify studies to be included in the current systematic review and metaanalysis. Randomized controlled trials in patients with KOA comparing tDCS with placebo were included. The outcomes defined were measures of physical function (questionnaires, gait, or physical performance). The Risk of Bias tool was used to assess bias in the randomized controlled trials, whereas the PEDro scale was applied for methodological quality, and the certainty of evidence for each outcome was assessed through GRADE. Results for each outcome were synthesized using meta-analysis (random-effects model, I2-test for heterogeneity) and a subgroup analysis was performed to improve the sensitivity of the results and to explore potential moderating factors of the effect sizes.

RESULTS

Ten studies with good to excellent quality were included, analyzing a total of 628 participants. Regarding physical function, tDCS showed a favorable effect (ES: -0.58; 95%CI -0.82, -0.33; I2: 52.1%) with a low risk of bias and low to moderate certainty of evidence. The concurrent application of physiotherapy interventions and tDCS improved the effects on pain and function. Applying physiotherapy interventions, as well as adding peripheral currents, increased the effect sizes (ES: -0.95, k = 3, p = .018; ES: -0.95, k = 4 p = .001, respectively). The pattern of application of the tDCS, either daily or in alternate days, did not moderate the effect size (p = .619). Meta-regression revealed that the number of tDCS sessions did not moderate the effect size either (p = .242).

CONCLUSION

The tDCS might be a promising therapeutic approach to enhance physical function in subjects affected with KOA. However, further systematic reviews with meta-analyses should be performed with standardized and proven-efficacy physiotherapy programs, as well as with long-term results, to ascertain whether the improvement may be sustained over time. This study provides valuable insights into optimizing tDCS interventions for enhanced outcomes in the management of KOA.Protocol available via PROSPERO [CRD42023440676].

Effects of active and sham tDCS on the soleus H-reflex during standing

Weak transcranial direct current stimulation (tDCS) is known to affect corticospinal excitability and enhance motor skill acquisition, whereas its effects on spinal reflexes in actively contracting muscles are yet to be established. Thus, in this study, we examined the acute effects of Active and Sham tDCS on the soleus H-reflex during standing. In fourteen adults without known neurological conditions, the soleus H-reflex was repeatedly elicited at just above M-wave threshold throughout 30 min of Active (N = 7) or Sham (N = 7) 2-mA tDCS over the primary motor cortex in standing. The maximum H-reflex (Hmax) and M-wave (Mmax) were also measured before and immediately after 30 min of tDCS. The soleus H-reflex amplitudes became significantly larger (by 6%) ≈1 min into Active or Sham tDCS and gradually returned toward the pre-tDCS values, on average, within 15 min. With Active tDCS, the amplitude reduction from the initial increase appeared to occur more swiftly than with Sham tDCS. An acute temporary increase in the soleus H-reflex amplitude within the first minute of Active and Sham tDCS found in this study indicates a previously unreported effect of tDCS on the H-reflex excitability. The present study suggests that neurophysiological characterization of Sham tDCS effects is just as important as investigating Active tDCS effects in understanding and defining acute effects of tDCS on the excitability of spinal reflex pathways.

Motor Evoked Potential Amplitude in Motor Behavior-based Transcranial Direct Current Stimulation Studies: A Systematic Review

Motor evoked potential amplitude (MEP_amp) is frequently measured in transcranial direct current stimulation (tDCS) studies that target the primary motor cortex (M1), and a subset of these studies involve motor behavior. This systematic review explored the role of MEP_amp as an indicator of neural change in M1-targeted tDCS studies involving motor behavior (i.e., motor practice and/or evaluation of motor performance) in healthy individuals, and examined the association between changes in motor performance and MEP_amp. We executed our search strategy across four bibliographic databases. Twenty-two manuscripts met eligibility criteria. While anodal tDCS combined with motor practice frequently increased MEP_amp, MEP_amp outcomes did not necessarily align with changes in motor performance. Thus, MEP_amp may not be the most appropriate indicator of neural change in tDCS studies that aim to improve motor performance.

Computation of group-level electric field in lower limb motor area for different tDCS montages

OBJECTIVE

Transcranial direct current stimulation (tDCS) injects a weak electric current into the brain via electrodes attached to the scalp to modulate cortical excitability. tDCS is used to rebalance brain activity between affected and unaffected hemispheres in rehabilitation. However, a systematic quantitative evaluation of tDCS montage is not reported for the lower limbs. In this study, we computationally investigated the generated electric field intensity, polarity, and co-stimulation of cortical areas for lower limb targeting using high-resolution head models.

METHODS

Volume conductor models have thus been employed to estimate the electric field in the brain. A total of 18 head models of healthy subjects were used to calculate the group-level electric fields generated from four montages of tDCS for modulation of lower limbs.

RESULTS

C1-C2 montage delivered higher electric field intensities while reaching deeper regions of the lower-limb motor area. It produced a uniform polarization on the same hemisphere target with comparable intensities between hemispheres but with higher variability.

CONCLUSIONS

Proper montage selection allows reaching deeper regions of the lower-limb motor area with uniform polarization.

SIGNIFICANCE

First systematic computational study providing support to tDCS experimental studies using montages for the lower limb while considering polarity factor for balancing brain activity.

Effects of intervention combining transcranial direct current stimulation and foot core exercise on sensorimotor function in foot and static balance

Objective

This study aimed to examine the effects of combining transcranial direct current stimulation (tDCS) and foot core exercise (FCE) on the sensorimotor function of the foot (i.e., toe flexor strength and passive ankle kinesthesia) and static balance.

Methods

In this double-blinded and randomized study, 30 participants were randomly assigned into two groups: tDCS combined with FCE and sham combined with FCE (i.e., control group). The participants received 2 mA stimulation for 20 min concurrently with FCE over 4 weeks (i.e., three sessions per week). After the first two groups completed the intervention, a reference group (FCE-only group) was included to further explore the placebo effects of sham by comparing it with the control group. Foot muscle strength, passive ankle kinesthesia, and static balance were assessed at baseline and after the intervention.

Results

Compared with the control group and baseline, tDCS combined with FCE could increase toe flexor strength (p < 0.001) and decrease the passive kinesthesia threshold of ankle eversion (p = 0.002). No significant differences in static balance were observed between tDCS + FCE and control groups. The linear regression models showed an association towards significance between the percent changes in metatarsophalangeal joint flexor strength and the anteroposterior average sway velocity of the center of gravity in one-leg standing with eyes closed following tDCS + FCE (r2 = 0.286; p = 0.057). The exploratory analysis also showed that compared with FCE alone, the sham stimulation did not induce any placebo effects during FCE.

Conclusion

Participating in 4 weeks of intervention using tDCS in combination with FCE effectively enhances toe flexor strength and foot–ankle sensory function.

Effects of 4 Weeks of High-Definition Transcranial Direct Stimulation and Foot Core Exercise on Foot Sensorimotor Function and Postural Control

Objective: This study aimed to examine the effects of 4 weeks of high-definition transcranial direct current stimulation (HD-tDCS) and foot core exercise (FCE) on foot sensorimotor function (i.e., toe flexor strength and passive ankle kinesthesia) and postural control.

Methods: In total, 36 participants were randomly assigned into three groups as follows: HD-tDCS, FCE, and the control group. A total of 12 training sessions were performed over 4 weeks (i.e., three sessions per week) in the laboratory. The HD-tDCS group received 20-min HD-tDCS with a current density of 2 mA, and the FCE group completed short foot exercise, towel curls, toe spread and squeeze, and balance board training. Participants in the control group just maintained the activities what they usually did and did not receive any interventions. Foot muscle strength, passive ankle kinesthesia, and postural control were assessed at baseline and post-intervention.

Results: HD-tDCS induced a greater decrease in the percentage changes in the passive kinesthesia thresholds of ankle inversion (p &lt; 0.001) and eversion (p = 0.013) than the control group. Compared with the control group, a significant increase in the percentage change in the metatarsophalangeal joint flexor strength was found in the HD-tDCS group (p = 0.008) and the FCE group (p = 0.027), and a significant increase in the percentage change in toe flexor strength was observed in the FCE group (p = 0.015). Moreover, FCE induced a greater reduction in the percent changes in the medial–lateral average center of gravity sway velocity in one-leg standing with eyes open (p = 0.033) and the anteroposterior average center of gravity sway velocity in one-leg standing with eyes closed (p &lt; 0.001) than control.

Conclusion: This study demonstrated that 4 weeks of HD-tDCS and FCE induced distinct benefits on foot sensorimotor function and the standing postural control performance in healthy young adults. HD-tDCS could improve the metatarsophalangeal joint flexor strength and the passive kinesthesia thresholds of ankle inversion and eversion. Meanwhile, FCE could also enhance foot muscle strength and enhance postural control performance in one-leg standing.