Sangeetha Madhavan

Neuromuscular Control of the Knee During a Resisted Single-Limb Squat Exercise

Background Closed kinetic chain exercises such as single-limb squats are preferred for knee rehabilitation. A complete understanding of the neuromuscular control of the knee during the single-limb squat is essential to increase the efficiency of rehabilitation programs. Hypothesis Performing a controlled single-limb squat with resistance to knee flexion and extension will increase the coactivation of the hamstring muscle group, thus reducing the quadriceps/hamstrings ratio. Study Design Descriptive laboratory study. Methods A total of 15 healthy human subjects (7 women, 8 men) performed controlled single-limb squats in a custom mechanical device that provided resistance to both flexion and extension. Subjects performed the task at 3 levels of resistance, set as a percentage of body weight. Surface electromyographic recordings from 7 muscles (gluteus medius, rectus femoris, vastus medialis oblique, vastus lateralis, biceps femoris, semitendinosus, and medial gastrocnemius) were collected during the task. Results Biceps femoris activity during knee flexion increased from approximately 12% maximum voluntary isometric contractions during low resistance (0% body weight) to approximately 27% maximum voluntary isometric contractions during high resistance (8% body weight). Although the quadriceps had greater activity than the hamstrings at all levels of resistance, the quadriceps/hamstrings ratio declined significantly with resistance (F2,27 = 29.05; P=. 012) from 3.0 at low resistance to 2.32 at the highest resistance. Conclusions Performing controlled resisted single-limb squats may help to simultaneously strengthen the quadriceps and facilitate coactivation of the hamstrings, thus reducing anterior tibial shear forces. The coactivation may also increase the dynamic control of the knee joint. Clinical Relevance The typical single-limb squat exercise performed in the clinic does not usually control for bidirectional resistance and knee joint excursion. As seen in this study, controlled single-limb squats at increased levels of resistance help to increase the coactivation of the hamstring muscles, which is essential to optimize neuromuscular control of the knee.

Focal and bidirectional modulation of lower limb motor cortex using anodal transcranial direct current stimulation

BACKGROUND Because we are interested in non-invasive transcranial brain stimulation as an adjuvant to post-stroke walking therapy, we applied direct current stimulation (tDCS) preferentially to either the left or right lower limb motor cortex (M1) in two separate sessions and assessed the resulting modulation in both cortices. OBJECTIVE/HYPOTHESIS We hypothesized that tDCS applied preferentially to one lower limb M1 of healthy subjects would induce between-hemisphere opposite sign modulation. METHODS Transcranial magnetic stimulation (TMS) with the coil offset 2 cm either side of vertex was used to assess the percent change in rectified motor evoked potential (MEP) area recorded bilaterally from vastus lateralis (VL) and tibialis anterior (TA) of 10 subjects during weak tonic contraction. RESULTS ANOVA revealed an up-regulation of the target cortex and a down-regulation of the non-target cortex (p = 0.001) and no effects of hemisphere (left, right) or muscle (TA, VL). Significant modulation was evident in 78% of VL and TA muscles (all p < 0.05). Excitability increased in 60%, but decreased in 18%. For 43% when excitability increased, a simultaneous decrease in excitability was evident in homologous muscle responses providing support for our hypothesis. CONCLUSIONS The results indicate a modest effectiveness and focality of anodal tDCS when applied to lower limb M1, suggesting in a human model that the strength and depth of polarizing cortical currents induced by tDCS likely depend on inter-individual differences in the electrical properties of superficial brain structures.

The Effects of Peroneal Nerve Functional Electrical Stimulation Versus Ankle-Foot Orthosis in Patients With Chronic Stroke A Randomized Controlled Trial

Background. Evidence supports peroneal nerve functional electrical stimulation (FES) as an effective alternative to ankle-foot orthoses (AFO) for treatment of foot drop poststroke, but few randomized controlled comparisons exist. Objective. To compare changes in gait and quality of life (QoL) between FES and an AFO in individuals with foot drop poststroke. Methods. In a multicenter randomized controlled trial (ClinicalTrials.gov #NCT01087957) with unblinded outcome assessments, 495 Medicare-eligible individuals at least 6 months poststroke wore FES or an AFO for 6 months. Primary endpoints: 10-Meter Walk Test (10MWT), a composite of the Mobility, Activities of Daily Living/Instrumental Activities of Daily Living, and Social Participation subscores on the Stroke Impact Scale (SIS), and device-related serious adverse event rate. Secondary endpoints: 6-Minute Walk Test, GaitRite Functional Ambulation Profile (FAP), Modified Emory Functional Ambulation Profile (mEFAP), Berg Balance Scale (BBS), Timed Up and Go, individual SIS domains, and Stroke-Specific Quality of Life measures. Multiply imputed intention-to-treat analyses were used with primary endpoints tested for noninferiority and secondary endpoints tested for superiority. Results. A total of 399 subjects completed the study. FES proved noninferior to the AFO for all primary endpoints. Both the FES and AFO groups improved significantly on the 10MWT. Within the FES group, significant improvements were found for SIS composite score, total mFEAP score, individual Floor and Obstacle course time scores of the mEFAP, FAP, and BBS, but again, no between-group differences were found. Conclusions. Use of FES is equivalent to the AFO. Further studies should examine whether FES enables better performance in tasks involving functional mobility, activities of daily living, and balance.

Enhancing motor skill learning with transcranial direct current stimulation - a concise review with applications to stroke

In the past few years, there has been a rapid increase in the application of non-invasive brain stimulation to study brain-behavior relations in an effort to potentially increase the effectiveness of neuro-rehabilitation. Transcranial direct current stimulation (tDCS), an emerging technique of non-invasive brain stimulation, has shown to produce beneficial neural effects in consequence with improvements in motor behavior. tDCS has gained popularity as it is economical, simple to use, portable, and increases corticospinal excitability without producing any serious side effects. As tDCS has been increasingly investigated as an effective tool for various disorders, numerous improvements, and developments have been proposed with respect to this technique. tDCS has been widely used to identify the functional relevance of particular brain regions in motor skill learning and also to facilitate activity in specific cortical areas involved in motor learning, in turn improving motor function. Understanding the interaction between tDCS and motor learning can lead to important implications for developing various rehabilitation approaches. This paper provides a concise overview of tDCS as a neuromodulatory technique and its interaction with motor learning. The paper further briefly goes through the application of this priming technique in the stroke population.

A paradox: After stroke, the non-lesioned lower limb motor cortex may be maladaptive

What are the neuroplastic mechanisms that allow some stroke patients to regain high‐quality control of their paretic leg, when others do not? One theory implicates ipsilateral corticospinal pathways projecting from the non‐lesioned hemisphere. We devised a new transcranial magnetic stimulation protocol to identify ipsilateral corticospinal tract conductivity from the non‐lesioned hemisphere to the paretic limb in chronic stroke patients. We also assessed corticospinal tract degeneration by diffusion tensor imaging, and used an ankle tracking task to assess lower limb motor control. We found greater tracking error during antiphase bilateral ankle movement for patients with strong conductivity from the non‐lesioned hemisphere to the paretic ankle than for those with weak or no conductivity. These findings suggest that, instead of assisting motor control, contributions to lower limb motor control from the non‐lesioned hemisphere of some stroke survivors may be maladaptive.

Polarity Independent Effects of Cerebellar tDCS on Short Term Ankle Visuomotor Learning

BACKGROUND Transcranial direct current stimulation (tDCS), an emerging technique of noninvasive brain stimulation, has shown to produce beneficial neural effects in consequence with improvements in motor behavior. There are not many studies examining the use of tDCS for lower limb motor control and learning. Most studies using tDCS for facilitating lower limb motor coordination have applied tDCS to the lower limb motor cortex (M1). As the cerebellum is also critically involved in movement control, it is important to dissociate the effect of tDCS on the cerebellum and M1 with respect to lower limb motor control before we begin the application of tDCS as a neuromodulatory tool. OBJECTIVE/HYPOTHESIS The purpose of this study was to determine the effects of cerebellar vs. motor cortical tDCS on short term ankle visuomotor learning in healthy individuals. METHODS Eight healthy individuals practiced a skilled ankle motor tracking task while receiving either facilitatory anodal tDCS to cerebellum, inhibitory cathodal tDCS to cerebellum, facilitatory anodal tDCS to M1, inhibitory cathodal tDCS to M1 or sham stimulation. Pre- and post-measures of changes in cortical excitability of the tibialis anterior muscle and measures of tracking accuracy were assessed. RESULTS Anodal cerebellar, cathodal cerebellar, and anodal M1 stimulation improved target-tracking accuracy of the ankle. This was not dependent on the observed changes in motor cortical excitability of the tibialis anterior muscle. CONCLUSION(S) Polarity independent effects of tDCS on cerebellum were observed. The present study shows that modulation effects of tDCS can occur because of changes in the cerebellum, a structure implicated in several forms of motor learning, providing an additional way in which tDCS can be used to improve motor coordination.

Corticospinal tract integrity correlates with knee extensor weakness in chronic stroke survivors

OBJECTIVE Muscle weakness develops rapidly after stroke, adversely affecting motor performance, and contributing to reduced functional ability. While the contributions of structural and functional alterations in skeletal muscle to post-stroke weakness have been well described, the relationship between motor pathway integrity, measured using both radiological and electrophysiological techniques, and post-stroke muscle weakness is not clear. This study sought to determine the role of corticospinal tract (CST) integrity on knee extensor weakness in chronic stroke survivors. METHODS Knee extensor strength and activation testing were performed at 90° of knee flexion using an interpolated triplet technique. CST integrity was evaluated using data obtained from Diffusion Tensor Imaging and transcranial magnetic stimulation. RESULTS Recordings in nine stroke subjects indicated substantial knee extensor weakness and activation deficits in the paretic legs of the stroke survivors. Regression analysis revealed that asymmetry in CST integrity was strongly related to between-leg differences in knee strength. CONCLUSIONS The results of this study suggest a strong link between CST integrity and lower extremity strength, and add to the growing evidence of substantial knee extensor weakness and activation impairments in stroke survivors. SIGNIFICANCE The findings from this study further our understanding of the anatomical and neurophysiological contributions to motor impairments after stroke, which may benefit clinicians and researchers in the field of stroke rehabilitation.

Timing-dependent priming effects of tDCS on ankle motor skill learning

Transcranial direct current stimulation (tDCS) has gained increasing interest in neurorehabilitation with its ability to modulate cortical excitability, and thereby influence neural plasticity and functional recovery. While the beneficial effects of tDCS on motor learning and function have been recognized, there is no clear consensus regarding the timing of the tDCS priming protocol in relation to the intervention especially with respect to lower limb motor learning. Depending on the time of priming in relation to the training task, the neural mechanisms of priming (gating vs. homeostatic plasticity) are different and thereby subsequently affect motor learning. Hence, the aim of this study was to examine the interaction of tDCS with subsequent vs. concurrent motor learning using an ankle visuomotor skill learning paradigm. Twelve healthy participants were tested under three stimulation conditions: (1) anodal tDCS prior to the motor task (tDCS-before), (2) anodal tDCS during the motor task (tDCS-during) and (3) sham tDCS during the motor task (tDCS-sham). Results revealed that tDCS application during practice of a skilled motor task increased motor performance compared to tDCS applied prior to motor practice. Both tDCS groups demonstrated enhanced motor learning when tested 24 hours after practice. We conclude that the priming effects of tDCS are timing dependent, and maybe a critical regulatory feature in determining outcomes of priming with tDCS.

Motor priming in neurorehabilitation.

Priming is a type of implicit learning wherein a stimulus prompts a change in behavior. Priming has been long studied in the field of psychology. More recently, rehabilitation researchers have studied motor priming as a possible way to facilitate motor learning. For example, priming of the motor cortex is associated with changes in neuroplasticity that are associated with improvements in motor performance. Of the numerous motor priming paradigms under investigation, only a few are practical for the current clinical environment, and the optimal priming modalities for specific clinical presentations are not known. Accordingly, developing an understanding of the various types of motor priming paradigms and their underlying neural mechanisms is an important step for therapists in neurorehabilitation. Most importantly, an understanding of the methods and their underlying mechanisms is essential for optimizing rehabilitation outcomes. The future of neurorehabilitation is likely to include these priming methods, which are delivered prior to or in conjunction with primary neurorehabilitation therapies. In this Special Interest article, we discuss those priming paradigms that are supported by the greatest amount of evidence, including (i) stimulation-based priming, (ii) motor imagery and action observation, (iii) sensory priming, (iv) movement-based priming, and (v) pharmacological priming. Video Abstract available. (see Supplemental Digital Content 1, http://links.lww.com/JNPT/A86) for more insights from the authors.

Long-Term Follow-up to a Randomized Controlled Trial Comparing Peroneal Nerve Functional Electrical Stimulation to an Ankle Foot Orthosis for Patients With Chronic Stroke:

Background. Evidence supports peroneal nerve functional electrical stimulation (FES) as an effective alternative to ankle foot orthoses (AFO) for treatment of foot drop poststroke, but few long-term, randomized controlled comparisons exist. Objective. Compare changes in gait quality and function between FES and AFOs in individuals with foot drop poststroke over a 12-month period. Methods. Follow-up analysis of an unblinded randomized controlled trial (ClinicalTrials.gov #NCT01087957) conducted at 30 rehabilitation centers comparing FES to AFOs over 6 months. Subjects continued to wear their randomized device for another 6 months to final 12-month assessments. Subjects used study devices for all home and community ambulation. Multiply imputed intention-to-treat analyses were utilized; primary endpoints were tested for noninferiority and secondary endpoints for superiority. Primary endpoints: 10 Meter Walk Test (10MWT) and device-related serious adverse event rate. Secondary endpoints: 6-Minute Walk Test (6MWT), GaitRite Functional Ambulation Profile, and Modified Emory Functional Ambulation Profile (mEFAP). Results. A total of 495 subjects were randomized, and 384 completed the 12-month follow-up. FES proved noninferior to AFOs for all primary endpoints. Both FES and AFO groups showed statistically and clinically significant improvement for 10MWT compared with initial measurement. No statistically significant between-group differences were found for primary or secondary endpoints. The FES group demonstrated statistically significant improvements for 6MWT and mEFAP Stair-time subscore. Conclusions. At 12 months, both FES and AFOs continue to demonstrate equivalent gains in gait speed. Results suggest that long-term FES use may lead to additional improvements in walking endurance and functional ambulation; further research is needed to confirm these findings.