The use of transcranial magnetic stimulation to evaluate cortical excitability of lower limb musculature: Challenges and opportunities

Abstract

Neuroplasticity is a fundamental yet relatively unexplored process that can impact rehabilitation of lower extremity (LE) movements. Transcranial magnetic stimulation (TMS) has gained widespread application as a non-invasive brain stimulation technique for evaluating neuroplasticity of the corticospinal pathway. However, a majority of TMS studies have been performed on hand muscles, with a paucity of TMS investigations focused on LE muscles. This perspective review paper proposes that there are unique methodological challenges associated with using TMS to evaluate corticospinal excitability of lower limb muscles. The challenges include: (1) the deeper location of the LE motor homunculus; (2) difficulty with targeting individual LE muscles during TMS; and (3) differences in corticospinal circuity controlling upper and lower limb muscles. We encourage future investigations that modify traditional methodological approaches to help address these challenges. Systematic TMS investigations are needed to determine the extent of overlap in corticomotor maps for different LE muscles. A simple, yet informative methodological solution involves simultaneous recordings from multiple LE muscles, which will provide the added benefit of observing how other relevant muscles co-vary in their responses during targeted TMS assessment directed toward a specific muscle. Furthermore, conventionally used TMS methods (e.g., determination of hot spot location and motor threshold) may need to be modified for TMS studies involving LE muscles. Additional investigations are necessary to determine the influence of testing posture as well as activation state of adjacent and distant LE muscles on TMS-elicited responses. An understanding of these challenges and solutions specific to LE TMS will improve the ability of neurorehabilitation clinicians to interpret TMS literature, and forge novel future directions for neuroscience research focused on elucidating neuroplasticity processes underlying locomotion and gait training.

Specific motor cortex hypoexcitability and hypoactivation in COPD patients with peripheral muscle weakness

“…We used a figure-of-eight coil to stimulate the leg motor cortex. Although a double-cone coil is more conventional to stimulate the leg motor cortex as the magnetic stimulus penetrates less deeply for a given intensity [57], the figureof-eight coil takes the advantage to be more focal [58], and there is no clear rationale to give priority to one over the other in the literature [57]. Apart from these considerations, some strict procedures helped us to increase the validity of the data provided by the figure-of-eight coil.…”

Planning face, hand, and leg movements: anatomical constraints on preparatory inhibition

“…To ensure that the CS excitability changes observed in Exp 1 and 2 were not specific to MEPs elicited in a hand muscle, we targeted the TA muscle of the right leg in Exp 3. MEPs are more difficult to elicit from leg muscles: Not only is the leg region in the depth of the sulcal, but the motor representations of leg muscles may contain fewer or weaker corticospinal projections (Kesar et al 2018). Given this challenge, the thresholding phase of Exp 3 also served as a screening procedure: We recruited 23 participants to identify 12 individuals for whom we were able to consistently elicit MEPs in the right TA.…”

Effect of acute transcranial magnetic stimulation on intracellular signalling in human skeletal muscle

“…However, despite our best efforts in targeting the quadriceps femoris muscle for stimulation, EMG, and especially peak forces, were generally low. Stimulating muscles of the lower limb with TMS can be difficult, as the somatotopic organization of the motor cortex results in a smaller specific area suitable for stimulation for muscles of the thigh compared with, for example, hand muscles (25). This was exemplified by a relatively high degree of activation of both antagonists at the level of the knee joint (BF) and in the lower leg (TA, Gn) in addition to the intended quadriceps muscle.…”