Abstract
In this study, we investigated how modulation in corticospinal excitability elicited in the context of standing tasks varies as a function of age and between muscles. Changes in motor evoked potentials (MEPs) recorded in tibialis anterior (TA) and gastrocnemius lateralis (GL) were monitored while participants (young, n = 10; seniors, n = 11) either quietly stood (QS) or performed a heel raise (HR) task. In the later condition, transcranial magnetic stimulation (TMS) pulses were delivered at three specific time points during the task: 1) 250 ms before the “go” cue (preparatory (PREP) phase), 2) 100 ms before the heel rise (anticipatory postural adjustment (APA) phase), and 3) 200 ms after heel rise (execution (EXEC) phase). In each task and each phase, variations in MEP characteristics were analysed for age and muscle-dependent effects. Variations in silent period (SP) duration were also examined for certain phases (APA and EXEC). Our analysis revealed no major difference during QS, as participants exhibited very similar patterns of modulation in both TA and GL, irrespective of their age group. During the HR task, young adults exhibited a differential modulation in the PREP phase with enhanced responses in TA relative to GL, which was not seen in seniors. Finally, besides differences in MEP latency, age had little influence on MEP modulation during the APA and EXEC phases, where amplitude was largely a function of background muscle activity associated with each phase (i.e., APA: TA; EXEC: GL). No age or muscle effects were detected for SP measurements. Overall, our results revealed no major differences between young adults and healthy seniors in the ability to modulate corticospinal facilitation destined to ankle muscles during standing tasks, with maybe the exception of the ability to prime muscle synergies in the preparatory phase of action.
Highlights
In humans, transcranial magnetic stimulation (TMS) allows for non-invasive exploration of variations in corticospinal excitability associated with task performance
The large ‘‘muscle’’ effect reflected the fact that the gastrocnemius lateralis (GL) exhibited, on average, higher tonic activity during quiet standing (QS) than the tibialis anterior (TA) in both age groups
TMS responses were very similar in TA and GL, irrespective of the age group for the QS task
Summary
Transcranial magnetic stimulation (TMS) allows for non-invasive exploration of variations in corticospinal excitability associated with task performance. This is due, in part, to the difficulty in eliciting reliable TMS responses in some leg muscles [1] and to the fact that lower limbs are primarily involved in tasks dealing with posture and locomotion, which were traditionally thought to depend largely on brainstem and spinal cord circuitry [2] Evidence from both animal studies and neuropathological case studies in humans suggests a major role for cortical structures in influencing postural and locomotor neuronal networks [3]. One of the major roles ascribed to cortical structures in postural control is to adjust ‘‘central set’’ by priming brainstem and spinal cord circuits in advance to generate appropriate responses to a given context [3] These observations confirmed the critical role of descending projections arising from the motor cortex in modulating lower limb responses during postural and stepping tasks, especially with regard to the generation of APA
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