Aging is associated with structural and functional changes in the central nervous system that may negatively impact motor control, postural stability, upper-limb coordination, and adaptive motor behavior. These functional alterations are partly attributed to changes in cortical excitability and reduced efficiency of intracortical inhibitory and sensorimotor circuits within the primary motor cortex (M1). Such modifications may contribute to a decline in neuroplasticity, defined as the capacity of the brain to reorganize in response to internal or external stimuli.
Neuroplasticity plays a critical role in motor learning and adaptation and is therefore essential for maintaining functional autonomy in older adults. Identifying interventions capable of enhancing or preserving neuroplasticity with aging represents an important clinical and societal challenge. Aerobic physical exercise has emerged as a promising non-pharmacological strategy to modulate cortical excitability and promote neuroplasticity.
The primary objective of this study is to assess the effect of a single session of moderate aerobic exercise on neuroplasticity induced by transcranial direct current stimulation (tDCS) in older adults, compared with young adults. The secondary objective is to examine the influence of exercise on intrinsic intracortical circuits and sensorimotor integration mechanisms within M1.
Study Design :
This is a monocentric exploratory study where each participant will complete three experimental sessions separated by at least one week :
Session 1 and Session 2 (randomized crossover design):
* Condition A : 20 minutes of moderate aerobic exercise followed by tDCS.
* Condition B : 20 minutes of seated rest followed by tDCS. The order of these two conditions will be randomized. Each participant serves as their own control.
Session 3:
* Assessment of the effects of aerobic exercise alone (without tDCS).
Experimental Procedures :
After providing written informed consent, participants undergo baseline assessments including questionnaires documenting physical activity level, handedness, and pain level.
Participants are comfortably seated, and surface electromyography (EMG) electrodes are placed over the abductor pollicis brevis (APB) muscle of the dominant hand. Transcranial magnetic stimulation (TMS) is delivered over M1 corresponding to the dominant hand representation. The optimal stimulation site (hotspot) and resting motor threshold (RMT) are determined.
Corticospinal excitability is assessed using single-pulse TMS at increasing intensities to construct input-output (I/O) recruitment curves. Motor evoked potentials (MEPs) are recorded from the APB muscle. A Boltzmann sigmoidal function is used to model I/O curves and extract parameters including slope, plateau, and S50. The primary outcome measure is the slope of the I/O curve, reflecting corticospinal recruitment gain.
Neuroplasticity is evaluated by comparing corticospinal excitability before and after a 20-minute session of anodal tDCS applied over the dominant M1 (2 mA). Changes in I/O curve parameters after tDCS, with or without prior physical exercise, serve as indicators of induced plasticity.
Intracortical inhibitory and facilitatory mechanisms are assessed using paired-pulse TMS protocols :
* Short-interval intracortical inhibition (SICI)
* Intracortical facilitation (ICF)
* Long-interval intracortical inhibition (LICI)
Sensorimotor integration is evaluated using paired peripheral nerve stimulation (median nerve at the wrist) combined with TMS to measure :
* Short-latency afferent inhibition (SAI)
* Long-latency afferent inhibition (LAI)
* Afferent-induced facilitation (AIF) Outcome measures are expressed as ratios of conditioned to unconditioned MEP amplitudes.
Aerobic Exercise Intervention :
Moderate aerobic exercise consists of 20 minutes of cycling on an ergometer at 60 to 70 revolutions per minute. Exercise intensity is individualized to correspond to approximately 50% of heart rate reserve (moderate intensity), monitored throughout the session.
The estimated duration of each experimental session is as follows :
* Session 1 (2h05) : Information, consent, and questionnaires (10 minutes), electrode placement (5 minutes), baseline recordings (35 minutes), 20 minutes of aerobic exercise or seated rest (randomized), 20 minutes of tDCS, and 35 minutes of post-intervention recordings.
* Session 2 (1h05) : Same procedure as Session 1 without information, consent, or questionnaires. Participants complete the alternate condition (exercise or rest), followed by tDCS and post-intervention recordings.
* Session 3 (1h35) : Preparation (5 minutes), baseline recordings (35 minutes), 20 minutes of aerobic exercise, and 35 minutes of post-exercise recordings.
