Overview

This study will investigate how aging and spinal cord injury (SCI) affect the transmission and plasticity of the corticospinal and reticulospinal tracts, which are important pathways for motor control. The study includes two experiments. Experiment 1 will assess corticospinal and reticulospinal tract transmission to lower-limb muscles in older adults with and without SCI. Experiment 2 will examine how aging and spinal cord injury affect the body's ability to strengthen nerve pathways involved in movement. The investigators will compare two pathways that help control leg movement, the corticospinal and reticulospinal pathways, to determine whether aging affects these pathways differently and whether spinal cord injury has a greater effect on the corticospinal pathway.

Eligibility

Inclusion Criteria:

• Male and females between 19-85 years;
• Chronic SCI (≥ 1 years post-injury);
• T10 or above - American Spinal Injury Association (AIS) Grade C, or D;
• Ability to perform a small voluntary ankle dorsiflexion, knee extension, and ankle plantarflexion (as detected by presence of voluntary EMG activity in the rectus femoris, tibialis anterior, and soleus muscles) with at least one leg;
• Ability to tolerate standing position;
• Ability to initiate at least one step without the assistance of a therapist;
• If ambulatory, ability to walk at a minimum speed of 0.1 m/s on a treadmill. Inclusion criteria for controls:
• Male and females (19-85 years);
• Able to perform dorsiflexion;
• Able to walk.

Exclusion Criteria:

• Uncontrolled medical problems including pulmonary, cardiovascular, or orthopedic disease;
• Any debilitating disease before the SCI that caused exercise intolerance;
• Premorbid, ongoing major depression or psychosis, altered cognitive status;
• History of head injury or stroke;
• Pacemaker;
• Metal plate in skull;
• History of seizures;
• Receiving drugs acting primarily on the central nervous system, which lower the seizure threshold, such as antipsychotic drugs (chlorpromazine, clozapine) or tricyclic antidepressants;
• Pregnancy;
• Ongoing cord compression or a syrinx in the spinal cord or spinal cord disease, such as spinal stenosis, spina bifida or herniated cervical disk;
• implanted defibrillators or implanted neurostimulators;
• have large ischaemic scars;
• diagnosed sleep deprivation.

Exclusion criteria for controls:

\- Same as for subjects with SCI.