RecruitingClinical trialPaired Stimulation Plasticity for Motor Recovery in Humans With and Without Spinal Cord InjuryThis 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.