What are the neuroplastic mechanisms that allow some stroke patients to regain high-quality control of their paretic leg, when others do not? One theory implicates ipsilateral corticospinal pathways projecting from the non-lesioned hemisphere. We devised a new transcranial magnetic stimulation protocol to identify ipsilateral corticospinal tract conductivity from the non-lesioned hemisphere to the paretic limb in chronic stroke patients. We also assessed corticospinal tract degeneration by diffusion tensor imaging, and used an ankle tracking task to assess lower limb motor control. We found greater tracking error during antiphase bilateral ankle movement for patients with strong conductivity from the non-lesioned hemisphere to the paretic ankle than for those with weak or no conductivity. These findings suggest that, instead of assisting motor control, contributions to lower limb motor control from the non-lesioned hemisphere of some stroke survivors may be maladaptive.