Sammanfattning: This thesis reflects two recent changes relevant to the field of rehabilitation medicine. Firstly, the COVID-19 pandemic illustrated that new disorders may benefit from the involvement of rehabilitation medicine. Secondly, advances in neurobiology pave new ways for therapeutic interventions among disorders already well known in the specialty, i.e. neuromodulation for improving motor function in chronic spinal cord injury (SCI).Paper I is a cross-sectional study on 23 adults with chronic clinically complete SCI (verified using standardized neurological examination) exploring the prevalence of residual functional connectivity, using a battery of neurophysiological tests. Unequivocal indications of residual translesional connectivity (i.e." discomplete" SCI) were found in 17% of participants. Applying more liberal criteria, the potentially discomplete group comprised 39-52%.Paper II is a prospective interventional study where five adults with cervical SCI received a series of paired associative stimulation (PAS) treatments. Lesions in this study were incomplete, rather than discomplete. PAS combines peripheral nerve stimulation and navigated transcranial magnetic stimulation (TMS), both targeted towards the neural structures subserving one arm. The aim was to corroborate findings from earlier studies suggesting that spinally targeted PAS improves upper extremity motor function in chronic incomplete SCI. Manual muscle testing scores in the targeted extremity increased by a mean 0,5 points (0-5 scale). Grip strength, assessed with hand dynamometry, increased by 3 kg in the stimulated hand and by 2 kg in the contralateral hand. Performance and Satisfaction of individually specified goals, as reflected by the Canadian Occupational Performance Measure (COPM), increased by 2 points. Improvements remained one month after stimulation.Paper III: The Linköping COVID-19 study (LinCoS) included all 745 individuals hospitalized for COVID-19 during the first wave of the pandemic (March 1st-May 31st, 2020) in our region. Survivors of this cohort with persisting symptoms corresponding to the diagnosis of a Post-COVID-19 Condition (PCC) at a screening telephone interview four months post-discharge, were invited to a comprehensive, interdisciplinary clinical assessment which 85% attended (n=158). Persisting impairments were reported and corroborated, including cognitive impairment (reported by almost 50% and corroborated by neuropsychological testing in almost 40%), weakness in extremities and pain (reported by about a third of subjects, respectively), as well as a large number of other (respiratory, visual, auditory, sensory) symptoms. In total, 16% of survivors in the total regional cohort was deemed to require further rehabilitative interventions at the clinical assessment.Paper IV: In follow-up telephone interviews at 2 years post-discharge of individuals with PCC at five months post-discharge, 84% of patients reported persisting problems affecting everyday life, although significant improvements were seen in symptom prevalence and impact compared with the situation at four months post-discharge. Cognitive, sensorimotor symptoms and fatigue were the most common persisting symptoms, and 21% had been readmitted to hospital in the interim period. Half of those on sick leave related to PCC at four months were still on sick leave. No differences in outcomes were evident between previously Intensive Care Unit (ICU)-treated and non-ICU-treated individuals.Summary: PAS improved upper extremity motor function in patients with incomplete SCI, corroborating prior studies. Evidence of discomplete SCI was demonstrated using standard neurophysiological techniques in a substantial subset of individuals with chronic, clinically complete SCI. Thus, PAS may also be a potential therapeutic intervention for people with such lesions, but this requires further investigation. PCC is highly prevalent in the medium to long term (4-24 months) after hospital-treated COVID-19. To ensure adequate care and rehabilitation, extensive examination of patients with symptoms indicating PCC is warranted.In conclusion, this thesis explores new frontiers in rehabilitation medicine by suggesting new possibilities for therapeutic interventions as well as identifying rehabilitation needs in new conditions that may benefit from our attention. Results of the studies on COVID-19 and SCI reported in this thesis, suggest the potential both for expansion of the specialty in terms of new diagnostic categories and the inclusion of new treatment modalities.