Inclusion Criteria:  Previous COVID-19 infection Persistent symptoms for pain, fatigue, weakness, or poor gait and balance after infection assessed by critical care clinicians on the initial consultation Willing to attend clinic for assessments  Exclusion Criteria:  Severe cognitive decline reduces their ability to interact with the TENS mobile app Major visual or hearing weakness reduces the ability to interact with TENS mobile app Unable to walk independently for a distance of 10 meter Major foot problems such as active lower extremity wounds, major foot deformity (e.g., Charcot Foot), previous major amputations, and claudication Demand-type cardiac pacemaker, implanted defibrillator, or other implanted electronic devices; and any conditions that may interfere with outcomes or increase the risk of the use TENS based on the judgement of clinicians