  Objective: We present a simple parameter, calculated from a single wearable
sensor, that can be used to objectively measure disease severity in people with
myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) or Long COVID. We
call this parameter UpTime. Methods: Prior research has shown that the amount
of time a person spends upright, defined as lower legs vertical with feet on
the floor, correlates strongly with ME/CFS disease severity. We use a single
commercial inertial measurement unit (IMU) attached to the ankle to calculate
the percentage of time each day that a person spends upright (i.e., UpTime) and
number of Steps/Day. As Long COVID shares symptoms with ME/CFS, we also apply
this method to determine Long COVID disease severity. We performed a trial with
55 subjects broken into three cohorts, healthy controls, ME/CFS, and Long
COVID. Subjects wore the IMU on their ankle for a period of 7 days. UpTime and
Steps/Day were calculated each day and results compared between cohorts.
Results: UpTime effectively distinguishes between healthy controls and subjects
diagnosed with ME/CFS ($\mathbf{p = 0.00004}$) and between healthy controls and
subjects diagnosed with Long COVID ($\mathbf{p = 0.01185}$). Steps/Day did
distinguish between controls and subjects with ME/CFS ($\mathbf{p = 0.01}$) but
did not distinguish between controls and subjects with Long COVID ($\mathbf{p =
0.3}$). Conclusion: UpTime is an objective measure of ME/CFS and Long COVID
severity. UpTime can be used as an objective outcome measure in clinical
research and treatment trials. Significance: Objective assessment of ME/CFS and
Long COVID disease severity using UpTime could spur development of treatments
by enabling the effect of those treatments to be easily measured.
