J.L. Newton 1 J. Pairman 1 K. Hallsworth 1, 2 S. Moore 1,2 T. Plo TZ 3 and M.I. Trenell 1, 2 1. UK National Institute for Health Research Biomedical Research Centre in Ageing & Age-related Disease, Institute for Ageing and Health, Newcastle University, Newcastle, UK, 2. MRC Centre for Brain Ageing & Vitality, Newcastle University, Newcastle upon Tyne and 3. School of Computing Science, Newcastle University, Newcastle upon Tyne, UK Free access Abstract Background: Chronic fatigue syndrome (CFS) is a common debilitating condition associated with reduced function and impaired quality of life. The cause is unknown and treatments limited. Studies confirm that CFS is associated with impaired autonomic regulation and impaired muscle function. Aim: Define the relationship between sedentary behaviour, physical activity and autonomic regulation in people with CFS. Design: Cohort study. Methods: Physical activity was assessed objectively in 107 CFS patients (Fukuda) and age, sex and body mass index (BMI)-matched sedentary controls (n = 107). Fatigue severity was determined using the Fatigue Impact Scale in all participants and heart rate variability performed in the CFS group. Results: The CFS group had levels and patterns of sedentary behaviour similar to non-fatigue controls (P > 0.05). Seventy-nine percent of the CFS group did not achieve the WHO recommended 10000 steps per day. Active energy expenditure [time >3 METs (metabolic equivalents)] was reduced in CFS when compared with controls (P < 0.0001). Physical activity duration was inversely associated with resting heart rate (P = 0.04; r2 = 0.03), with reduced activity significantly associating with reduced heart rate variability in CFS. There were no relationships between fatigue severity and any parameter of activity. Thirty-seven percent of the CFS group were overweight (BMI 25–29.9) and 20% obese (BMI >=30). Conclusion: Low levels of physical activity reported in CFS represent a significant and potentially modifiable perpetuating factor in CFS and are not attributable to high levels of sedentary activity, rather a decrease in physical activity intensity. The reduction in physical activity can in part be explained by autonomic dysfunction but not fatigue severity.