OBJECTIVE: To examine the relationship between overweight combined with low muscle mass and the cardiac autonomic nervous system using heart rate variability (HRV) in healthy workers. METHODS: A total of 1,150 workers were included, with a mean age of 43.55 +/- 11.45 years. The subjects were classified as low muscle mass if their appendicular skeletal muscle mass was below the 50th percentile of the study sample. Similarly, subjects were classified as overweight if their body mass index was above 25 kg/m(2). Electrocardiography recordings were obtained for 5 min, and the time-domain and frequency-domain indices of HRV were analyzed. RESULTS: Compared with the high muscle mass and non-overweight (HMM) group, the low-frequency power and the standard deviation of normal-to-normal intervals were significantly decreased in both the overweight and high muscle mass (OHMM) group and the overweight and low muscle mass (OLMM) group. The significantly decreased high-frequency (HF) power and square root of the mean squared differences of successive differences, which reflects efferent parasympathetic activity, was indicative of reduced parasympathetic modulation in the OHMM and OLMM groups. In addition, the OLMM group had a lower HF power than did the OHMM group. CONCLUSIONS: This study suggests that HRV is reduced in overweight combined with low muscle mass group than overweight and HMM group.