The following electronic only articles are published in conjunction with this issue of Heart.   Physical activity is a major contributor to the ultra low frequency components of heart rate variability J M Serrador, H C Finlayson, R L Hughon Objective—To investigate the link between changes in level of physical activity and the pattern of heart rate variability during long term ambulatory monitoring. Design—Heart rate variability was measured simultaneously with a quantitative indicator of muscle activity by electromyography (EMG) in five men and five women while they did activities typical of daily life or while they rested for 2–3 hours. Spectral and cross spectral analyses were performed on both variables with standard fast Fourier transform. Results—There was a marked reduction in spectral power in the ultra low frequency band (< 0.003 Hz) on going from active to rest conditions for both heart …