Changes in the Hurst exponent of heartbeat intervals during physical   activities
  The fractal scaling properties of the heartbeat time series are studied in a controlled ergometric regime using the Hurst rescaled range R/S analysis. The long-time "memory effect" quantified by the value of the Hurst exponent $H>0.5$ is found to increase during progresive physical activity at healthy subjects in contrast to those having stable angina pectoris (SAP), where it is decreasing. We argue that this finding may be used as a useful new diagnostic parameter for short heartbeat time series. 
M. Martinis, A. Kne\v{z}evi\'c, G. Krsta\v{c}i\'c, and E. Vargovi\'c   (Rudjer Bo\v{s}kovi\'c Institute, Zagreb, Croatia)