Critical Scale-invariance in Healthy Human Heart Rate
  We demonstrate the robust scale-invariance in the probability density function (PDF) of detrended healthy human heart rate increments, which is preserved not only in a quiescent condition, but also in a dynamic state where the mean level of heart rate is dramatically changing. This scale-independent and fractal structure is markedly different from the scale-dependent PDF evolution observed in a turbulent-like, cascade heart rate model. These results strongly support the view that healthy human heart rate is controlled to converge continually to a critical state. 
Ken Kiyono, Zbigniew R. Struzik, Naoko Aoyagi, Seiichiro Sakata,   Junichiro Hayano, Yoshiharu Yamamoto