Estimation of maximal heart rate using the relationship between heart rate variability and exercise intensity in 40-67 years old men - Karavirta, L; Tulppo, M P; Nyman, K; Laaksonen, D E; Pullinen, T; Laukkanen, R T; Kinnunen, H; Hakkinen, A; Hakkinen, K (Eur J Appl Physiol, 2008)