With the race season coming to a close, and the Ironman World Championships just around the corner, many athletes will be squeezing in as many key sessions as possible during this critical training period. But as we sit on a knife’s edge between training “too much” and “too little”, it’s useful to have some objective measurement to ensure we’re maximizing the adaptation from every training session we undertake.

Recently, I was fortunate enough to have contributed to a heart rate variability guided training study with lead author Alejandro Javaloyes (1). The study compared an eight-week cycling training programme prescribed according to either pre-defined block periodization (BP) or guided using heart rate variability (HRV). That is, subjects completed either a mixed programme set out in advance or a programme adjusted on a day-to-day basis via daily heart rate variability measurements using the smartphone application ‘HRV4Training’.

HRV is a measurement of the variability in the time...