A Polar WearLink 31 coded transmitter (Polar Electro, Kempele, Finland) was
worn on the chest with an elastic belt. HR data were wirelessly transmitted to the
ActiTrainer and stored as average beats per minute over 1-minute epochs. ActiLife
software was used to download and time-match the HR values with the accelerometer
data. Recorded HR values were transformed into percentage of HR reserve utilizing the
formula %HRR = [(HRactivity - HRrest)/ (est. HRmax – HR rest)]*100. HRmax was
estimated using the formula HRmax = (208 – 0.7 * age).26 Time spent in activity (cut-off
value > 20% HRR) and time spent in moderate/vigorous activity (cut-off value > 40%
HRR) were calculated