The R and T waves amplitudes of the electrocardiogram recorded during the
exercise test undergo strong modifications in response to stress. We analyze
the time series of these amplitudes in a group of normal subjects in the
framework of functional data, performing reduction of dimensionality, smoothing
and principal component analysis. These methods show that the R and T
amplitudes have opposite responses to stress, consisting respectively in a bump
and a dip at the early recovery stage. We test these features computing a
confidence band for the trend of the population mean and analyzing the zero
crossing of its derivative.
Our findings support the existence of a relationship between R and T wave
amplitudes and respectively diastolic and systolic ventricular volumes
