A method of obtaining a continuous measurement of cardiac pre-ejection period for an individual for a plurality of cardiac cycles occurring over a time n, where n comprises a plurality of contractions c of the individual's heart, comprising: acquiring a time-dependent electrocardiogram waveform using a first body-worn sensor apparatus configured to detect signals indicative of electrical activity of the individual's heart for time n; acquiring a time-dependent vibration waveform using a second body-worn sensor apparatus configured to detect signals indicative of vibrations having a frequency between 5 and 35 Hz caused by compression waves produced due to contraction of the individual's heart for time n; transmitting the time-dependent electrocardiogram waveform and the time-dependent vibration waveform to a processing apparatus; and processing the time-dependent electrocardiogram waveform and the time-dependent vibration waveform with the processing apparatus by (i) time synchronizing the time-dependent electrocardiogram waveform and the time-dependent vibration waveform, characterized in that the processing with the processing apparatus further comprises (ii) recursively determining an estimated time of aortic valve opening for each contraction c of the individual's heart during time n by processing the time-dependent vibration waveform using a state estimator algorithm which segments the time-dependent vibration waveform into a moving time window of length l comprising a plurality of contractions of the individual's heart which includes contraction c, and calculates the estimated time of aortic valve opening for contraction c from the data within the pre-determined moving window, and (iii) recursively determining a pre-ejection period for each contraction c of the individual's heart during time n by determining a time difference between a fiducial point in the time-dependent electrocardiogram waveform indicating the onset of electrical stimulation of the ventricles during contraction c and the estimated time of aortic valve opening determined for contraction c. acquiring a time-dependent electrocardiogram waveform using a first body-worn sensor apparatus configured to detect signals indicative of electrical activity of the individual's heart for time n; acquiring a time-dependent vibration waveform using a second body-worn sensor apparatus configured to detect signals indicative of vibrations having a frequency between 5 and 35 Hz caused by compression waves produced due to contraction of the individual's heart for time n; transmitting the time-dependent electrocardiogram waveform and the time-dependent vibration waveform to a processing apparatus; and processing the time-dependent electrocardiogram waveform and the time-dependent vibration waveform with the processing apparatus by (i) time synchronizing the time-dependent electrocardiogram waveform and the time-dependent vibration waveform, characterized in that the processing with the processing apparatus further comprises (ii) recursively determining an estimated time of aortic valve opening for each contraction c of the individual's heart during time n by processing the time-dependent vibration waveform using a state estimator algorithm which segments the time-dependent vibration waveform into a moving time window of length l comprising a plurality of contractions of the individual's heart which includes contraction c, and calculates the estimated time of aortic valve opening for contraction c from the data within the pre-determined moving window, and (iii) recursively determining a pre-ejection period for each contraction c of the individual's heart during time n by determining a time difference between a fiducial point in the time-dependent electrocardiogram waveform indicating the onset of electrical stimulation of the ventricles during contraction c and the estimated time of aortic valve opening determined for contraction c. (i) time synchronizing the time-dependent electrocardiogram waveform and the time-dependent vibration waveform, characterized in that the processing with the processing apparatus further comprises (ii) recursively determining an estimated time of aortic valve opening for each contraction c of the individual's heart during time n by processing the time-dependent vibration waveform using a state estimator algorithm which segments the time-dependent vibration waveform into a moving time window of length l comprising a plurality of contractions of the individual's heart which includes contraction c, and calculates the estimated time of aortic valve opening for contraction c from the data within the pre-determined moving window, and (iii) recursively determining a pre-ejection period for each contraction c of the individual's heart during time n by determining a time difference between a fiducial point in the time-dependent electrocardiogram waveform indicating the onset of electrical stimulation of the ventricles during contraction c and the estimated time of aortic valve opening determined for contraction c.