Multi-sensor system for estimating blood pulse wave characteristics

Detecting two occurrences of a single blood pulse wave using two heart activity sensors associated with two different body locations, e.g. heart and wrist; computing, based on these detections synchronised to a common clock, time characteristics of the blood pulse wave, e.g. a pulse transit time (PTT); and computing, based on the time characteristics, a metric representing a physiological condition of the human body, e.g. blood pressure. The sensors are e.g. an electrocardiogram (ECG) sensor 10, ballistocardiogram (BCG) sensor 16, or photoplethysmogram (PPG) sensor 12 and may be provided on a single apparatus, e.g. a watch. An altitude sensor may be provided. PPG measurements of insufficient signal quality may be discarded. Also discloses determining the PTT from a single PPG sensor.