Seismocardiogram (SCG) is the recording of body vibrations induced by the heart beat. SCG contains information on cardiac mechanics, in particular heart sounds and cardiac output. In this paper we present a new wearable device for SCG recordings during long term monitorings, and the results of a validation test in 4 subjects. The system is based on the integration of the MagIC smart shirt (i.e., a textile-based wearable system for the assessment of ECG and respiratory movements), and an external triaxial MEMS accelerometer positioned on the left clavicle. SCG was estimated as the average of accelerations occurred in each heart beat. The SCG components due to the valve closure and to recoil forces following the heart contraction (ballistocardiogram) were extracted by high-pass (>18 Hz) and band-pass (0.6-20 Hz) filters respectively. Then the difference between the I and J waves of the ballistocardiogram (|I-J| index, possibly related to the cardiac output) was identified by an ad-hoc procedure and compared with the model flow indirect estimation of cardiac output. Validation on 4 volunteers showed that: 1) our wearable system provides statistically consistent estimates of both heart-sound related vibrations and recoil movements; 2) reliable estimates of the |I-J| index can be obtained by considering about 1 minute of SCG recording in stationary conditions; and 3) changes of the |I-J| index during exercise correlate well with changes of cardiac output estimated by the model flow.