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Top1. Introduction
Ballistocardiography (BCG) measures the vibration of the human body due to cardiac and circulatory function. The existence of BCG signals was first reported by Gordon in 1877 (Gordon, 1877). In the mid 19th century, BCG was extensively researched especially by Isaac Starr, who published some of the first scientific papers on it (Starr, Rawson, Schroeder, & Joseph, 1939). Due to its noninvasiveness, BCG measurement sensors can be imperceptibly embedded into different everyday objects. In fact, there have been reports of embedding measurement equipment of the BCG signal in different modified scales (Gonzalez-Landaeta, Casas, & Pallas-Areny, 2008; Inan, Etemadi, Wiard, Giovangrandi, & Kovacs, 2009), a chair (Junnila, Akhbardeh, Barna, Defee, & Varri, 2006), a wheelchair (Han, Kim, Cha, & Lee, 2008; Postolache, Girao, Mendes, & Postolache, 2009), and a bed (Heise & Skubic, 2010; Mack, Patrie, Suratt, Felder, & Alwan, 2009). Lately, new approaches to the measurement of BCG signals have emerged and contributed to the research of BCG. Three of these are accelerometer-based seismocardiography (SCG) (Zanetti, 1990) and sternal acceleration BCG (SAB) (McKay, Gregson, McKay, & Militzer, 1999), and radar-based mechanocardiography (MCG or R-BCG) (Postolache, Madeira, Girão, & Postolache, 2010; Tavakolian, Zadeh, Chuo, Vaseghi, & Kaminska, 2008).
It has been proposed that the different approaches to BCG measurement can be further divided into precordial vibration recordings, such as SCG, and circulatory reaction recordings, such as the conventional BCG (Tavakolian, Ngai, Blaber, & Kaminska, 2011). Precordial vibration recording has also been proposed to be a feasible tool in cardiac time interval detection whereas circulatory reaction recording could be more useful in deriving cardiac output or stroke volume (Tavakolian et al., 2011). Conventional BCG and its newer variants have been combined with other noninvasive measurements such as ECG, photoplethysmography (PPG), and respiration measurements in different devices and test setups (Chuo, Tavakolian, & Kaminska, 2011; Han et al., 2008; He, Winokur, & Sodini, 2011; Junnila et al., 2006; McKay et al., 1999; Pinheiro, Postolache, & Girão, 2009; Shin, Hwang, Chang, & Park, 2011; Tavakolian, Vaseghi, & Kaminska, 2008). Combining precordial vibration recordings or circulatory reaction recordings with other bio-signals is expected to both enhance the analysis of vibrations of the heart and circulation, and help in deriving variables that are troublesome to obtain otherwise (e.g., continuous systolic blood pressure).