You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Ballistocardiography and Seismocardiography: A Review of Recent Advances 
Chapter Title:  D. Importance of Sensor Location, Axis Selection and Orientation 
1. Abstract of Cited Paper (BIB001):  Simultaneous seismocardiogram (SCG) and ultra-low frequency ballistocardiogram (BCG) signals are recorded. Preliminary results from the BCG helped tag which waves on the SCG are related to the rapid systolic ejection and aortic valve closure events. These results agreed with and further confirmed previous findings using the echocardiogram. This is the first reported work on comparisons of SCG and BCG signals and provides a setup to study the effects of arterial circulation on the morphology of the SCG signal. 
2. Abstract of Cited Paper (BIB002):  Seismocardiogram (SCG) is the recording of the minute body accelerations induced by the heart activity, and reflects mechanical aspects of heart contraction and blood ejection. So far, most of the available systems for the SCG assessment are designed to be used in a laboratory or in controlled behavioral and environmental conditions. In this paper we propose a modified version of a textile-based wearable device for the unobtrusive recording of ECG, respiration and accelerometric data (the MagIC system), to assess the 3d sternal SCG in daily life. SCG is characterized by an extremely low magnitude of the accelerations (in the order of g × 10−3), and is masked by major body accelerations induced by locomotion. Thus in daily life recordings, SCG can be measured whenever the subject is still. We observed that about 30 seconds of motionless behavior are sufficient for a stable estimate of the average SCG waveform, independently from the subject's posture. Since it is likely that during spontaneous behavior the subject may stay still for at least 30 seconds several times in a day, it is expected that the SCG could be repeatedly estimated and tracked over time through a prolonged data recording. These observations represent the first testing of the system in the assessment of SCG out of a laboratory environment, and open the possibility to perform SCG studies in a wide range of everyday conditions without interfering with the subject's activity tasks. 
3. Abstract of Cited Paper (BIB003):  Seismocardiography (SCG) is a non-invasive measurement of the vibrations of the chest caused by the heartbeat. SCG signals can be measured using a miniature accelerometer attached to the chest, and are thus well-suited for unobtrusive and long-term patient monitoring. Additionally, SCG contains information relating to both cardiovascular and respiratory systems. In this work, algorithms were developed for extracting three respiration-dependent features of the SCG signal: intensity modulation, timing interval changes within each heartbeat, and timing interval changes between successive heartbeats. Simultaneously with a reference respiration belt, SCG signals were measured from 20 healthy subjects and a respiration rate was estimated using each of the three SCG features and the reference signal. The agreement between each of the three accelerometer-derived respiration rate measurements was computed with respect to the respiration rate derived from the reference respiration belt. The respiration rate obtained from the intensity modulation in the SCG signal was found to be in closest agreement with the respiration rate obtained from the reference respiration belt: the bias was found to be 0.06 breaths per minute with a 95% confidence interval of -0.99 to 1.11 breaths per minute. The limits of agreement between the respiration rates estimated using SCG (intensity modulation) and the reference were within the clinically relevant ranges given in existing literature, demonstrating that SCG could be used for both cardiovascular and respiratory monitoring. Furthermore, phases of each of the three SCG parameters were investigated at four instances of a respiration cycle-start inspiration, peak inspiration, start expiration, and peak expiration-and during breath hold (apnea). The phases of the three SCG parameters observed during the respiration cycle were congruent with existing literature and physiologically expected trends. 
4. Abstract of Cited Paper (BIB004):  Abstract Seismocardiogram (SCG) is the measure of the micro-vibrations produced by the heart contraction and blood ejection into the vascular tree. Over time, a large body of evidence has been collected on the ability of SCG to reflect cardiac mechanical events such as opening and closure of mitral and aortic valves, atrial filling and point of maximal aortic blood ejection. We recently developed a smart garment, named MagIC-SCG, that allows the monitoring of SCG, electrocardiogram (ECG) and respiration out of the laboratory setting in ambulant subjects. The present pilot study illustrates the results of two different experiments performed to obtain a first evaluation on whether a dynamical assessment of indexes of cardiac mechanics can be obtained from SCG recordings obtained by MagIC-SCG. In the first experiment, we evaluated the consistency of the estimates of two indexes of cardiac contractility, the pre-ejection period, PEP, and the left ventricular ejection time, LVET. This was done in the lab, by reproducing an experimental protocol well known in literature, so that our measures derived from SCG could have been compared with PEP and LVET reference values obtained by traditional techniques. Six healthy subjects worn MagIC-SCG while assuming two different postures (supine and standing); PEP was estimated as the time interval between the Q wave in ECG and the SCG wave corresponding to the opening of aortic valve; LVET was the time interval between the SCG waves corresponding to the opening and closure of the aortic valve. The shift from supine to standing posture produced a significant increase in PEP and PEP/LVET ratio, a reduction in LVET and a concomitant rise in the LF/HF ratio in the RR interval (RRI) power spectrum. These results are in line with data available in literature thus providing a first support to the validity of our estimates. In the second experiment, we evaluated in one subject the feasibility of the beat-by-beat assessment of LVET during spontaneous behavior. The subject was continuously monitored by the smart garment from 8 am to 8 pm during a workday. From the whole recording, three data segments were selected: while the subject was traveling to work (M1), during work in the office (O) and while traveling back home (M2). LVET was estimated on a beat-by-beat basis from SCG and the RRI influence was removed by regression analysis. The LVET series displayed marked beat-by-beat fluctuations at the respiratory frequency. The amplitude of these fluctuations changed in the three periods and was lower when the LF/HF RRI power ratio was higher, at O, thus suggesting a possible influence of the autonomic nervous system on LVET short-term variability. To the best of our knowledge this case report provides for the first time a representation of the beat-by-beat dynamics of a systolic time interval during daily activity. The statistical characterization of these findings remains to be explored on a larger population. 
5. Abstract of Cited Paper (BIB005):  Seismocardiography (SCG) is the measurement of vibration of the precordium due to cardiac movement. SCG is considered to be of value when cost-effective cardiac and circulatory assessment is needed. SCG is normally recorded in the dorso-ventral axis from the sternum, while it has been shown that the superior-inferior axis also has consistent waveforms and might be usable in stroke volume evaluation. Thus, it seems that a system focusing on the superiorinferior axis could be very useful. In this article, a portable measurement system for the measurement of the superiorinferior axis of the SCG is described, and its functioning is demonstrated. Four subjects were measured in the sitting position to verify the system’s functioning. The measurements demonstrated the system’s capability to capture accurate cardiovascular data. Future work includes the validation of the measurement of the superior-inferior of the SCG signal against an established method such as echocardiography.

Literature Review:
For both BCG and SCG, the measurement location has a significant bearing on the morphology, amplitude, and clinically relevant features of the signal. For the SCG, since it is a measure of local vibrations, the precise location of the sensor on the chest impacts the measured signal BIB004 BIB001 BIB005 . A widely used placement has been on the sternum BIB003 , BIB002 , BIB006 . Pandia et al. found that the second heart sound was more pronounced when the SCG was measured on the left side of the chest compared to the sternum BIB004 . For BCG signals measured using an accelerometer, the same is true; an accelerometer placed on the foot will not measure the same BCG signal as one placed on the head, thus stressing the importance of a clear description of, and thoughtfulness regarding, the sensor location on the body. An additional crucial issue is the orientation of the acceleration axis. BCG or SCG accelerations in the dorso-ventral direction will not be identical to those in the lateral (right-to-left) or headto-foot direction; consequently, depending on the purpose of the measurement the axis should be chosen accordingly or a three-axis accelerometer should be used. In spite of the major role played by the selection of the measurement axes, the axes orientation, and the sensor location, from the review of the existing literature it appears that information on these aspects is often missing, making difficult the understanding of the experimental setup and the interpretation of results. Thus, as detailed in Section VI, a standardization on these issues is deemed necessary, and in the meantime, it is advisable that the above pieces of information are clearly stated in any scientific communication dealing with BCG and SCG. Fig. 2 shows a compilation of photos depicting several examples of modern BCG and SCG acquisition hardware, enabling data acquisition in a variety of settings, including in bed, in the home, outdoors, and in microgravity. These systems are discussed below in detail.