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:  C. 3-D Ballistocardiography and Microgravity Studies 
1. Abstract of Cited Paper (BIB001):  Terrestrial ballistocardiographic (BCG) measurements are typically performed in only one or two axes because of the coupling between the subject and the ground. An appropriate physiological interpretation of these BCG signals therefore assumes that the information in the unmeasured axis is either understood, or able to be ignored. BCG signals from measurements in microgravity can be made in all three axes and permit examination of these assumptions. Such microgravity measurements show that lung volume significantly affects the BCG signals, predominately in the head-to-foot direction. Further, the maximum accelerations recorded following systole are poorly captured by coronal plane measurements as the greatest displacements occur in the sagittal plane. These results suggest a need to carefully consider the influence of the motion in the unmeasured plane when interpreting terrestrial BCG signals.

Literature Review:
As the sections on instrumentation earlier in this review have indicated, measurements of BCG (in particular) are constrained by the coupling of the body to the ground, a direct result of the influence of gravity. As such, full 3-D recordings of the BCG are difficult in the terrestrial environment, and much of the focus has been on accelerations in the coronal plane (the XY plane as defined in the section on measurement axes). Given this limitation, it is therefore not surprising that the idea of measuring the BCG in a subject in free-fall (weightlessness, zero-G, microgravity) was an obvious target of investigation. The first such experiment was performed in the 1960s in parabolic flight, with the subject strapped into a "tub," which was itself instrumented to record the BCG  . Despite the limited periods of microgravity available (typically ∼20 s) and the subject restraints, recordings of good quality were obtained. Spaceflight represents the other obvious environment in which the "true" 3-D BCG can be recorded. The earliest recordings were made by the Soviets on Saluyt-6  and consisted of a series of five recordings were performed in two crew members of a long duration mission on days 46, 71, 98, 133, and 175. A piezoelectric sensor, attached close to the center of mass, recorded ballistic forces in the feet-to-head axis during breath holding experiments. Individual changes were seen during the mission with maximum amplitude of the IJ wave occurring on day 133. Measurements were also made during the Spacelab-1 mission aboard the Space Shuttle in 1983  . These experiments were conducted in two subjects at two occasions during this short duration spaceflight and showed an increase of the overall systolic accelerations along the longitudinal axis in microgravity. Perhaps the best-analyzed dataset of the BCG in spaceflight came from measurements made during the Spacelab D-2 mission in 1993. During that flight, extra time became available (due to an extension of the overall mission length), and an experiment was hastily conceived, approved, implemented, and performed to measure 3-D BCG in a free-floating subject. Parenthetically, this may be one of the fastest spaceflight experiments ever developed with the time from concept, to collection of the data (including approval of an institutional review board) was only 4-5 days, surely some sort of record. The experiment utilized data from a free-floating subject instrumented with an ECG and wearing a snuggly fitting suit that measured respiratory motion using an impedance plethysmograph (see Fig. 4 ). This instrumentation was a part of the Anthrorack series of human studies managed by the European Space Agency. The second cruicial piece of instrumentation was a set of high-fidelity triaxial accelerometer that were attached to the vehicle and used for measuring the accelerations imparted by crew activity in the Spacelab. The sensor package was detached from the vehicle and taped to the lumbar region of the subject, near to the (presumed) center of mass. Data were then recorded as the subject remained stationary and free floated in the center of the Spacelab, providing a continuous recording, free of interruptions of 146 s. In order to synchronize the two separate data streams, collisions with the Spacelab structure, which disrupted signals in both data streams, were used as posthoc event source BIB001 . The data from the D-2 study and some subsequent studies provided valuable insight into several aspects of the BCG. In particular there were four major conclusions derived from this dataset. 1) Lung volume greatly influences the accelerations recorded, especially in the longitudinal (head-to-foot) body axis (see Fig. 5 ), with the implication being that there is better coupling between the heart and the body in the longitudinal axis at higher lung volumes BIB001 . Interestingly, the actual direction of respiratory motion (mid inspiration versus mid expiration) had only minimal influence of the BCG. 2) Data derived from short periods of microgravity in parabolic flight are largely equivalent to data obtained in sustained microgravity BIB002 . 3) The BCG has a plane of symmetry that is primarily sagittal. This suggests that 2-D recordings performed in a supine subject (i.e., coronal recordings) fail to capture a significant portion of the effect of the blood ejection on the body, complicating their interpretation BIB002 . 4) The accelerations that are recording in a 2-D system are only modestly correlated with the true 3-D accelerations that actually occur, again complicating their interpretation BIB002 . BCG flight experiments were also an integral part of the Russian cardiovascular research program for the orbital station MIR. BCG along the head-to-foot direction was measured in three crew members during the second MIR mission in 1988 and compared to SCG recordings. Significant changes of the BCG amplitudes (HI, IJ, JK) during the long-term flight were described together with large inter individual differences. The first true 3-D-BCG recordings were made during the sixth MIR mission in 1990 in two crew members on flight days 56 and 110. Three new piezoelectric sensors were used placed in perpendicular planes in a small cylindrical box with a diameter of 40 mm and a height of 20 mm. The sensitivity of the sensor was 20 mV/m/s 2 . The sensor was placed between the scapulae using rubber belts and a metallic plate. The special amplifier (BCG-3) was connected to the recording unit "Gamma-1," and the data were transmitted telemetrically to the ground station. In summary, no dramatic changes in the vector sum were detected. Maximum forces ranged from 5.85 to 10.18 N. However, profound individual changes of the shape, amplitude, and timing of the BCG, especially in the lateral and dorso-ventral plane have been found. Finally, combined BCG and SCG measurements have been made every month in space during the 14 months space flight of Valeri Poljakov, 15th to 17th MIR missions (Russian-Austrian flight experiment "Pulstrans")  .