Twenty-Year Studies with the Ballistocardiograph
Authors: STARR ISAAC , WOOD FRANCIS C. 
Published: June 2012

A group of 211 healthy persons, gathered together from 23 to 17 years ago to provide normal standards for the ballistocardiograms, has been followed to the present time. This study is concerned with the group as a whole, or with subgroups. The study of individuals will appear in later communications.
With a few minor exceptions, the group, at the time of the initial test, had ballistocardiograms normal in form. The death rate of the group during the period of the study was much lower than was to be anticipated from standard life expectancy tables for the state of Pennsylvania.
Although the ballistocardiograms of this group were normal in form, there was great variation in the amplitude of the records. Since these records were calibrated in terms of force, this can be interpreted as due to differences in the force of the heart's contraction. Those whose hearts contracted with little force at the initial test later suffered from death and cardiac disability, chiefly coronary heart disease, in far greater numbers than those whose hearts contracted strongly.
The interpretation of this striking finding is bound up with that of another; in our data, as in that of others, there is strong correlation between the ages of the subjects and the amplitudes of their ballistocardiograms. The heart tends to weaken and beat with less coordination as it grows older.
Exact expressions can be given to the "normal" rate of decline of cardiac function with the years by the slope of the regression between age and ballistocardiogram amplitude found in the group of those who remained healthy for 17 years after the test. The normal rate of cardiac decline, as age advances, agrees closely with that calculated for several bodily functions. The cardiac decline seems a little more rapid than that of these other bodily functions, but the difference is small and its significance is doubtful.
In our data, among those who later developed undoubted heart disease, there was no significant tendency for the older people to have smaller ballistocardiograms than the younger.
From the chronologic age of each subject who later developed undoubted heart disease, and the age-amplitude regression of those who remained healthy, one can calculate for each "cardiac" subject, the amplitude expected if he had been in perfect health. On the average this expected value far exceeds the values found. So most of these hearts were performing abnormally for their age although manitest clinical evidence of heart disease had not yet appeared.

When the effect of age is eliminated by pairing the results directly, the relation between the amplitudes of the initial ballistocardiograms and the later development of heart disease remains significant for
p
= 0.05. Indeed, in such pairs with similar ages, those who later developed heart disease had initial ballistocardiograms that averaged 25 per cent smaller than those of their mates who remained healthy for the next 17 years. This is additional evidence that the hearts of those who were to develop myocardial disease were, at the initial test, like the hearts of much older normal people; that is, the physiologic age of their hearts far exceeded the chronologic age.