Mr. Charles Henry, director of the laboratory for investigating the physiology of the sensations at the Sorbonne, has just designed a new registering dynamometer which allows the energy of the muscles of the hand to be measured, at the same time eliminating the effect of pain, that prevents the exertion of the full strength in the various types of spring dynamometer. This very exact apparatus, shown in its completed form in our photographs, will give very precise results concerning the influence of various diets on man. The role of foods is twofold: They should furnish the material for the growth or repair of the tissues, and at the same time should make up the expenditures of energy of the organism, whether these expenditures are in the form of heat or in the form of mechanical work. Certain foods, like the albumens and the fats, contribute both to the repair of the tissues and to the production of energy. Others, such as the hydrocarbons, seem to serve entirely as energy furnishers. But within what limits 473 does each one of these classes of products attain one or the other ends of rational alimentation? The only way to answer this question is by determinations of the energy expended combined with chemical researches. Unhappily, exact apparatus for measuring the applicable energy of a muscular system has been lacking up to the present. Springs of elliptical form, which serve in physiology as dynamometers, are more or less inexact measurers of exertion, because the pressures exerted by the fingers on the different parts of the ellipse oppose each other, and this force of defop mation is not registered by the needle. Moreover, these dynamometers do not indicate the duration of the pressure, an essential thing to know, because men of relatively feeble strength but with rapid muscular co-ordination can produce, all things being equal, higher pressures than other men who are relatively stronger, but who are slower. A muscle is not tired out when it has reached its maximum pressure, since it maintains this pressure a certain time, and does not give way until the end of a short interval. This makes it necessary to record variable and decreasing pressures. Verdin has indeed transformed into dynamographs the dynamometers of Duchene de Boulogne, applying to them a system of air transmission by means of Marey drums, which, by the play of a lever, register pressures on turning cylinders. These dynamographs constituted an important theoretical progress over the old dynamometers, but the metallic springs of various forms that they utilized had also the double disadvantage of only responding to the normal pressures of the steel blade, and utterly ignoring the lateral pressures. They indicated at each instant but a small stress, because of the pain caused by the rigidity of the metal. For example, the fatigue curves obtained "with the dynamometer of Mr. Charles Henry present a normal form very different from those furnished by spring dynamographs. The new apparatus consists of a spherical bulb of rubber ending in a shoulder, that permits it to be joined to a metallic tube in such a way as to allow only the spherical portion of the envelope to be de-formable. This bulb is full of mercury, which under the pressure of the hand or of the fingers, mounts to different levels in the metallic tube. A mass of iron which is raised by the mercury communicates its movement through a thread rolled around a convenient reduction pulley (1-6) to a pen, which traces the pressures on the recording cylinder. This cylinder is covered with paper squared in millimeters, and is made to move by clockwork, which gives it the uniform velocity of one millimeter a second. Upon pressing the bulb, the subject obtains at each instant the maximum pressure, until he is exhausted. With the system of Mr. Henry there is no deformation of the tracings, and since the transmitting device is inalterable, the curves are always comparable. Moreover, the double graduation fixed on the card indicates the total pressure and the work done. The area of each of the recorded curves measures what is called the "static work," or in other words, the product of the mean pressure and the duration of the effort of the flexor muscles of the fingers. Now, Mr. Henry and Miss Joteyko have shown experimentally that to be able to calculate in kilogramme-meters the work performed by the same expenditure of energy, that is to say, the available energy of the muscles, the number of kilogramme seconds represented by the static area must be divided by 120. Thanks to his dynamometer, the inventor obtained in the course of several experiments measures of static work varying between 99 and 490, the available energy corresponding being between 0.8 and 4. The mean ratio of "static work" to maximum pressure equaled 3.3, while according to the spring dynamographs of Charles Verdin the same ratio was 66.3, or twenty times greater. Moreover, the fatigue curves obtained with the new apparatus show notable differences from those furnished by the older forms. At first, the decrease is much slower; then at the end a more rapid decrease of the exertion with the time is indicated. This proves that the spring dynamographs should be given up, since the factor of pain keeps the subject from giving at each moment his maximum pressure. The muscle reposes to an unknown degree, and the static areas formerly calculated lack exact significance.Translated from the French of Jacques Boyer for the SCIENTIFIC AMERICAN.
This article was originally published with the title "The New Henry Dynamometer" in Scientific American 97, 26, 472-473 (December 1907)