|BIU SANTE HISTOIRE DE LA SANTE MEDICA ETIENNE-JULES MAREY|
Bibliothèque numérique Medic@ : Jules Marey
The invention of biomechanical techniques
E-J Marey considered himself a "medical engineer", or an "engineer of life". This name is particularly apt if one considers the kinetic and dynamic techniques he designed, developed and continuously perfected, with the efficient assistance of many collaborators.
By definition, kinetics is concerned with motion, as characterised by displacement, speed and acceleration. Chronophotography, the kinetic technique invented by E-J Marey, detects displacement. Using optic transduction like the renowned photographer, Eadweard J. Muybridge (1830-1904), his technique detects and records the displacement of body segments, that is, it captures body diagrams in motion. Over a period of just a few years, between 1882 and 1888, E-J Marey makes a number of improvements: displacement is first shown by silhouettes and then by geometric shapes, for the first time in history. Characterised by the uniqueness of the point of view, the technique breaks down the motion on a single photographic plate ("still plate chronophotography"), which records successive shots, and makes it possible to see the various phases in a single glance. He initiates a new technical phase in 1888, with the use of a mobile paper band, then photosensitive film with no perforations and a camera, which constitute "film chronophotography", the basis of cinematography.
The invention of chronophotography had a major impact on many fields (occupational, sport and pathological motions). Many complementary methods were developed during the same period, making it possible to study the motion of various parts of the body (trunk, pelvis, jaw, etc.), and to measure performance (trajectory of the tip of a sword, height of a jump, path followed when walking, etc.).
During the same period, E-J Marey studied dynamic techniques, that is, the measurement of forces, which, by definition, determine motion. He designed and created the first force platform in the history of biomechanics, the "dynamometric table" (E-J Marey, 1883 ), which was also called a "dynamographic platform" or "dynamograph". This is a measurement platform, used to record the actions performed by a subject at rest or in movement. In other words, it measures the reaction forces on the ground, and therefore the global effect of muscle actions, when the external forces applied on the subject are limited to his weight and the ground. reaction
In all, E-J Marey made decisive technical progress in the detection and graphical representation of motion, opening the way for scientific study.
The definition of a scientific method for the study of motion
Contrary to the false impression created by a quick reading of E-J Marey’s work, his technical innovations, made with such care and talent, are part of a broader perspective, that of a method of experimental analysis. His scientific method involves capturing, recording, and explaining all possible movements, in order to deduce general laws.
The method, which he recommends and applies, comprises several stages: analysis of the movement, often with a simultaneous use of kinetic and dynamic techniques, then attempt at quantification, and finally interpretation. This is, for example, how a jump is analysed, or locomotion, whose analysis proposes interpretation in terms of muscular properties. Finally, whenever possible, an analogue model is created, i.e. a synthesis, in order to test the validity of the results.
The method is clearly biomechanical, in the modern sense of the term, although, during this pioneering period, certain limits hamper its application. Thus, E.J. Marey's knowledge of mechanics is modest, and the results, primarily of a qualitative order. Still, he defined and implemented a scientific method for the study of motion, based for the first time on experimentation, with a clearly stated purpose: to establish objective explanations. This method can be qualified as "global", in that measurements are taken on the periphery of the body, and therefore result from an integration of more elementary phenomena.
The reasons for E-J Marey’s scientific choices
E-J Marey’s methodological choices are far from circumstantial: they are based on two positions of principle.
The first arises from his philosophical convictions: E-J Marey shares the positivistic ideas of Auguste Comte, and strongly opposes the theory of vitalism. His interest in mechanics is a consequence of this, serving his ambition to correlate motion to the forces producing it, both in humans and in many animal species. Thus, he develops a scientific method for analysing motion, in order to obtain the means to find an objective explanation.
His second position of principle is no less strongly stated. Since he believed that vivisection would perturb natural functions (and not permit the understanding of the regular play of life), he promotes an approach based on data taken from the periphery of the body. This is one of the originalities of E-J Marey, which opposes him radically with the reductionist approach of Claude Bernard and of most of his contemporaries.
The conjunction of these two positions of principle explains the prudence which inspired E-J Marey to study the "act of will", as he called voluntary motion, and why the physiological interpretations he developed are limited chiefly to the action of muscles, primarily the role of muscular elasticity.
A century has passed, and some of the battles fought by E-J Marey have been relegated to the history of sciences. No one now contests that the laws of physics apply to biological phenomena, and in particular to the motion of humans and animals. But, although we have come a long way since that time, thanks to technical progress, to the evolution of problematics and to accumulation of knowledge, it is no less remarkable to observe the indelible mark E-J Marey has left: do we not continue to study motion according to the method he elaborated? Moreover, the advancement of knowledge in the field of biological sciences, and in that of physiology and psychology in particular, opens up a whole new field of investigation to Marey's method, that of cognitive biomechanics, focussing on understanding the process of motion control.
1. De la mesure des forces dans les différents actes de la locomotion. In CRAS, 97 : 782-786 et 820-825. (tiré à part).
© Simon Bouisset. Tous droits réservés.
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