History of motion analysis

The study of human and animal movement is an intriguing topic that has always fascinated the curiosity of artists and researchers. Across the years, many publications appeared with the aim of figuring out the principles of movement and understanding its biomechanical causes and effects. Such studies may finally answer the age-old question: how can such a perfect machine, that is, the human body, achieve such smooth movements in a so simple and natural a manner? Human/animal biomechanics involves multiple anatomical systems (nervous,muscular, visual, auditory, etc.) and requires strong coordination between the systems and the limbs, resulting in smooth and elegant movements. Such motion may be very simple in the effect, but very complex from the mechanical point of view. Examples range from animal quadrupedal walking or human bipedal walking (which requires maintaining balance), to the most extreme sport performances (Ancillao 2016).

For years, engineers tried to reproduce natural movement strategies by means of machines or robots, but no one has ever succeeded in equalling such a complex, smooth, and beautiful motor performance. A close and remarkable attempt is the one achieved by the Dutch artist Theo Jansen who created some wind-powered kinetic sculptures, named Strandbeest (that means Beach Beasts; see also www.strandbeest.
com). These sculptures are made of PVC, wood, and fabric airfoils and are able to collect the power given by the wind. By means of advanced mechanical design, wind power is stored in flywheels and transferred to several legs that move sequentially, achieving a walking effect (Fig. 1.1). These beasts are able to coordinate the motion between several moving units, reproducing a multilegged walking pattern which makes them free to walk around on the sand by themselves, powered by the wind. The first biomechanical studies took advantage of photography techniques. In fact, it was observed that it is possible to obtain a representation of motion by taking sequential pictures at a fixed time interval. This technique was subsequently called chronophotography. The most famous attempts were the works of the English photographer, Eadweard Muybridge (9 April 1830–8 May 1904), that placed some arrays of cameras along the perimeter of the place where the motion took place (e.g. a horse racetrack). An advanced triggering system allowed taking sequential photographs of animals and people performing motor tasks. His works were greatly appreciated for their value both artistic and scientific.

Muybridge’s most famous work, ‘The Horse in Motion’ (Fig. 1.2), was inspired by a biomechanical question that was popular at that time: is there a moment in which all four feet of a trotting horse are off the ground at the same time? Till that time, in fact, most artists used to paint horses with one foot always on the ground. The question was intriguing, because the forward movement logically required a moment of complete loss of contact with the soil, but no one had ever observed it, as the human eye is not fast enough to catch the moment. To answer that question, Muybridge took sequential shots of a galloping horse using an array of 12 cameras placed along a racetrack (Muybridge 1878). The images clearly showed that there was a time in the running stride when the horse did actually have all four hooves off
the ground (Fig. 1.2, 1st line). Other famous Muybridge works are ‘The Woman Walking Downstairs’ and ‘Two Women Kissing,’ shown in Figs. 1.3 and 1.4, where the author used the chronophotography technique to capture human motion. Such work provides an
objective evaluation of the posture and the overall motor strategy involved in that
action.

The ‘Woman Walking Downstairs’ (Fig. 1.3) is considered one of the first attempts to study the biomechanics of the human body while performing everyday tasks, whereas the ‘Two Women Kissing’ (Fig. 1.4) is considered the first-ever filmed kiss.

In 1882, the French scientist and photographer Étienne-Jules Marey (5 March
1830–15 May 1904) designed a device capable of taking 12 consecutive frames in a second. This device was a shotgun modified to capture light on a photosensitive disc. The device was called a chronophotographic gun (Figs. 1.5 and 1.6). The resulting sequence of pictures, when properly played, was able to reproduce motion and was the precursor of cinematography, invented by the Lumière brothers in 1895.

Marey’s research interest was mainly focused on the motion of animals (horses,
birds, cats, and others) and human locomotion (Marey 1874). His most famous work is the movie that demonstrates how falling cats always land on their feet
(Fig. 1.7). He also conducted very similar studies on chickens and dogs finding out that they could do almost the same (Marey 1894). Marey conducted studies on the biomechanics of human walking as well (Fig. 1.8), and improved the photographic technique by adding markers on the subject’s body. These markers resulted in
bright dots or lines on the developed film, allowing an accurate identification of body segments and landmarks and their evolution over time (Fig. 1.9). In the following years, biomechanical analysis became more accurate thanks to the advances in photographic techniques and cinematography. The possibility of an objective evaluation of human motor performance captured the interest of clinicians and today it is considered a valid method to study motor performance and to support the diagnosis with quantitative data (Whittle 1996). The most notable advance was the introduction of 3D tereophotogrammetry
techniques that allow us numerically to reconstruct the position of a point by
observing it from two different points of view. It is the same process occurring in
the human brain which is able to reconstruct the depth of field by means of the
different images provided by the two eyes (Fig. 1.10).