Quantitative analysis of motor activities with an elevated "aesthetic content": the case of Tai Chi Ch'uan

The main purpose of this Thesis was to describe the learning process of a complex motor task: the First Lu of Tai Chi Ch’uan (TCC). TCC is a branch of traditional Chinese martial arts and, as is the case for all martial arts, one of its most important characteristic is the deep interaction between mind and body: the body is naturally relaxed, the mind is tranquil but alert, the body movements are slow, smooth, and well coordinated. It is composed by a complex sequence of movements that needs several years of practice to be learned and which is characterized by “an high aesthetic content”. There are different styles of TCC, the more diffused ones are Chen, Yang, Wu and Sun. In this Thesis we focused our attention on the Yang style. In the Yang school the main exercise is composed by 108 posture (or figures) divided into four sections called “Lu”. In this Thesis we analyzed in detail the Fist Lu of the Yang style, composed by 22 postures. We tried to provide a general description of this martial art focusing our attention on the bioenergetic, biomechanic and perceptive aspects of this “form of exercise”. Motor control literature proved that muscular, cardiac, ventilatory, electromyographyc and mechanical variables can be used as indicators of skill and, in some cases, as indexes of learning (however these studies are generally referred to discrete movements or, at most, to cyclic tasks). Moreover, the skill level seems to be a “quantity” that can be perceived by anyone even if the observer has no specific competence in that discipline. When observing a skilled movement a “sense of beauty” is also perceived. Indeed millions of people like to watch the Olympic games not only because of the competition excitement, the national pride and the media coverage but also because of the simple pleasure of looking at movements that could be considered as an example of “perfection”. Thus, there seems to be a relationship between the technical skill of the performer and the sense of “beauty” perceived by an observer (whether an expert or not in that discipline). But, which are the variables (bioenergetical and/or biomechanical) of human movement that allow an observer to recognize the level of the athlete’s performance? Which are the features of human movement that are perceived as “beautiful”? Are these related to those that reflect the level of expertise? Is beauty in the eyes of the beholder or are there common objective characteristics of motion associated to the idea of “beauty”? In this work we attempted to answer these questions by means of a multidisciplinary approach. In this Thesis we tried to understand which were the variables involved in the learning process of a complex and not repetitive sequence of movements. To do this a heterogeneous group of 23 TCC practitioners, with a level of expertise from one to twenty-two years, were analyzed; they were recruited on a voluntary basis through different TCC schools and the inclusion criteria included that subjects had practiced the Yang style of TCC for at least one year and could correctly and properly complete the First Lu (the first 22 postures of TCC). In this Thesis, after a general introduction of the topics of interest (Chapter 1), the results obtained in the three studies we performed are presented (Chapter 2, 3, and 4). The general discussion and the conclusions are reported in Chapter 5. STUDY 1 - The aim of this study was to give a biomechanical description of the First Lu of Tai Chi Ch’uan and to investigate which kinematic variables associated to the body centre of mass (BCOM) movements could describe the learning process of this complex motor task. Twenty-three subjects (10 experts: 15.0 ± 5.1 yrs of practice; 13 beginners: 2.3 ± 1.1 yrs of practice) were recruited through the same TCC school (Yang style). Practitioners were asked to perform the exercise twice and kinematic data were collected through a stereophotogrammetryc system. From the values of displacement of BCOM, velocities, accelerations and jerk were calculated. Significant differences between groups (p<0.05) were observed for most of the parameters of interest: experts performed the movements at a significantly lower pace (229 and 175 s), with wider movements (22.1 vs. 19.2 m) of total BCOM displacement and with a lower jerk (1.82 vs. 2.57 m.s-3) than beginners. According with the theories of motor learning, variability of movement decreased with the increase of expertise. The bandwidth for displacement and acceleration of BCOM (1 SD about the average values, considered as an indicator of control and accuracy of movement) was indeed lower in experts than in beginners. Based on kinematic data, external mechanical work and power were computed. External mechanical work (WEXT) was calculated by means of two independent methods (the “single mass model” and the “segmental model”) and was found not to differ between groups; on the other hand, mechanical power (W’EXT = WEXT / TEXE) was significantly larger in experts compared to beginners due to the significant difference in TEXE. The values of WEXT and W’EXT were found to be 10 times lower than those reported for walking at the self selected speed (about 1 m.s-1), according to the fact that average speed maintained during the execution of the First Lu of TCC is ten times lower (about 0.1 m.s-1). STUDY 2 - The aim of this study was to give a bioenergetic description of the First Lu of Tai Chi Ch’uan and to investigate which metabolic and electromyographic variables are involved in the learning process of this complex motor task. Twenty-three subjects, divided into two groups based on their expertise, were recruited through the same TCC school (Yang style). Even if no differences in oxygen uptake (about 12 ml.min-1.kg-1) and expired ventilation (about 21.7 l.min-1) were detected between groups, results show that expert and non expert TCC practitioners can be differentiated based on the “quality” instead of the “quantity” of their breathing: expert practitioners breathed slowly (11.0 and 15.4 b.min-1, respectively) and more deeply (2.28 and 1.53 l, respectively) than beginners. TCC is thus an exercise of moderate intensity but it is characterized by a long duration of co-activation of the lower limb muscles (from 50 to 80 % of total exercise duration). We expected that longer co-contraction times were related to higher metabolic demands, but this was not verified. It is well known that one of the main causes of inefficiency is the presence of a large % of co-contractions and indeed the values of mechanical efficiency as measured in this study are rather low (from 0.10 to 0.16, compared to 0.25 at the muscular level). TCC is thus an un-efficient “type of locomotion” and this is also demonstrated by the high values of energy cost (about 35 J.m kg-1) that are three times larger that those reported for walking (about 12 J.m kg-1) at the same “average velocity” (about 0.10 m.s-1). STUDY 3 – In the third study we performed a perceptive analysis (visual perception) of this type of movement. Two groups of subjects were recruited: one group of practitioners (10 experts: with more than 8 years of practice; 15 non experts: with less than 4 years of practice) and one group of observers (26 experts of this motor task and 30 naïve observers). The observers were asked to rate the performance of practitioners for aesthetic (beauty/grace and fluidity/continuity) and technical (skill and balance control) qualities of movement. Data reported in this study indicate that: 1) there are indeed features of human motion (or at least of TCC) that are recognised as “beautiful”: fluency, rhythm and technical skill; and 2) differences in the perception of movement are indeed recognizable between expert and naïve observers. Thus, as for the aesthetic evaluation of a static object (e. g. visual arts), beauty in human motion seems to be a function of both the quality of the “object” and of the observer. We also found a significant relationship between metabolic parameters (e. g. the breathing pattern) and aesthetic/technical scores and this further suggest that action performance is the final mergence of the coalition of all these components. Data reported in this Thesis indicate that any observer should be able to recognise the “level” of a movement performance (TCC, in our case) even if he has no specific competence in that discipline, since skilled performance is often associated with a perception (or idea) of grace and beauty, which are movement qualities that anyone seems to be “able to rate”. In this Thesis we also tried to understand which are the quantitative indexes that could characterize the performance of expert and non-expert TCC practitioners and thus play a significant role in the learning process of this complex sequence of movements. It is reasonable to think that these variables influence also the judgement of the observers (both technical and aesthetical). This was done through a kinematic, metabolic and EMG analysis. Longer time of execution (228 vs. 174 s), lower velocities (0.10 vs. 0.12 m.s-1), wider movements (23.8 vs. 21.1 m), and a better control of movement (lower SDs) were the main kinematic features that characterized the performance of an expert TCC practitioner in comparison to a beginner. The increase in the time of execution was found to depend, in part, from the breathing pattern; TCC is considered as a form of “meditation in movement” thus, as in other forms of meditation, a deeper and slower breathing pattern is generally achieved with practice. Indeed, expert practitioners were characterized by a slower breathing frequency (11.0 vs. 15.4 b.min-1) and a larger tidal volume (2.28 vs. 1.53 l) than beginners; moreover, in experts, the number of breaths was found to be smaller (36 vs. 44) and closer to the number of postures (22) than in non experts (in TCC opening movements should correspond to an inspiration, closing movements to an expiration). Metabolic data indicated that TCC is an exercise of moderate intensity ad that no significant differences could be detected (between expert and non expert practitioners) for the values of ventilation (about 21.7 l.min-1), heart rate (about 116 bpm) and oxygen consumption (about 12.0 ml.min-1.kg-1). Data reported in this Thesis also indicate that this “form of exercise” is characterized by long co-activation times of the lower limb muscles (from 50 to 80 % of total exercise duration). It is well known that one of the main causes of inefficiency is the presence of a large proportion of co-contractions; in fact mechanical efficiency in TCC was found to be of about 0.10 - 0.15 (compared to 0.25 at the muscular level). TCC also appears to be a very expensive “form of locomotion”, with values of energy cost being three times greater with respect to walking (34 vs. 11 J.m-1.kg-1) at the same “average velocity” (about 0.1 m.s-1). In conclusion, we found metabolic and kinematic indexes able to explain and describe the improvement of “skill” with the years of practice. We also demonstrated that technical skill is strictly related to the perception of beauty. This study therefore underlines the interplay between technical, metabolic, kinematic and perceptive data.