2.1 Definition of Trunk Strength
In order to discuss the body trunk strength, we first describe the definition of posture, movement, motion, action in kinematics.
Posture: a posture consists of two elements: attitude and position. Attitude represents the relative positional relationship among each part of the body such as head, trunk, limb, and it can be measured through joint angle. On the other hand, position is used to represent the relationship between the body axis and gravity, and it can be indicated by standing, supine (face up), and so on.
Movement: a movement refers to temporal change of posture. In other words, it is described as a change of attitude and position.
Motion: a unit that analyzes the behavior as a task that is specifically carried out by a movement.
Action: a unit when taking into consideration the context of the meaning and intention shown by a movement.
Body trunk is the torso which has the following three roles: (1) supporting and maintaining postures, (2) the foundation for producing movement, (3) serving as an axis. Several methods were proposed to increase the mass of surface muscle group (e.g., rectus abdominis muscle) and deep muscle groups (e.g., transverse abdominal muscle) [6–8], such as plank, elbow-to, and breathing techniques.
It is worth mentioning that the quality of movement is more important than the amount of movement, which is a key point in improving people’s healthy life expectancy in the super-aged society in the future. Bad motion patterns caused by bad postures may lead to musculoskeletal pain syndrome. The prevention and treatment of pain is directly related to the activity in everyday life, therefore it is a very effective and efficient means for health promotion and preventive care. Based on this rationality, the MSI (Movement System Impairment) approach was proposed, which reduces mechanical stress by correcting the movements and motions and thereby enables the prevention and treatment of pain . In addition, Shumway-Cook and Woollacott proposed a system theory which advocates that movement is not simply the result of muscle-specific exercise program or uniform reflection, but rather the result of the dynamic interaction among perception system, cognition system, and musculoskeletal system . This theory has been applied in rehabilitation and has been proved effective in practice. For instance, the capability of independent walking in the elderly was improved after motion control exercise with repeated movement such as walking, even though muscle strength was not really improved, the capability of independent walking was increased. In physically expressive sports such as dance sports, it is widely known that smooth usage from deep muscles to the surface muscles is more important than muscle mass. And the core stability is the result of motion control and the muscle function of the Lumbar spine, pelvis, and hip joint complex.
Therefore, it is considered that system theory as well as MSI approach should be applied to health promotion and preventive medicine. In other words, rather than evaluating single function, such as muscle strength and range of motion, it is important to establish an easy and proper approach to evaluate and visualize the quality of movement pattern in a comprehensive manner by considering neuromuscular coordination. In this way, it will become possible to learn and acquire the optimal movement pattern without injury or secondary dysfunction. However, many services of health promotion and prevention activities still solely put emphasis on strength training.
2.2 Typical Trunk Movement
According to the WDSF technique book “Rumba” , there are four types of trunk movement.
Left and right horizontal movement of pelvis (Fig. 1 left). While keeping the pelvis and shoulder line horizontal, move pelvis left and right horizontally.
Left and right tilt of pelvis (Fig. 1 center). The left side of the body is compressed vertically while the right side of the body is stretched. Keep the left shoulder close to the left hip and the right shoulder away from the right hip. The right side is the same.
Back and forward tilt of pelvis (Fig. 1 right). For forward tilt, tilt the pelvis forward by moving the upper part of the pelvis forward and the bottom part backward, and vice versa for back tilt.
Trunk torsion. While keeping the pelvis and shoulder line horizontal, rotate around the vertical axis of the trunk.
The trunk can also make various movements by opening and closing the chest. In the case of standard dance, many movements involve the conjunction move of limbs and head. In the case of Latin dance, many movements only move the lower part of the body smoothly while keeping the upper part instable. In this paper, we focus on trunk torsion that may occur in walking and rotational motion, which in characterized by the angle change in the vertical axis of the shoulder line and the hip line.
2.3 Existing Trunk Models
The most detailed trunk model is the musculoskeletal model . As it models trunk components in great details, it can be used to describe complex movements. However, it is difficult to apply this on torsion movements due to large number of parameters involved. In addition, the trunk model for walking focuses on the waist and the lower limbs, which is not suitable for the purpose of our study. Even in the case of whole body models, the trunk is usually modeled as mass-point or cylinder, and there is no model of trunk torsion movements. Although there are models for body torsion while walking, they cannot be applied to the cases of repeated trunk torsions.