Highly experienced athletes from the German Wushu Federation were invited to participate. The participants had to fulfill the following criteria: aged 18 years or above, at least 5 years of training experience in Tai Chi, and at least fifth place at the German Tai Chi championship. Exclusion criteria were injuries, illnesses or health conditions that preclude participation in sporting activities, and allergies to adhesives (because of the use of reflective markers for the biomechanical analysis).

After ethical approval was obtained, the athletes were recruited by email and signed a written informed consent.

Three healthy males and five healthy females (age: 44 ± 8 years, height: 1.70 ± 0.07 m, body mass: 65.2 ± 11.7 kg) participated in this study. Training experience ranged from 6 to 30 years. All athletes achieved first place at the German Tai Chi championship at least once. One participant ranked first, and another sixth, at the European Tai Chi championships, and one athlete ranked sixth at the Tai Chi World championships. Except for one who had not graduated, all athletes had a graduation level of 3–6 in Tai Chi according to the Chinese Wushu Duanwei System. The characteristics of the participants are shown in Table 1.

Three-dimensional kinematic and kinetic data were collected at 250 Hz and 1,000 Hz, respectively, using a 13-camera motion capture system (Qualisys AB, Gothenburg, Sweden) and two integrated force plates (60 × 120 cm; AMTI, Watertown, MA, USA). In total 57 reflective markers were placed bilaterally to participants head, upper body, upper limbs, and lower limbs, based on the Cleveland Clinic Marker Set (Motion Analysis Corp, Santa Rosa, CA, USA), which uses an anatomical landmark calibration technique.

After a 15–20-min individual warm-up, each participant was equipped with reflective markers. Subsequently, each participant performed a 10 m-walk over the two force plates to calibrate the individual measuring instruments. Since only two force plates were available, the whole Peking style routine (shown in Figure 1) had to be separated into single movements. Moreover, repetitive movements were performed only once. Each participant performed all 24 movements twice: first as the CV (corresponding to national/international championships) and second, after a short break, as the HS (with shorter and higher stances, that means a smaller distance between the feet and thus less knee flexion). To define the length and height of the stances, as well as the range of motion of the upper body and upper limbs, the bow stance (Gong Bu) taken in the movement called Part the Wild Horse‘s Mane was used. All other movements were adjusted according to this movement. The average dimensions for the two versions are shown in Figure 2.

All raw kinematic and kinetic data were labeled and cut using QTM (Qualisys, Göteborg, Sweden). Further analysis was performed using Visual3D (C-Motion Inc., Germantown, MD, USA). Raw kinematic data were filtered with a 15 Hz Butterworth filter, and raw kinetic data with a 25 Hz Butterworth filter. For the statistical analysis, IBM SSPS Statistics 27 (IBM Corp., Armonk, NY, USA) was used. Thoracolumbar rotation was calculated from the torsion of shoulder axis and pelvis. For the single movements, the mean and standard deviation values for thoracolumbar rotation to the left and right were calculated, and the total range of motion of the thoracolumbar rotation was determined. The range of motion of the CV and HS were compared using the Wilcoxon signed-rank test for paired differences. Movements performed on both sides were also compared using the Wilcoxon signed-rank test for paired differences, to examine side differences within the same performance (CV or HS). Differences were considered significant at p < 0.05. In order to determine which movements comprised large thoracolumbar rotation, the range of motion was compared with thoracolumbar rotation during the 10 m-walk at the beginning of the trial, which revealed a mean thoracolumbar rotation of 20.0° ± 7.7°.

The means and standard deviations of thoracolumbar rotation for all movements are shown in Table 2. In total, 18 out of 27 movements have a range of motion larger than 20°: White Crane Spreads Its Wings, Brush Knee and Step Forward (right), Step Back and Repulse Monkey (left), Step Back and Repulse Monkey (right), Grasp the Sparrow‘s Tail (left), Grasp the Sparrow‘s Tail (right), Wave Hand in the Clouds, Strike to Ears with Both Fists, Left Heel Kick, Lower Body and Stand on One Leg (left), Lower Body and Stand on One Leg (right), and Seal Tightly. Five of them have a range of motion at least twice as large as 20° (i.e. > 40°): Brush Knee and Step Forward (left), High Pat on Horse, Fair Lady Weaves with Shuttle (right), Fair Lady Weaves with Shuttle (left), and Pick Up the Needle from the Sea Bottom.

Seven movements of the Peking style routine are performed on both sides: Part Wild Horse's Mane, Brush Knee and Step Forward, Step Back and Repulse Monkey, Grasp the Sparrow's Tail, Heel Kick, Lower Body and Stand on One Leg, and Fair Lady Weaves with Shuttle. No significant lateral differences (between the left and right sides) were observed for any of these movements (Table 3), with the exception of Brush Knee and Step Forward (difference: 20.1° ± 4.8°) and Fair Lady Weaves with Shuttle (difference: 8.4° ± 5.6°).

The means and standard deviations of thoracolumbar rotation for all movements are shown in Table 2. In total, 5 of 27 movements have a range of motion lower than 20°: Beginning, Fan Back, Turn Body, Deflect, Parry, and Punch, Cross Hands, and Closing. Ten movements have a range of motion larger than 40°: Brush Knee and Step Forward (left), Brush Knee and Step Forward (right), Grasp the Sparrow‘s Tail (left), Grasp the Sparrow‘s Tail (right), Wave Hand in the Clouds, High Pat on Horse, Strike to Ears with Both Fists, Fair Lady Weaves with Shuttle (right), Fair Lady Weaves with Shuttle (left), and Pick Up the Needle from the Sea Bottom.

No significant lateral differences were detected for any of these movements (Table 4), except for Fair Lady Weaves with Shuttle (difference: 13.0° ± 4.8°).

Comparison of the two versions revealed a significant difference in 16 of the 27 movements (Table 2). However, the differences in the range of motion for most of the movements relative to normal thoracolumbar rotation were relatively small. The largest differences ranged between 10.1° [Pick Up the Needle from the Sea Bottom) and 19.7° (Fair Lady Weaves with Shuttle (left)], for the following eight movements: White Crane Spreads Its Wings, Brush Knee and Step Forward (right), Grasp the Sparrow‘s Tail (left), Grasp the Sparrow‘s Tail (right), Strike to Ears with Both Fists, Fair Lady Weaves with Shuttle (right), Fair Lady Weaves with Shuttle (left), and Pick Up the Needle from the Sea Bottom.

Five movements in the HS and 10 in the CV have a range of motion exceeding 40°. These five movements in the HS, which are also in the CV, comprise Brush Knee and Step Forward (left), High Pat on Horse, Fair Lady Weaves with Shuttle (right), Fair Lady Weaves with Shuttle (left), and Pick Up the Needle from the Sea Bottom. The additional five movements in the CV are Brush Knee and Step Forward (right), Grasp the Sparrow‘s Tail (left), Grasp the Sparrow‘s Tail (right), Wave Hand in the Clouds, and Strike to Ears with Both Fists. Comparing the HS and CV, eight movements were found with notable differences between 10.1° and 19.7°. For the remaining movements, only small differences were observed. The seven movements performed on both sides showed no significant lateral differences in either the HS or CV, except for Brush Knee and Step Forward and Fair Lady Weaves with Shuttle.

According to Aumüller et al. (2010), the maximum thoracolumbar range of motion is about 100°. Therefore, the following degrees of rotation correspond to percentage values. Thirteen movements of the HS are performed with a rotation between 20° and 40°, and five movements range between 40° and 55°. For the CV, most movements show a rotation above 20°, and 10 movements even exceed 40°. However, this version is typically used by athletes only.

In the study of Schenkman et al. (1997), which examined thoracolumbar rotation in four subgroups (healthy young, healthy elders, functionally limited elders, and individuals with Parkinson's disease), the largest thoracolumbar rotation measured in the functionally limited elders (49.6°) and Parkinson's disease (55.5°) subgroups was similar to the range of motion values for the five HS movements mentioned above (ranging from 44.1° to 55.0°). For the healthy young and healthy elders subgroups, the differences from the measured thoracolumbar rotation (87.6° and 70.8°, respectively) were larger, but the movements reached about 50% of the maximum rotation. Based on these values, the following five single movements, which are all performed on both sides, can be used for improving thoracolumbar rotation in cases where single movements are practiced instead of the entire Peking style routine: Brush Knee and Step Forward, Step Back and Repulse Monkey, Grasp the Sparrow‘s Tail, Wave Hand in the Clouds, and Fair Lady Weaves with Shuttle. There are three movements with considerable thoracolumbar rotation, but they have distinct disadvantages that may make them unsuitable: High Pat on Horse can hardly be practiced as a single movement, and Pick Up the Needle from the Sea Bottom and Lower Body and Stand in One Leg may be difficult to perform for individuals with limited mobility.

Wen et al. (2018) reported 58.9% greater knee flexion range of motion and a 40.2% higher peek knee extension moment when performing the bow stance in a lower rather than higher position. Since the upper limb and upper body movements are proportional to the movements of the lower limbs, we expected a large difference in the range of motion of thoracolumbar rotation between the HS and CV; surprisingly, however, the difference was not very large. Although there was a significant difference for 16 movements, half of them showed only minor numerical differences (in the single-digit range), which can be considered negligible with respect to a the maximal thoracolumbar rotation of 100° (Aumüller et al., 2010). It can be concluded that Tai Chi movements have a considerable effect on thoracolumbar rotation, even if the movements are performed with shorter and higher positions.

In two of the seven movements of the Peking style routine performed on both sides, a lateral difference (between left and right) was observed: Brush Knee and Step Forward and Fair Lady Weaves with Shuttle. In the first case, the difference can be explained by the fact that the movement started from the end of the previous movement, White Crane Spreads Its Wings, necessitating a rotational movement. In the second case, the opposite occurred: the necessary rotation starting from the end of the previous movement, Stand on One Leg (right), up to Fair Lady Weaves with Shuttle (right) was smaller than the rotation needed between Fair Lady Weaves with Shuttle (right) and Fair Lady Weaves with Shuttle (left). If this is taken in consideration, the lateral difference for these two movements disappears.

This biomechanical study is the first to investigate the biomechanical characteristics of all 24 movements of the complete Peking style routine. Therefore, the main strength of this study is that all common Tai Chi movements were analyzed in terms of thoracolumbar rotation.

All movements were performed by athletes with several years of experience in the practice of Tai Chi, and thus with high proficiency. This strength, however, could also constitute a limitation, because the athletes are not used to performing movements with shorter and higher stances, as required for the HS. Thus, a third version with even shorter and higher stances, as performed by unfit older people and individuals with chronic diseases, was not conducted. It must also be taken into account that the study included a very small group of athletes.

A further potential limitation is the fact that the Peking style routine had to be separated into single movements, because only two force plates were available. However, the movements were executed highly reliably, and in a very similar manner to the sequence of the Peking style routine.

Another important aspect to consider is that this is solely a biomechanical study, investigating whether the range of motion of thoracolumbar rotation during Tai Chi movements is large enough at all to have the potential to improve the spine flexibility. Due to the nature of the study factors such as training duration and training frequency that are relevant for improving physical fitness as they are considered in intervention studies, were not taken into account. Therefore based on this study, further intervention studies need to be conducted to directly investigate the potential of Tai Chi practice to increase spinal flexibility.