Vertical Breast Displacement in Asian Women During Exercise: influence of Bra Type, Size and Different Parts of the Breast

All of the participants chosen in this research had a relatively small breast size (lower than D cup according to the Chinese bra sizing system), because it was reported that Asian women tended to have smaller breasts with a higher density [24, 25, 26]. Seven healthy active women (mean ± SD (standard deviation)): age = 20±1 year; breast circumference = 86±3 cm; upper-breast circumference = 86±3 cm; under-breast circumference = 73±2 cm; band size = 72±3 cm) who had no history of pregnancy, breast cancer or breast surgery were recruited for this study. The specific measurements of the participants are shown in Table 1. According to the professional bra fitting criteria, each participant was fitted by an experienced bra fit assessor. Two of the participants were cup A (breast circumference minus under-breast circumference was 10–12 cm), three were cup B (breast circumference minus under-breast circumference was 12–14 cm), and the remainnig two were cup C (breast circumference minus under-breast circumference was 14–16 cm).

A sports bra (NIKE^TM, China) (Figure 1) designed for medium intensity exercise was chosen for this experiment. Two sizes of this style of bra, M and L, were used for the trials. The cups of the sports bra were made from recycled polyester and spandex blend fabric (88%/12%). The mesh fabric was made from polyester and spandex blend fabric (80%/20%). Both the front and back sides of the liner were made from polyester (100%).

Fig. 1

The structure of the everyday bra chosen for the experiment (COSMO LADY, China), is shown in Figure 2. The cup lining is made from polyester and cotton blend fabric (65%/35%). The wing lining is made from polyamide and spandex blend fabric (84%/16%).

Fig. 2

This experiment was conducted in the human motion and ergonomics laboratory of the institute. Three-dimensional motion capture systems (NDI Optotrak Investigator) with three infrared cameras (200 Hz) were employed to track the motion of the active tracking markers attached to the body and bra. The whole testing process included the following steps: warm-up, marker attachment and data collection. The origin of the laboratory coordinate system was set in the right rear of the treadmill, which was established with the X-axis positive forward, the Y-axis positive to the left, and the Z-axis positive upward (Figure 3).

Fig. 3

2.3.2

Marker attachment

Assuming breast motion is symmetrical, only left breast motion data were collected. Participants were asked to take off their own bra and put on the experimental bra in the changing room. To quantify breast motion relative to trunk motion, six infrared LED (light emitting diode) markers (20 mm diameter and 3.1 grams each) were attached to predetermined positions on the body with medical tape (Cofoe, China) and on bra points with double-sided tape (Huajiu, China), as shown in Figure 4.

Fig. 4

The suprasternal notch was used to eliminate trunk motion with six degrees of freedom. The trunk angles (lean) during the four exercises were ignored because they were relatively small in the vertical direction, compared to the horizontal direction [1]. Only the vertical breast displacement was analysed in this paper, because Scurr et al. reported that vertical breast displacement dominates in three-dimensional displacement during high speed running (above 8 km/h) [30]. Vertical breast displacement was calculated via vertical relative breast coordinates, which were the difference between the Z coordinates of the left breast nipple and those of the suprasternal notch. In each gait cycle, minima vertical relative breast coordinates were subtracted from maxima vertical relative breast coordinates to calculate vertical breast displacement in each trial. The mean and SD values were calculated for each of the four exercise modalities in the two bra conditions (everyday bra and sports bra).

The normality of all data on vertical breast displacement was checked using the Shapiro-Wilk test. A one-way analysis of variance (ANOVA) design with the breast part (five marked points) as the independent variable was used to find whether there was a significant difference in vertical displacement among the different parts of the breast. To determine whether any significant difference exists, Bonferroni post hoc analyses were conducted. A multi-way ANOVA design with three within factors (exercise modality, bra type and breast size) was then conducted to reveal whether there was a main effect on or interactions with vertical breast displacement. Statistical analyses were conducted using the Statistical Package for the Social Sciences (Version 25.0; SPSS Inc., Chicago, IL). The significance level was set at 0.05.

It was found that the vertical movement trajectories of the five points followed a very similar path in the two supported conditions. The vertical displacement of the 5 markers are shown in Figure 5. The mean vertical displacement of the nipple was greatest in each bra condition (sports bra: 14.0 mm; everyday bra: 19.6 mm). The least mean vertical displacement occurred at marker 3 (M3) whilst wearing the everyday bra, whereas this occurred at M4 whilst wearing the sports bra. Nevertheless, the results of the one-way ANOVA showed no significant difference in the displacement of the five marked points (p > 0.05).

Fig. 5

As no significant difference was found among the five markers, the vertical displacement of the nipple was used to represent the vertical displacement of the whole breast. The mean (±SD) of vertical breast displacement was calculated for each of the four exercise modalities in the two bra conditions (Figure 6). There was a dramatic increase in vertical breast displacement from walking (ranging from 4.0 mm to 13.4 mm) to jogging (ranging from 6.6 mm to 35.1 mm). The statistical results showed no significant interaction effect on vertical breast displacement. However, the results of multi-way ANOVA showed a significant effect of exercise modality on vertical breast displacement (p<0.05). Bonferroni post-hoc analyses revealed significant differences in vertical breast displacement between walking and each of the other three exercise modalities (p< 0.05). Nevertheless, no significant difference was found in vertical breast displacement between running and jogging, running and rope skipping, and jogging and rope skipping.

Fig. 6

Vertical breast displacement ranged from 7.4 mm to 25.7 mm in the everyday bra condition, and ranged from 6.5 mm to 18.8 mm in the sports bra condition. The results of multi-way ANOVA in each exercise modality revealed a significant effect of bra type on vertical breast displacement (p < 0.05). In other words, the vertical breast displacement while the participants were wearing the everyday bra was significantly greater than when wearing the sports bra.

The breast size of the participants varied from cup A to cup C, which was relatively small. The results of the multi-factor ANOVA showed that breast size had no significant effect on vertical displacement in each condition (p > 0.05).

The results of this study showed no significant difference among different parts of the breast, which means that the second hypothesis of this study is rejected. What was found in this study confirmed the results of Mason et al.[21], but was inconsistent with those reported by Arch et al.[22] Arch found that the average maximum marker displacement was greater than that of the marker closest to the nipple, and maximum marker displacement values were located either above or below the nipple medially. This inconformity could be caused by the differences in the bras used in the two studies (the seamless-style bra used in Arch et al.'s study versus the encapsulation bra used in this one), or by the differences in breast density because of participants’ ethnicity in the two studies (Caucasian women in Arch et al's study versus Asian women in this one). The finding of no significant difference among breast parts was also in contrast to Lu et al.'s research [8], because the maximum of vertical breast displacement occurred at a point 4 cm directly above the nipple in Lu's research, which might be caused by the different local coordinate system used in the measurement process. This suggests that conclusions drawn under the different coordinate systems may be different.

The results showed a significant difference in vertical breast displacement between rope skipping and walking, partially accepting the first hypothesis. Besides, there were significant differences in vertical breast displacement between walking and jogging, and walking and running, confirming the findings by Mason et al. and Scurr et al.'s research [9, 21, 30]. These findings may be explained by the different levels of intensity of the four modalities. The intensity of walking is low, while that of running and rope skipping are both high. When using trunk movement to represent the intensity of exercise, significant differences in vertical trunk movements were found among walking, jogging/running and rope skipping (Figure 7). It is worth noting that the small difference in the degree of trunk forward lean during the four exercises was ignored in this study.

Fig. 7

However, no significant difference was found between rope skipping, jogging and running. Namely, when the intensity of trunk motion was higher than jogging, the vertical breast displacement did not increase to comply with the intensity. According to Scurr et al.'s research [30], the vertical breast displacement remained similar when the speed of the running tasks was above 8 km/h in both bra and no bra conditions, which was consistent with this study. Ligamentous structures in the breast limit the motion to some extent, resulting in breast displacement peaking at a certain state instead of continually rising with the landing force. In this study, breast displacement might peak in the jogging condition.

It was also suggested that results of breast motion research analysed in treadmill running conditions (at the speed of 7.5~10 km/h) might be applicable for rope skipping, as no significant difference in vertical breast displacement was found between these two exercise modalities. This finding helps to inform the requirements for breast support garments to reduce breast pain during rope skipping.

The results showed that regardless of the modality of exercise used, vertical breast displacement in the sports bra condition was significantly decreased compared to the everyday bra condition. That is, in terms of reducing vertical breast displacement, sports bras are more effective than everyday ones. This finding is consistent with previous studies [31, 32]. According to Lu et al.'s research, the tension caused by stretched fabrics and the compressed cup pad made the sports bra more advantageous [8]. During exercise, the kinetic energy of the breast was converted into the tensile energy of the fabric and pad material, along with the compression energy of the pad material [8]. This might be the mechanism by which vertical breast displacement was reduced when wearing a sports bra. The findings also confirmed the notion that wearing a sports bra during exercise is essential for women, as it could reduce breast displacement, and thus relieve the discomfort caused by excessive displacement[2, 21, 29].

The result of the insignificant effect of breast size on vertical breast displacement was in contrast to the third hypothesis, and inconsistent with Boschma et al. and Lorentzen et al., who believed that breast displacement increases as along with the cup size [2, 33]. Although increased breast size results in increased breast mass, breast displacement does not change significantly as breast mass increases if breast density is at a high level [18]. Since participants in this study were Asian women, they have high breast density, which may be the cause of the result. As stiffness is the most important parameter for minimising breast displacement [18], this result may also be explained by the high breast stiffness in the participants. It was reported that Asian women have high density breasts which contain a high percentage of glands inside, leading to high breast stiffness [3, 34]. This finding means that Asian women with small breasts require a similar level of breast support during exercise, irrespective of breast size.

A limitation of this study is that we only compared the vertical breast displacement, lacking comparisons of the velocity and acceleration of breast movement. Previous research has identified those factors influencing breast discomfort, including speed/acceleration of movement during exercise and ground reaction forces generated by different breast support conditions [19, 35, 36]. Future investigations might consider comparing these three factors between rope skipping and other exercise modalities under different breast support conditions. Besides, a limitation exists in the handling of data. The marked points representing breast motion were not set directly on the breast, but on the surface of the bra. If the bra has significant mass, there might be a relative displacement between the bra and the breasts during exercise, and therefore the results may be skewed. Moreover, it was also recognised that this study was restricted by the small number of participants, which might have led to less representation or larger errors, such as large standard deviation in the mean vertical breast displacements of the five markers. The small number of participants might also have caused the results obtained to be burdened with a high risk of making a mistake when assessing null statistical hypotheses.

In spite of these limitations, up to now, this is the first research to have explore vertical breast displacement during rope skipping, and the first to compare vertical breast displacement during rope skipping with that during walking, jogging and running. The data and findings obtained in this research are therefore essential for the design of special sports bras for rope skipping, which is hopeful for reducing excessive breast displacement and thus alleviating breast discomfort.