Bra–breast forces generated in women with large breasts while standing and during treadmill running: Implications for sports bra design
Highlights
► Breast movement and discomfort are barriers to exercise for women with large breasts. ► Women with large breasts have a wide range of breast volumes (283–1345 ml). ► High support sports bras significantly decrease bra–breast force. ► Breast mass and force are important biomechanical considerations for bra design.
Introduction
Large breasts can contribute to numerous negative health outcomes in women, including upper limb, neck, back and head pain (Greenbaum et al., 2003; Kaye, 1972; Ryan, 2000; Wood et al., 2008). These problems have been found to be severe enough to force women with large breasts to seek reduction mammaplasty (Greenbaum et al., 2003; Kaye, 1972; Maha, 2000; Ryan, 2000; Wilson and Sellwood, 1976), as well as limit their ability to participate in physical activity (Bowles et al., 2008; Gehlsen and Albohm, 1980; Lorentzen and Lawson, 1987; Mason et al., 1999). Inhibiting physical activity can perpetuate a reverberating cycle of these problems, as reduced energy expenditure associated with decreased physical activity can contribute to weight gain and, in turn, increased breast mass. Interestingly, bra manufacturers report that the size of women's breasts is increasing over time, with the average bra size in the UK and Australia reported to have increased from a 12B to a 14C (Mintel Group, 2001; www.smh.com.au/lifestyle/life/boom--bust-20100719-10gx9.html, accessed February 2012) and bra sales of D to G cup sizes in the UK reported to have risen by 50% year-on-year since 2005 (www.guardian.co.uk/lifeandstyle/2010/may/16/womens-breasts-are-getting-bigger, accessed February 2012).
Bras act as external supports for the breasts. Therefore, the breast mass that bras are required to support, particularly for women with large breasts during physical activity, is an important consideration for bra design. Despite the relevance of breast mass to bra design and the musculoskeletal problems experienced by women with large breasts, the relationship between breast mass and bra size and the bra–breast forces generated in women with large breasts during both upright standing and physical activity are yet to be quantified.
Previous research relating to sports bra design has predominantly been limited to kinematic data, which has shown vertical breast displacement during physical activity to be associated with exercise-induced breast discomfort (Gehlsen and Albohm, 1980; Lorentzen and Lawson, 1987; Mason et al., 1999; Starr et al., 2005). That is, the greater the vertical breast movement, the greater the breast discomfort (Gehlsen and Albohm, 1980; Lorentzen and Lawson, 1987; Mason et al., 1999; Starr et al., 2005). Although sports bras have been found to be better than fashion bras in reducing both vertical breast displacement and exercise-induced breast discomfort (Gehlsen and Albohm, 1980; Lorentzen and Lawson, 1987; Starr et al., 2005), research suggests that only 41% of women (Bowles et al., 2008) and 13% of adolescent females (McGhee et al., 2010) wear a sports bras during exercise. Vertical breast displacement has also been found to increase with increments in bra size (Lawson and Lorentzen, 1990). Therefore, breast mass, and in turn breast kinetics, may be important considerations not only for women with large breasts to appreciate when purchasing a supportive bra, but may also offer a scientific basis upon which to improve current bra designs. Improved breast support could assist to increase the comfort of women with large breasts when they exercise and, in turn, promote their involvement in physical activity.
Two published studies were found that previously investigated breast kinetics (Gefen and Dilmoney, 2007; Haake and Scurr, 2010), although neither directly measured the two components underlying breast force (mass and acceleration) in women with large breasts. Gefen and Dilmoney (2007) derived rather than directly measured both components, whereby breast mass values were assumed and breast acceleration data were taken from a previous study (Mason et al., 1999). However, this previous study had limitations of low participant numbers (n = 3), involved only young participants who had small breasts, and derived breast acceleration data from breast displacement data, which had been manually digitised for only two running strides, performed for only 5–7 s (Mason et al., 1999). In their dynamic model of the breast during exercise, Haake and Scurr (2010) used breast mass data from a previous study and, although they directly measured breast acceleration, this was only for one participant, who did not have large breasts.
During exercise such as treadmill running, the breasts and trunk have been described as moving in somewhat of a sinusoidal pattern (Haake and Scurr, 2010), with a time delay between breast movement and trunk movement (Haake and Scurr, 2010). The lowest point of downward vertical trunk displacement during treadmill running coincides with heel strike (McGhee and Steele, 2006; Haake and Scurr, 2010), whereby the trunk stops moving vertically downward suddenly once the heel strikes the ground. The lowest point of vertical breast displacement occurs a short time later and results in a “slap” of the breast against the anterior thorax (Haycock, 1987), as the breast is descending while the trunk is ascending. The jarring effect of this movement of the breasts against the trunk and the net forces generated at this time has been associated with exercise-induced breast discomfort, particularly in women with large breasts.
The net forces associated with breast movement during physical activity include the force of gravity (mg) acting on the breasts and the driving force of the trunk, which are restrained by the stiffening and dampening forces of the anatomical restraints of the breasts and the bra. The vertical component of the bra–breast force (breast mass × breast acceleration) therefore equals the sum of the driving forces (gravity and driving force of the trunk) and the restraining forces (anatomical restraints of the breasts and the bra) (Haake and Scurr, 2010). The anatomical restraints of the breast include the overlying skin and fine bands of fibrous tissue within the breast, which are connected to the pectoral fascia (Cooper's Ligaments) (Mason et al., 1999). The net force generated during physical activity is consequently moderated by both breast mass and breast acceleration. The greater the breast mass, the greater the net force; therefore, women with large breasts will generate greater forces than their smaller breasted counterparts. The limited published breast acceleration data has shown breast acceleration to be moderated by both the level of external breast support provided by a bra (high versus low) and the type of physical activity (walking versus jogging and running) (Mason et al., 1999). When adopting a static upright posture, the net bra–breast force creates an anterior torque on the thorax (Greenbaum et al., 2003), which is also greater in women with large breasts compared to women with small breasts.
Both the range and magnitude of the bra–breast force in women with large breasts is essential information for bra design to ensure that bras are designed to cater for the loads generated during both upright standing and physical activity. However, neither the range nor magnitude of the bra–breast forces generated in upright standing or during physical activity have previously been quantified in different levels of breast support commonly worn by women with large breasts. Therefore, the aim of this study was to determine the bra–breast force generated in women with large breasts across a range of bra sizes, during static upright standing and treadmill running, while these women wore different levels of breast support. It was hypothesised that lower bra–breast forces would be associated with higher levels of breast support.
Section snippets
Participants
Fifteen women (mean age = 31 years; range 19–44 years) who were professionally sized (McGhee and Steele, 2010) to wear a D+ bra cup, were recruited as representative of women with large breasts. The participant number provided an 85% power to detect a delta score of 6.12 with a significance level of 0.05 (two tailed) based on the maximum standard deviation (SD) of the vertical component of the bra–breast force during the downward breast movement during treadmill running for each condition (high
Breast volume and mass
The mean breast volume of the participants' left and right breasts was 757 ml and 765 ml (range 283–1345 ml (size 14D to 12G)). This equated to a mean breast mass of 591 g and 598 g for the left and right breast, respectively. Breast volumes and masses, and their corresponding bra sizes in the one style and make of bra (New Legend sports bra, Berlei, Pacific Brands, Victoria, Australia), are presented in Table 1.
Vertical breast displacement and acceleration
Vertical breast displacement and acceleration data from which the left net
Breast volume and mass
The range of breast volumes of the participants was consistent with previous studies (Loughry et al., 1987; Nahabedian and Galdino, 2003; Sigurdson and Kirkland, 2006; Smith et al., 1986). Furthermore, the breast volumes in relation to bra size and isolated cup size were also consistent with two of the three previous studies that have reported the relationship between these variables (Sigurdson and Kirkland, 2006; Smith et al., 1986), both of which measured breast volume by similar methods
Conclusion
The wide range of breast masses of women who were all considered to be large breasted, and the magnitude of the bra–breast force during upright posture and during physical activity, particularly during the downward phase of the breast cycle, are important biomechanical considerations for bra design and bra choice, to limit force generation and breast discomfort. This wide range, together with the positive correlation between breast mass and vertical breast displacement, suggests that adequate
Author disclosure statement
No competing financial interests exist for DE McGhee, JR Steele, WJ Zealey or G Takacs.
Acknowledgements
The authors thank Steve Cooper from the Science Workshop, University of Wollongong, for constructing the breast volume measurement system. This research was funded by the University of Wollongong, Faculty of Health & Behavioural Sciences Early Research Career Grant and the New South Wales Sporting Injury Committee.
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