Breast-torso coordination during treadmill running as a function of breast support

Background: A time-lag between the breast and torso during running has been shown to be one of the variables most effected by breast support [1] and can result in a “breast slap”, which may be related to breast pain [2]. Formal methods of calculating relative phase between the continuous motion of oscillators are well-established and may offer deeper insight of breast-torso coordination than discrete time lag. Vector coding is one such method that has the advantage of being inherently interpretable [3].

Aim: To quantify breast-torso coordination in running in three different breast support conditions.

Methods: Twelve healthy female participants with a breast size of UK 34D (age=23±5 years, height=1.70±0.04 m, mass=70±6 kg, mean±SD) ran on a treadmill at 10 km.hr-1. Nipple and torso positional data were recorded for 10 seconds at 240 Hz using electromagnetic sensors (Micro Sensor 1.8™, Liberty, Polhemus, USA). Participants ran with Lululemon® Enlite (encapsulation style) and Nike™ Medium (compression style) sports bras in a random order and then with no bra. Data were filtered using a Lowpass Butterworth filter at 13 Hz. Vector coding analysis was performed on data divided into cycles. Coordination patterns were categorised into bins of breast dominancy (67.5° ≤ µ < 112.5°, 247.5° ≤ µ < 292.5°), torso dominancy (0° ≤ µ < 22.5°, 157.5° ≤ µ < 202.5°, 337.51° ≤ µ < 360°), in-phase (22.5° ≤ µ < 67.5°, 202.5° ≤ µ < 247.5°) and anti-phase (112.5° ≤ µ < 157.5°, 292.5° ≤ µ < 337.5°) [4]. Breast support conditions were compared using one-way repeated measures ANOVA (α = 0.05) for each bin.

Results: Figure 1. shows greatest difference in mean phase angle between no bra and all sports bra conditions occurred at ~40%-70% and 90% of the gait cycle. There was a main effect of bra support on the coordination pattern frequencies for each phase bin (p<0.001). The breast and torso were significantly more in-phase in both sports bras compared to no bra and in the encapsulation style bra vs. the compression style bra.

Conclusions: There was an effect of breast support on phase angle across the gait cycle, advocating the value of examining continuous breast-torso coordination rather than at only discrete time points. In both sports bras there was a transition after 40% and 90% of the gait cycle, at approximately take-off with each foot, to another state of in-phase coordination in the flight phase. Whereas the breast and torso were barely in-phase during the flight phase of the gait cycle with no bra.