OBJECTIVE: Therapeutic ultrasound (TUS) is the most common physiotherapy treatment for inflammatory conditions of the lactating breast. However, effective parameters for treatment are unknown, and based on musculoskeletal evidence. This study's aims were to determine the difference in heat perception (using TUS) between lactating breast and calf muscle tissue and the range of intensities required for heating perception in healthy lactating women. METHOD: This repeated measures study recruited lactating mothers who exclusively breastfed infants aged 6 months or younger who responded to social media posts and flyers placed in medical offices. TUS was sequentially applied to the breast and calf, starting from 1Wcm RESULTS: Fifty mothers (mean age, BMI = 31.6 years, 26.5), first perceived warmth at lower intensities in the breast (Z = -3.637,p <
0.002), but there was no difference between locations for most tolerable warmth (Z = -1.165,p = 0.244). Factors associated with higher perception of first warmth were antidepressant use (β = 0.369[95 %CI:0.103-0.635],p = 0.007) and calf location (β = 0.286[95 %CI:0.055-0.516],p = 0.015). Higher body mass index was the only factor associated with higher perception of tolerable warmth (β = 0.024[95 %CI:0.004-0.044],p = 0.017). The range of intensities required to perceive heating in the breast were 1-2.5Wcm CONCLUSIONS: As perception of breast and calf warmth differs, TUS parameters used to treat musculoskeletal conditions may not be appropriate for inflammatory conditions of the lactating breast. Additional studies are needed to determine the tolerability and effectiveness of TUS using different intensities when treating women with ICLB.