Garmin HRM-FIT Owner's Manual page 8

Band and strap width
The width of the sports bra band under the bust is approximately
from 19 to 50 mm (from 0.75 to 2 in.). A long band may position
the heart rate monitor too low on the torso for accurate heart
rate. A narrow band may not have enough material to properly
clip onto the heart rate monitor.
Zipper
The sports bra does not have a zipper or other embellishment in
the front that might interfere with the clips.
4 Heart Rate Monitor Instructions