FIG. 1 is a top perspective view of a heart rate monitor showing my new design;

FIG. 2 is a top view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a front view thereof; and,

FIG. 6 is a back view thereof.

The bottom which is not shown in the drawings forms no part of the claimed design. The subject matter shown in broken lines also forms no part of the claimed design.