The Design consists of the features of shape, ornamentation and configuration, of the LOW-PROFILE FITNESS MONITORING CAPSULE shown in the drawings. Portions shown in stippled lines form no part of the design. Drawings of the Design are included, in which: Figure 1 is an isometric view of the first variant of the low-profile fitness monitoring capsule. Figure 2 is a front view of the first variant of the low-profile fitness monitoring capsule. Figure 3 is a back view of the first variant of the low-profile fitness monitoring capsule. Figure 4 is a top view of the first variant of the low-profile fitness monitoring capsule. Figure 5 is a right side view of the first variant of the low-profile fitness monitoring capsule. Figure 6 is a left side view of the first variant of the low-profile fitness monitoring capsule. Figure 7 is a bottom view of the first variant of the low-profile fitness monitoring capsule. Figure 8 is a first oblique bottom view of the first variant of the low-profile fitness monitoring capsule. Figure 9 is a second oblique bottom view of the first variant of the low-profile fitness monitoring capsule. Figure 10 is a third oblique bottom view of the first variant of the low-profile fitness monitoring. Figure 11 is a bottom view of the first variant of the low-profile fitness monitoring capsule showing a section line for Figure 12. Figure 12 is a cross-sectional view of the first variant of the low-profile fitness monitoring capsule of Figure 11 taken along line 12-12 of Figure 11. Figure 13 is an isometric view of the second variant of the low-profile fitness monitoring capsule. Figure 14 is a front view of the second variant of the low-profile fitness monitoring capsule. Figure 15 is a back view of the second variant of the low-profile fitness monitoring capsule. Figure 16 is a top view of the second variant of the low-profile fitness monitoring capsule. Figure 17 is a right side view of the second variant of the low-profile fitness monitoring capsule. Figure 18 is a left side view of the second variant of the low-profile fitness monitoring capsule. Figure 19 is a bottom view of the second variant of the low-profile fitness monitoring capsule. Figure 20 is an oblique bottom view of the second variant of the low-profile fitness monitoring capsule of Figure 13. Figure 21 is a top view of the second variant of the low-profile fitness monitoring capsule showing a section line for Figure 22. Figure 22 is a cross-sectional view of the second variant of the low-profile fitness monitoring capsule of Figure 21 taken along line 22-22 of Figure 21.