Garmin Vivoactive 3 Faq page 8

Certain Types of Activities Can Affect Heart Rate
Performance
Activities that cause flexing of the wrist (ex. rowing, weight lifting, or gripping a tennis racket)
may create an effect similar to wearing the watch too tightly and restrict blood flow. When this
occurs the watch may have a difficult time detecting an accurate heart rate.
Skin Tone and Tattoos Can Affect Heart Rate Accuracy
Skin tone has an effect as the melanin in the skin absorbs some of the light. Garmin designs our
watches to work on all skin tones.