How can heart rate be measured manually?

Measuring heart rate manually is done by assessing the pulse, specifically through the radial artery located at the wrist or the carotid artery at the neck. This method allows for a direct measure of the heart's pumping action as it sends blood through the arteries, creating a palpable rhythm. By pressing gently on these arteries, one can feel the pulse and count the beats over a specific time, usually for 15 seconds and then multiplying that number by 4 to get beats per minute.

Using this technique is effective because it gives an immediate and accurate measure of the heart rate, reflecting the body's demand for oxygen during different levels of activity. The other options do not provide a reliable method for measuring heart rate. Counting breaths relates to respiratory rate, feeling for muscle tension does not correlate with heart rate, and monitoring visual cues might give insights into exertion but doesn't provide a quantifiable heart rate.