“Stronger” hearts typically have a resting pump rate lower than that of weaker or less healthy ones. A healthy, athletic male might have a resting BPM of 60, while an otherwise healthy but post-partem female could be closer to 90.

Would both of these hearts expend the same energy pumping 120 BPM? Would the healthier heart be theoretically expending more as it is acting in double-time, or would the weaker one be working harder as it is already inefficient at pumping blood?