A recent study has shown that cardiac output is increased during mild dynamic exercise that involves lifting and extending the leg. 

Researchers had participants perform two minutes of dynamic leg exercise that involved alternating contracting and relaxing the quadriceps muscle for two seconds. As the quadriceps were contracted, the knee was extended and the heel lifted three to five centimeters off the ground. 

During the exercise, one leg was contracted while the other was relaxed. 

Bilateral weights (meaning equal on both sides) of two to five and a half kilograms were added to the ankles to increase muscular work. Throughout the leg exercise, heart rate was recorded and stroke volume was measured using Doppler ultrasound. 

In the study, heart rate increased by approximately forty percent, and stroke volume decreased by about five percent. What that means is, cardiac output increased by about thirty five percent, showing that mild dynamic exercise with a light resistance caused a small increase in cardiac output resulting from the decrease in stroke volume that was more than offset by the increase in heart rate. 

Miles and colleagues reported that stroke volume deceased by twenty percent during leg extension exercises that involved twelve repetitions to fatigue. In this study, the leg extension included a three second lifting motion, a one second pause, and a three second lowering motion. The stroke volume was assessed here using impedance cardiography, which is a noninvasive technology measuring total electrical conductivity of the thorax and its changes . Heart rate increased approximately fifty beats a minute, and cardiac output increased by approximately seventeen percent. 

Cardiac output responses to more intense training have been reported by Lentini and colleagues, who had healthy male subjects perform a double leg press to failure and ninety-five percent of their maximum strength. Stroke volume was determined using echocardiography, the use of an ultrasound to measure heart rate and stroke, during the lock phase and the lowering phase of the lift. Cardiac output increased significantly during the lifting phase, and increased further during the lockout phase. 

The increase in cardiac output, however, is modest in these workouts compared to an aerobic exercise. So while resistance exercise certainly will increase your cardiac output by a surprising amount,  it's not quite enough to excuse yourself from the treadmill or exercise bike entirely. 

At the end of the day, the quickest and cheapest ways to make sure you have better heart health will almost always be from either a brisk jog or bike ride.