Cardiac output is the product of stroke volume and heart rate. We have seen that with training, stroke volume increases, and heart rate decreases. As a result, during rest and submaximal exercise, cardiac output remains relatively unchanged following endurance training. However, at maximal exercise, cardiac output increases quite significantly following training.
This is because training causes a significant increase in maximal stroke volume, and maximal heart rate remains unchanged (220-age in years), so as a result, maximal cardiac output is increased.