Leo Rogier Verberne

Comparing two types of bicycles takes some doing

1. Heart rate

Bicycling is healthy. But the massive spills that occur during road races, sometimes resulting in permanent disability or even death, render the sport dangerous, even more dangerous than skiing. Accidents during road races occur so frequently that professional racers feel it is ‘all part of the game.’ In terms of health, it matters whether you ride a recumbent or a regular racing bicycle: the workload of the heart and the pressure on sexual organs differ per type of bicycle.

Residual volume
The amount of blood that is pumped into the vessels when the heart contracts is called the stroke volume. Which is approx. 70 ml in a healthy, untrained male adult with an average height and build, when he is at rest (1). However, at the end of a contraction, the heart chamber is not empty, a residual volume of 40 to 50 ml stays behind. So at the beginning of the contraction the heart chamber contained 110 to 120 ml (70 + 40/50). The heart frequency of an average male at rest is approx. 70 strokes per minute (1). Which means that his heart pumps around ± 4.9 liters of blood (70×70 ml) into the vessels every minute. This is called the heart minute volume or cardiac output. That cardiac output (and not the heart frequency) determines the endurance-performance that you can achieve.

Regular racing bicycle
The stroke volume of the heart and the heart frequency both increase during physical exertion. Should this adult male proceed to ride a regular racing bicycle at a speed of, for example, 20 km/hour, then the stroke volume of his heart could double from 70 to 140 ml. The extent to which the heart is filled at the beginning of the contraction could increase during his efforts from 120 ml (at rest) to 160 ml. The heart muscle contracts more vigorously as a result of the heart chamber being fuller. That makes the residual volume smaller. For example 20 ml instead of 40 ml at rest. When exerting oneself physically, the heart frequency too increases, from 70 strokes per minute (at rest) to, for example, 140 during cycling. Due to the combination of a larger stroke volume and increased heart frequency, the cardiac output increases from 4.9 to 19.6 liters (140×140 ml). And so, as a result of riding a regular racing bicycle at a speed of 20 km/hour, both the stroke volume and the heart frequency in this example are doubled and the cardiac output increases four times (19.6 instead of 4.9 liters of blood).

Recumbent racing bicycle
If that same average male rides a recumbent bicycle, then his legs are positioned at the same height as his heart (figure 1, left). This stimulates the flow of blood back to his heart. Which fills more optimally. At the same level of physical exertion (the same pedaling power), the blood volume in the heart chambers increases at the beginning of contraction to, for example, 200 ml on the recumbent bike instead of the 160 ml on the regular racing bicycle. As the heart fills more optimally, it contracts more vigorously and the residual volume at the end of contraction decreases to, for example, 10 ml instead of the 20 ml on the regular bicycle. And so the same male will have a stroke volume of 190 ml (200-10) on the recumbent bike instead of 140 ml on the regular racing bicycle (160-20) at the same level of physical exertion. Meanwhile, the oxygen requirement of the muscles is the same on both bicycles. Which means that the blood supply (so the cardiac output) must be equally large in both cases, namely 19.6 liters. In the case of the regular bicycle, the heart of this male needs 140 strokes to pump out 19.6 liters of blood (140x140 ml = 19.6). On the recumbent bike, his heart will achieve the same result with a mere 103 strokes per minute (103×190 ml = 19.6 liters). So the heart frequency is 36% higher on the regular racing bicycle (140/103). But a higher heart frequency entails a higher workload of the heart (2).

 Figure 1 The workload of the heart is higher at the same level of exertion (the same pedaling power) on a regular racing bicycle due to a higher heart frequency

Conclusions
1. Both the stroke volume of the heart as well as the heart frequency increase during physical exertion; combined, they increase the cardiac output and that determines the endurance-performance that you can achieve.
2. The stroke volume of the heart is lower on a regular racing bicycle compared to a recumbent bicycle; as a result, the heart frequency is higher at the same level of physical exertion.
3. A higher heart frequency means a higher workload of the heart.

Sources
1. Guyton & Hall; Textbook of Medical Physiology, 11th Ed. 2005; Unit III The Heart: Function of the Ventricles as Pumps , p 106. Unit IV The Circulation: Cardiac Output, Venous Return and Their Regulation, p 229-241
2. Nico Westerhof; persoonlijke mededeling 2009