Massive spills occur during road races on regular racing bicycles, sometimes resulting in permanent disability or even death. These accidents occur so frequently that professional racers feel it is ‘all part of the game.’ In terms of health, it matters whether you ride a regular or a recumbent racing bicycle: when riding in regular position the head is protruding; when riding recumbent the feet are protruding and take the hit in case of a crash or spill. Furthermore, when riding recumbent any pressure on the genital organs by a slim bicycle saddle is lacking. And the workload of the heart is less during physical strain in a lying position.
A 1. Heart rate
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 sits at rest (1). However, at the end of a contraction, the heart chamber is not empty, a residual volume of about 50 ml stays behind. So at the beginning of the contraction the heart chamber contains 120 ml (70 + 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 an adult male proceed to ride a regular racing bike 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 165 ml. As a result of the heart chamber being fuller the heart muscle contracts more vigorously. That makes the residual volume smaller. Which may be 25 ml instead of 50 ml at rest, for example. 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 165 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 25 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 (165-25) 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 (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
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 on a regular bicycle at the same level of physical exertion.
3. A higher heart frequency means a higher workload of the heart.
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
© Leo Rogier Verberne