Cycling in physiology

Authors


Cycling is the use of bicycles for transport or exercise. Little did its inventor Baron Karl von Drais in 1817 supposedly expect the then ‘Dandy-horse’-called vehicle, his beloved Draisienne, to become the daily vehicle of choice for an estimated billion of people worldwide. Quotes from Nobel Laureates such as Albert Einstein, William Golding, William Saroyan, Christopher Morely and, currently, Venkatraman Ramakrishnan document their affection. John F. Kennedy called it ‘a simple pleasure’ that ‘nothing compares to’ (Argonauts 2012), Leo Tolstoy happily taught cycling to himself at age 67, and many a fellow physiologist do we know who hits the road on two unmotorized wheels every morning. Cycling is not only fun, but has tremendously advanced our understanding of physiological processes and has become an indispensable tool for the study of exercise-related pathophysiologies.

In clinical trials on exercise, US-based centres are traditionally more inclined to using the treadmill, while in Europe, ergometer cycling is often used (e.g. Guzun et al. 2012). Trial results that directly compare both methods such as Zhang et al. (2010) are rare, however, show a slight preference for cycling (Badruddin et al. 1999) (Hsia et al. 2009) (Loftin et al. 2004) (Tuner et al. 2008).

Faria et al. have thoroughly and extensively reviewed physiological variables that are (1) predictive of cycling performance in athletes, (2) change in professional cyclists and (3) significantly affect athletic performance. They found that the power output at the Lactate threshold, a peak power output of >5.5 W kg−1, a high percentage of type I fibres in the vastus lateralis muscle, the maximum achievable lactate steady state and a lack in tachypnoeic shift are all relative predictors of performance in road cycling, while the predictive value of the VO2max is not considered a valid marker and must be combined with additional performance indicators to gain predictive credibility. Interestingly, rising lactate levels seemed, in this meta-analysis, to be advantageous to cycling performance, and a general superiority of peripheral adaptations in working muscle over central regulatory adaptations was evident.

Remarkably, a large meta-analysis showed that cardiac adaptations in athletes (athlete's heart) differ, depending on the static vs. dynamic nature of their sport of choice (Pluim et al. 2000).

Lucia et al. (2001) reviewed in detail the physiological responses of professional road cyclists, who cycle 30 000–35 000 km year−1 in training (yes, they really do), and whose physiological adaptations cannot be extrapolated from those seen in elite amateurs: Distinctive are an absence of the tachypnoeic shift, a relative reliance on fat for energy metabolism at high exercise intensities and an extreme fatigue resistance of the slow motor units. One illustrative example that comes to mind is ‘Big Mig’ Miguel Indurain Larraya, whose one-time unchallenged superiority in professional road cycling has, together with his mental strength, been attributed to a number of ‘outstanding’ physiological variables (F. Luft 2012, personal communication): an oxygen-carrying capacity of the blood of 7 L min−1, a VO2max of 88 mL kg−1 min−1, a maximum cardiac output of 50 L min−1, a total lung capacity of 7.8 L and a record resting heart rate of 28 bpm (Danish Cycling Union 2003). The two articles mentioned above (Faria et al. 2005) (Lucia et al. 2001), although very thorough and extensively researched, are representative of many papers published on cycling, which often take more the point of view of a coach/specialist in sports medicine who is trying to design the most effective training regimen. It is the special strength of a physiological journal such as Acta Physiologica (Oxford), to, beyond mere sports medicine, use cycling as a model activity to better understand the mechanisms of exercise-induced changes in physiology, from integrative approaches to the molecular level.

Fatigue development has repeatedly been a major topic in exercise physiology studies, and however, little is known about the molecular basis of fatigue development at the muscular level. Recently, it has been shown that excitation–contraction coupling-related processes play a major role in the development of muscle fatigue, which is mediated by Ca(2+)-release, while a single exercise session is sufficient to induce specific adaptions, for example in the Na(+), K(+)-ATPase (Green et al. 2011).

Cycling/training studies have contributed to new insights into systemic cardiovascular response and regulation. In prolonged training protocols, Hart et al. (2010) found a resetting of the carotid baroreflex that explains the reduction in arterial pressure and left ventricular function following exercise, while Helge (2010) describes interesting differences in substrate utilization between arm and leg, that is higher carbohydrate oxidation and lactate release in upper limb muscles. More light has been shed on the mechanisms underlying the beneficial effects of exercise on cardiovascular health (Baranowski et al. 2011) (Maarbjerg et al. 2011).

Muscle function and metabolism have been studied in detail, wherein new insights have been gained on muscle catabolic (Frayn 2010) (Lanza & Sreekumaran Nair 2010) and anabolic (Graham et al. 2010) (Mascher et al. 2011) metabolism, local blood flow regulation (Calbet & Joyner 2010) (Sheriff 2010) (Sarelius & Pohl 2010) and fibre composition (Canepari et al. 2010) (Schiaffino 2010).

For those of us who stand in awe during the Tour de France or the Giro d’ Italia, recent doping scandals have almost led to the assumption that the current impressive results can only be achieved at a high cost, namely the ever-increasing use of (illegal) substances to artificially increase performance. Crivelli et al. (2011) looked at the ergogenic properties and mechanism of action of a usual suspect, Salbutamol.

Cycling studies have provided us with valuable insights into the architecture and dynamics of the human locomotor system, such as Norrbom et al. (2010) looking into mitochondrial bioactivity and Nordsborg et al. (2010) studying skeletal muscle Na(+), K(+) pump mRNA expression after exercise.

Numerous studies are out there to show the beneficial effects on individual and societal health brought about by cycling. The average Dane, Dutchman or German cycles on 15% of all trips taken, while in Anglo-American countries, cycling levels are as low as 1.3% (UK) and 0.9% (USA) (Pucher & Buehler 2008). What keeps people from using the bike? Large analyses have shown that missing infrastructure outbalances cultural differences and the fear of/inability to ride a bicycle. Environmental exposure to pollutants is often feared by would-be city cyclists, effects of which are currently investigated in a number of studies to be published in Acta Physiologica. To the best of our knowledge, convincing study results demonstrate that individual and societal health benefits clearly outweigh potential harm (accidents, pollution), even in the big cities (Hartog et al. 2011). So get out your bikes and – pedal away!

‘Whenever I see an adult on a bicycle, I have hope for the human race’ – H. G. Wells

Conflict of interest

There is no conflict of interest.

A. B. P. wishes to thank Professor Friedrich Luft for sharing his immense scientific knowledge as well as his enthusiasm as a cyclist.