騎乘單車姿勢與安全

[觀念與技巧] Cycling 快速的踩踏較有效率

單車騎士如能在平路與山路上維持每分鐘踩80~85轉通常是最有效率。

實驗發現，在強度(最大耗氧量85%)相同，同樣騎30分鐘，以每 分鐘踩50轉和100轉騎車的騎士，兩人的耗氧率機近相同，心跳與呼吸率、力量與血液中的乳酸濃度都類似。但是每分鐘踩50轉(50 rpm)的騎士(用高速檔)較倚賴肌肉內儲存的碳水化合物作能量，每分鐘踩100轉的，則是消耗脂肪作為能量。

慢速收縮肌細胞在50rpm和100rpm時消耗的肌肉肝醣量相等。

快速收縮肌細胞（fast-twitch muscle-fiber）
  　50rpm:    消耗50%的肌肉中的肝醣(glycogen)
100rpm:    消耗30%的肌肉中的肝醣(glycogen)

運動員體能的維持，有賴於肌肉肝醣儲存量與高效率地身體送氧能力。許多運動的研究都是針對減少肌肉肝醣的消耗及增加肌肉肝醣的再生成，以期良好的表現。

=> 快速收縮肌纖維／細胞在較慢且出力的踩踏時，快速消耗肌肉肝醣。
=> 使用較多的肌肉細胞
=> 較高的氧氣消耗率
=> 效率差

慢速收縮肌細胞(slow-twitch muscle cell)每分鐘可收縮80至100次，因此對於低速檔位，較快的踩踏可以應付裕如。

矛盾之處是：實驗的結論是輕踩，高迴旋（踩踏）導致較多的脂肪細胞氧化，*儘管最大的脂肪燃燒通常與低速(較用力)的運動方式有關。

基本上，實驗顯示較高的脂肪分解是發生在100rpm時，慢速收縮肌纖維是負責快速，較輕力氣的肌肉收縮。慢速收縮肌纖維（比起快速收縮肌纖維）較能燃燒脂肪。

而高速踩踏也可以在快速收縮肌肉纖維裏面保留更多的肝糖，這樣在快到終點時可以有更大的爆發力。




Pedalling Technique: why pedalling fast is more efficient

Elsewhere we have explained that cyclists are usually more efficient on both hills and flat terrain when they pedal quickly (at about 80-85 rpm) rather than at slower cadences.

To determine the actual effects of slow and fast pedalling on leg muscle cells, scientists at the University of Wisconsin and the University of Wyoming recently asked eight experienced cyclists to cycle at an intensity of 85% VO2max for 30 minutes under two different conditions. In one case, the cyclists pedalled at 50 rpm while using a high gear. In the second case, the athletes pedalled in a low gear at 100 rpm. They were travelling at identical speeds in the two instances, so the athletes’ leg-muscle contractions were quite forceful at 50 rpm and moderate – but more frequent – at 100 rpm. As it turned out, the athletes’ oxygen consumption rates were nearly identical in the two cases, and heart and breathing rates, total rate of power production, and blood lactate levels were also similar.

However, the athletes broke down the carbohydrate in their muscles at a greater rate when the 50 rpm strategy was used, while the 100 rpm cadence produced a greater reliance on fat. The greater glycogen depletion at 50 rpm occurred only in fast-twitch muscle cells. Slow-twitch cells lost comparable amounts of glycogen at 50 and 100 rpm, but fast-twitch cells lost almost 50% of their glycogen at 50 rpm and only 33% at 100 rpm, even though the exercise bouts lasted for 30 minutes in each case. This rapid loss of carbohydrate in the fast-twitch cells during slow, high-force pedalling probably explains why slow pedalling is less efficient than faster cadences of 80-85 rpm. Basically, as the fast fibres quickly deplete their glycogen during slow, high-strength pedalling, their contractions become less forceful, so more muscle cells must be activated to maintain a particular speed. This activation of a larger number of muscle cells then leads to higher oxygen consumption rates and reduced economy.

Admittedly, this scenario – in which slow pedalling preferentially pulls the glycogen out of fast-twitch muscle cells – may sound a little odd to you! Fortunately, the paradox isn’t really too troubling; after all, slow pedalling rates are linked with high gears and elevated muscle forces, while fast cadences are associated with low gears and easy muscle contractions. Since fast-twitch fibres are more powerful than slow-twitch cells, the fast twitchers swing into action at slow cadences, when high muscular forces are required to move the bike along rapidly.

On the other hand, ‘fast’ pedalling rates of 80-100 rpm are not too hot for the slow-twitch cells to handle. Slow-twitch cells can contract 80-100 times per minute and can easily cope with the forces required to pedal in low gear. Another possible paradox in the Wisconsin-Wyoming research was that fast pedalling led to greater fat oxidation, even though maximal fat burning is usually linked with slow-paced efforts. Basically, the higher fat degradation at 100 rpm occurred because the slow-twitch cells handled the fast-paced, low-force contractions. Slow-twitch fibres are much better fat burners than their fast-twitch brethren!

Fortunately, there’s a bottom line to all this: during training and competition, cyclists should attempt to use fast pedalling rates of 80-85 rpm, both on the flat and on inclines. Compared to slower cadences, the higher pedalling speeds are more economical and burn more fat during exercise. Ultimately, the high pedalling rates also preserve greater amounts of glycogen in fast-twitch muscle fibres, leading to more explosive ‘kicks’ to the finish line in the closing moments of races.

(‘The Effect of Pedalling Frequency on Glycogen-Depletion Rates in Type I and Type II Quadriceps Muscles during Submaximal Cycling Exercise’, European Journal of Applied Physiology, vol. 65, pp. 360-364, 1992)

http://www.pponline.co.uk/encyc/0965.htm