Post date: Jan 6, 2015 6:19:05 PM

TWO TYPES OF MUSCLE FIBERS

The muscle cells contain two distinct types of muscle cells or fibers.

Type I (slow twitch, SO fibers) - These muscle cells shorten at a relatively slow speed and generate energy from both fats and carbohydrates via aerobic metabolism . They are the major muscle fiber in use at 70-80% VO2max. Type I cell characteristics include: 

• high concentration of mitochondria for aerobic metabolism 

• increased intracellular myoglobin (which gives the muscle its characteristic red color) to store and transport O2 

• low concentration of glycolytic enzymes used for anaerobic metabolism 

• relatively fatigue resistant 

Type II (fast twitch, FG fibers) - These muscle cells are less efficient than the slow twitch cells and are almost entirely dependent on glycogen as fuel. They are called into action for sprints when the athlete approaches 100% of their maximum performance (and are working in the anaerobic range above 100% VO2max). Type II cell characteristics include:

• low concentration of mitochondria 

• high concentration of ATP and glycolytic (ATPase) enzymes 

• a rate of shortening 3 to 5 times that of a type I muscle cell 

The relative proportion of type I and type II fibers within a muscle varies from person to person and is determined by genetics (ie inheritance from your parents). However, with limits, this ratio can be modified with exercise and training. Successful endurance athletes have a preponderance of slow twitch muscle fibers (up to 90% of the fibers in the calf in cross country skiiers) while sprinters have more fast twitch fibers. Short term studies in bicyclists (5 months) failed to show a change in the ratio of cell types (percentage of slow vs fast twitch fibers) in leg muscles, but a longer multi-year study has suggested that this ratio can change with time, continuing to change for at least 5 years with regular training. 

But even without a change in the ratio of cell types, there is no question that both slow and fast twitch fibers can markedly improve their metabolic capacity with training

Cycling Cadence: Slow cadence - Fast Twitch, Fast Cadence - Slow Twitch?

Many cyclists and triathletes ask about which cadence is "best". Lance Armstrong's victories in the Tour de France over the last seven years have caused a lot of interest in this question, as we saw a dramatic difference in pedaling styles between Tour contenders Ullrich, with his lower cadence, bigger gear style, and Armstrong, with his higher cadence, low gear form. With Armstrong's style appearing to be more effective with his mountain stage wins, many scientists and coaches looked further into why this higher cadence style may be more effective.

It has been reported in past studies that pedaling a higher cadences, (80+ rpm), is more efficient than slower cadences. One way to measure efficiency is by measuring oxygen consumption rates. This measures the metabolic "cost" of the exercise. In order to determine the cost of slower vs. faster cadences one study compared oxygen consumption rates, heart rates, breathing rates, power production and even blood lactate production of pedaling at 50 rpm vs. 100 rpm. In order to make the comparison, the athlete's speeds were kept constant across the two cadences.

The result was that all of these indicators were very similar between the two cadences for the thirty minute test. One significant difference however, was the greater glycogen depletion in the slower 50 rpm condition. Looking closer, the researchers saw that only the fast twitch muscle fibres used more glycogen when pedaling at 50 rpm than they did when pedaling at 100 rpm. The slow twitch muscle fibres lost comparable amounts of glycogen in both the 50 rpm and 100 rpm conditions.

The slower cadence resulted in fewer, but more forceful contractions required to maintain the constant speed. It may sound counter-intuitive, but the higher force requirements of the slower cadence results in the recruitment of more fast twitch muscle fibres, since these fibres are capable of producing more force than slow twitch fibres. The drawback is that fast twitch fibres consume more glycogen, and fatigue more quickly than slow twitch fibres. As result, over the course of a workout, fast twitch fibres will get depleted and will fatigue. More fast twitch fibres will need to be recruited as the duration increases, which results in an increase of the total number of muscle fibres activated.

In addition, research has found that the faster cadence results in greater fat oxidation. Basically since slow twitch fibres are more efficient fat burners than fast twitch fibres, higher cadences that resulted in greater slow twitch recruitment used less glycogen, which is very important in endurance exercise performance. Over the course of a longer duration, the higher oxygen costs, and faster glycogen depletion seen in slower cadences results in reduced efficiency as compared to fast cadences.

Of course few cyclists would ever pedal as slowly as 50 rpm, but there are still efficiency differences, although smaller, between cadences in the 70s and lower 80s as compared to high 80s and 90 rpm plus. Slow twitch fibres can easily handle cadences of 100 rpm, but there is another factor to consider. When many cyclists do "spin ups", or accelerations to a higher cadence, they find that they bounce on the saddle, and their pedaling form suffers. This is caused by a lack of neuro-muscular co-ordination at high cadences, which in turn negatively affects efficiency. The good news is that co-ordination is trainable, and with practice, pedaling at high cadences can be improved.

So the question remains: what is the best cadence? Examining high performance cyclists and triathletes seems to confirm that cadences of 85-95 rpm are optimal for most athletes, and for most terrain. With proper training at these cadences (at all intensities), the higher cadences will be more economical and thus will result in faster times on the bike. And there is an added bonus: since higher cadences result in more glycogen being spared, especially in fast twitch fibres, there is the ability to have a faster sprint to the finish.

Joel Filliol