Much of the info that will end up in this ‘Simple Science’ section will come from a magnificent book I have (all 2.5kg!!! of it) called Swimming Fastest by Ernest Maglischo. But don’t let the word ‘swimming’ put you off if you aren’t a swimmer. For most of this you can take out the word swimming and just plop in running, rowing, cycling, tennis, bob-sled, tiddly winks, whatever you are into. Only the middle section of the book is on physiology and energy systems, so don’t worry, if you print out the posts you won’t end up lying in bed for your nighttime reading with 2.5kg of pages scattered about. Besides which, although I make no apology for lifting info straight out of the book, in the interests of keeping everything nice and simple it has all been and will be radically condensed, hopefully without changing the meaning.
Ok, so this first bit is on fast twitch and slow twitch fibres:
Slow twitch (ST) fibres (red) contract 10 to 15 times per second (still sounds quite fast to me). Fast twitch (FT) fibres (white) contract 30 to 50 times per second. FT fibres also shorten more rapidly, and can shorten up to 6 fibre lengths per second. ST fibres have more endurance and they have more capacity for aerobic work, but their capacity for anaerobic metabolism is limited. ST fibres have more myoblobin, which is the substance that transports oxygen across the muscle cell. ST fibres also contain more mitochondria, the protein structures within muscle cells where aerobic metabolism occurs and ST fibres also have a greater concentration of the aerobic enzymes that catalyze the release of energy during aerobic metabolism. On the other hand FT fibres have a lower capacity for aerobic metabolism as they have less myoglobin, fewer mitochondria and a lower concentration of enzymes. FT fibres produce more lactic acid than ST fibres at equivalent workloads and so fatigue more quickly. They also use glycogen more quickly.
Now I thought this bit on endurance training was interesting and a bit unfair. Endurance training will increase the aerobic capacity of slow twitch and fast twitch fibres. Trained fast twitch fibres never reach the level of aerobic capacity of trained slow twitch fibres. An athlete can increase the aerobic capacity of fast twitch fibres, however, to a level that surpasses that of untrained slow twitch fibres. Conversely, strength and sprint training will increase the size and contractile speed of fast twitch and slow twitch fibres as well as their potential for rapid energy release. Fast twitch fibres however, possess a greater potential than slow twitch fibres for such increases. Although an athlete can increase contractile speed and force in slow twitch fibres that have been sprint trained, they never reach the level of even untrained fast twitch fibres. Sounds like sprinters get the best deal. I wish I was a sprinter!
There are 3 subgroups of FT fibres. Fta, FTb and FTc. We’re going to forget FTc as they, and what they do, are controversial and make up only about 3% of our fibres anyway. Fta fibres contract faster and with greater force than ST fibres and make up about 33% of our fibres. FTb fibres contract with about twice the force of Fta fibres and make up about 14% of our fibres. Roughly 50% of fibres are ST.
It’s not true that we only use ST fibres when we go slow and FT fibres when we go fast. ST are the first to contract. When the resistance increases, both the ST and FT fibres will contract to overcome it whether the movement is slow or fast.
It seems that although you can’t change an ST fibre into a FT fibre you can change the proportion of FTb’s and Fta’s. Most notably, FTb’s becoming Fta’s by an increase in the amount of myoglobin etc.
So that was ok wasn’t it? About 10 pages of the mighty tome condensed into one manageble post. The next section/s are some really easy back-to-basic stuff on the circulatory and respiratory systems.
