Slow twitch to fast twitch switching?

I’ve been reading the lactic acid tolerance training ebook and it’s very good so far! I have one question however. One of the claims in it were that ‘undetermined’ tissue and fast twitch fibres would readily become slow twitch fibres over time with the correct stimulus e.g. endurance training. However once they were slow twitch they were very difficult to switch back and this leads to the dulling of explosive qualities over time if you follow a programme with slower overdistance work for 400m training. I’d always thought the more endurance type work I do for the 400 in the off season would be offset by the faster work closer to competition. I was wondering if there was any truth to this and any literature? Thanks!

The lactate threshold ebook explanation of the fast to slow twitch fibre conversion matches other articles I have seen. Including the theory that the switch from fast to slow is more permanent than the reversing back. Although quantification in terms of the magnitude of the loss of explosiveness and time taken to convert is harder to find. Ditto any conversion back. I guess the question is who wants to be muscle biopsied to confirm how/when/how fast this works …

  1. However KitKat minimises this risk by warning against the dangers of an excessive period of base building - where the athlete ends up getting to the speed phase too late in their season.
  2. The concurrent approach to training also includes the use of hills and acceleration development during a base period, keeping the athlete in touch with speed even during early phase endurance training.
  3. Finally there is also the question of how to plan to build the endurance base. Reading examples of endurance blocks from athletes as diverse as Michael Johnson, Irene Schawinska and Kit Kat shows endurance development is acquired by highish volumes of tempo such as 12x200, or what I would describe as middle distance vo2max reps covering say 300m to 800m (ie efforts of 1 - 3mins). Although this is relatively slow running it is not steady state endurance running.

IMHO Following the above 3 points reduces the chance of damaging fibre conversion.

There are CNS recruitment, fiber types, and fiber hypertrophy, and the training effects are different:

(1) It is pretty well established in the literature that chronic overexposure to endurance loads causes the pumps to reduce the ability to support high intensity explosive activity, and for that reason some HI sprints, plyos, weights, and such that load the CNS need to be maintained through base period or the base period needs to be short as KK says.

(2) A large amount of endurance load–particularly if accompanied by little speed load–will cause type I fibers to hypertrophy and type IIx/IIa fibers to atrophy. Even if you emphasize short speed close to comp, if you build up significant type I hypertrophy relative to body mass, this will reduce the explosiveness and mass relative force as the type I fibers are still there for some time even if de-emphasized.

(3) The Down-transition of IIx => IIa will occur regardless of what load mechanism is used. You have to cut back sharply during comp/peaking as Charlie has explained to reverse this.

What training sessions had you been planning in the pre season period before you began faster work ?

I’d usually have 4 types of workouts for the pre season. 1) Speed work from 60-120 at around 90-95% with short hill accels at the start. 2) work at 400m pace so about 85/90% with varying distances and hill accels at the start, 1st week from 150-300, 2nd week 300-450, 3rd 450-600. 3) tempo work at 70% from 300-600m with volumes hitting around 4000m at peak. 4) A long hills session which is not run too hard maybe 75-80%. Each is done 3x in a 4 week cycle so I run about 3x a week. I do keep in touch with high intensity work all year but have quite a high volume of endurance work in the off season. Do you think this ratio would fend off against negative fibre conversion?

Why worry about fibre conversion when deriving a programme. Yes it happens and IKH has given a good summary when/why. Whilst it is directly relevant to sprint performance it is hard to measure/quantify etc.
However there are key training principles that have worked for many athletes in terms of actual performance and therefore their underlying impact on fiibre status must be favourable.

  • Surely it is better to derive your schedule from their training approaches rather than trying to figure out fibre implications.
    From a quick refresher of KItKats principles it looks to me as though he aims to run around 3 sessions per week at or faster than 400m race pace.
  • 1-2 speed development sessions of sprints from 40-60m and 1-2 per week of 400m pace work, using efforts of 150-300 at or close to target race pace.
    Plus long hills as a form of overdistance (overtime) and a method to develop triple extension/form.

I realise this is a great over simplification of Kitkat but clearly this has worked for many elite athletes and has great logic for any level of runner.
By association this must have benefits to fast/slow twitch fibre proportions.
Same principle for a CF speed model for 100/200 athletes. Follow the principles of the programme and the fibres take care of themselves.

Looks as though you have 2 sessions per week at or faster than target pace 1) and 2) - so not too bad.

If you have a problem, either in actuality or in your perception it sounds like it could be your high volume of off season endurance work. What is this and why are you doing it ?
If you are doing only low intensity running, eg for road races/ xc or just to keep fit aerobically then you may be degrading fast twitch capability. However it is not just fibre status that may decline it is also the tendency for poor foot strike, short stride length, loss of flexibility and mindset that are affected. So you have to decide if your achievements from endurance work is worthy of the likely decline in 400m sprinting.