In lay terms please, can anyone provide a brief summary of what our current understanding is about the capacity for adaptation of muscle fibre (types) and the kind of stimulus required to force adaptation - either towards the speed or alternatively towards the endurance end of the performance spectrum. Thanks, kk.
Put simply, all work causes type IIB fibers to become or to more closely resemble type IIA fibers. As volume is unloaded, the fibers will revert back and often overshoot so there is actually more type IIB than before training starting. This can only happen with proper volume regulation though.
There’s also the issue of fast twitch fibers (both type IIA and type IIB) converting to type I slow twitch fibers under excessive work loads or if they’re taxed in a manner that requires them to perform like slow fibers. Distance running or extremely high repetition weight lifting are good example.
And finally, there is evidence that it is the nature of the neural signal activating a muscle fiber that dictates its characteristics (fiber type). If the muscle fiber is primarily stimulated in ways a type II fiber would be used, it would in theory transform.
Summing everything up:
-Fastest to fast <—> any type of work (can be fixed w/proper unloading)
-Fast to slow <—> distance work, ultra high rep weights, basically any work much longer than 40 seconds in duration done with intensity (aerobic work essentially)
-Slow to fast <—> extremely intense work under 9 sec. in duration (applied in reasonable volumes and with unloading), this includes mainly plyos
From Athletic Performance and Vitamin D:
Vitamin D also increases the size and number of Type II (fast twitch) muscle fibers.
I have the whole study too, if someone is interested in the names of the referenced studies.
Thanks guys, so crucial to program design and coaching of any kind. Will read when I get time
Could you post the reference for the vitamin D and type II fibre hypertrophy please
Cannell, J.J. (2009). Athletic Performance and Vitamin D. Medicine & Science in Sports and Exercise, 41 (5).
much appreciated thank you