Not trying to cherry pick here, just combination of personal experience and scientific literature. If we simply look at all the exercises as ways to contract muscle fibres to trigger a response (to said stimulus), we are subjecting these muscles to a load for a certain amount of time (or Time Under Tension/Time Under Load - TUT/TUL). Second, we all have a mix of Type 1, Type 2A and Type 2B muscle fibres (here is where genes bless us with either a larger number of fast twitch (sprinters/bodybuilders or more slow twitch (marathoners). Most people along the Bell curve have a good mix of both. But it is highly likely those with a the average mix have muscle fibre types unevenly spread across the body (ie personal example: slow twitch predominant in legs, fast twitch predominant in back/shoulders). What does this all mean? That the various fibre types require different levels of TUT (and correspondingly heavier/lighter resistance) to be trained in the optimal load/time window. And this is why a prescribed training regime to suit the individuals genetic tolerances is required (unless, as stated in the article I posted, we have the blind leading the blind, and get too old by the time we find the ideal workout plan).
So TUT - what does that mean in practice and why do I say that CrossFit sessions delve into the aerobic realm (unless you can prove to me that there are predominantly heavy weight, low rep, short session CrossFit sessions). As I have mentioned before (and are discussed in greater detail in the pdf posted by biosci, the three fundamentals of exercise are intensity (effort), volume (TUT or number of sets), and frequency (the rate of occurrence of training sessions). To trigger the growth/strength gain response, the intensity needs to be high and invariable (ie 100% effort). If the effort is, say, 90-95%, there is no requirement for the muscles to overcompensate, regardless of the number of sets or frequency performed. The body wants to stay in homeostasis, ie unchanged, as muscle gain is taxing on the system and requires more energy to maintain (hence atrophy after a certain amount of inactivity back to a "comfort" state). Progress is possible in lower intensity programs, but is much slower.
Now, volume - or TUT. if we are dealing with strength training (and NOT endurance training - not sure what your goals are with CrossFit), TUT and muscle fibre tolerances are critical to understand. If we want to stay in an anaerobic environment, TUT can't last very long - minimal 20 seconds to avoid lifting too heavy and risking injury. 120 second is max unless you want to enter the aerobic environment. Take a stopwatch and count the total TUT of any particular muscle group in a WOD and check if you are, one, exerting maximal intensity to trigger a response, and two, within the anaerobic window of TUT. Fast twitch muscles respond best with heavy weight, 30-45 seconds TUT as they can exert great force but tire out quickly (use of glycogen stores). Mixed types work optimally within the 60-70 second TUT range. Slow twitch can take a lot more volume (at reduced load of course) as they work in the 70-120 or beyond range (use of oxygen for fuel but smaller than fast twitch).
Bottom line, again depending on what your goals are, the incredible amount of reps asked for in at least the CrossFit WODs I have seen (I am a novice to this particular flavour of KooAid, I admit), means two things: while intensity or effort may feel high (and for some exercises such as thrusters and muscleups very well is high), most in-shape pers can achieve the rep counts given (esp if broken into groupings 25/25/25/25 = 100). However, most WODs are very high volume and seem to be geared to aerobic stimulus, wherein the fast-twitch fibres are going to lose the most, with mixed a close second. They may become slow-twitch like, but will never realise max strength potential.
) and the lesser known Crossfit Endurance (www.crossfitendurance.com, WFS).
