Why fives in CrossFit?

In programming we frequently prescribe 5 repetitions work into the CrossFit program. But why is five a magical number for strength adaptation and changes in body composition?

 

To conclude that five reps is a good choice, first we look at all available options to rep ranges. The smallest number possible to complete a full rep is 1, the maximum is 33,000. We take 33,000 from a marathon runner, assuming a 1.27m stride on average equaling 1262 steps a mile and each step is a rep. 

So, from 1 to 33,000. 

On this spectrum, if an athlete is prescribed a heavy set of 3 on the deadlift there is a higher differential on the force production from rep 1 to 2 compared to 2 to 3.

If an athlete was prescribed 33,00 reps, the 32,999 would be marginally easier than the last. 
So diminishing returns occurs as rep ranges increases.

Or the difference in perceived effort, from one rep to the next, gets smaller as the reps in a set increase. Now, nobody would hinge at the hip for 33,000 reps. We take a range from 1-20, this is typically a load prescription with weights. 

So lets look at the difference between 1 rep and 20. 

At 1 single rep, the energy system used is ATP. It has the largest change in an athletes force production and creates the largest neurological adaptation. However a true 1 rep max requires athletes to dig deep into the CNS system and almost nobody can do this unless it has been trained specifically for a sport (weightlifting). This is why almost all 'CrossFitters' have a low 1RM in proportion to a 10RM, strength endurance is practiced more - because it is tested more in the sport.  

At 20 reps the energy system is glycolytic (carbohydrate burning), the adaptation is hypertrophy (increase in muscle mass). The build of of lactic means typically a CrossFitter cant complete a true 20rm. The traditional athletes that work 20rep sets are bodybuilders. They are seeking muscle size, instead of strength. So although bodybuilders are strong compared to the general population, they are weak compared a weightlifter or high level CrossFitter. 

So where is the sweet spot? Fives. 

At five reps there is a crossover between force production and lean muscle tissue increase. There is also a great crossover in energy system demand, so the body seeks sugar as an energy source.'Five's' are magic as both an experienced athlete and a new member to the gym will both get adaptation from this rep range. 

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Coach Nathan