Short and Sweet = Simple Science

How is it that a 10 minute crossfit workout can be more beneficial to one's fitness in comparison to a 90 minute workout? How is it that by working harder for a shorter duration we are able to burn more fat and build more muscle?

The answers to these questions are MITOCHONDRIA and HORMONES.

Mitochondria are responsible for energy production by way of oxygen utilization. As more oxygen becomes available at the cellular level, more substrate (fat or carbohydrate) is broken down as a result of the increased efficiency in the bioenergetic pathways. When you go through a really tough workout like FRAN that only lasts a matter of minutes, you are uptakeing an obseen amount oxygen which throws your mitochondria into overdrive resulting in the eventual breakdown of body fat. If the mitochondria are stimulated into working at high capacities (as they are in crossfit) they will most certainly become more efficient in both oxygen and fat utilization. Furthermore, brief workouts executed at high intensity can build up a OXYGEN DEBT in which once your workout is done, your mitochondria still require elevated levels of oxygen which means further fat breakdown. This debt can continue for a matter of hours meaning 5 hours after your 4 minute FRAN you'll still be burning fat!
Also, short and increase workouts tend to result in increased lactic acid levels. This decrease in cellular pH triggers a number of muscle-building hormones like growth hormone and testosterone. Also intense exercise can lead to elevated blood calcium levels resulting in improved cardiac contraction, and calcitonin secretion further resulting in increased bone density.

If your going to train, make it short and sweet; make it crossfit.