Wouldn’t be cool if they sold a car that had 3 engines? One that was optimized for coming off the line like greased lightning, one that was zippy but fuel efficient for whipping around town and yet a totally different third one that ran smooth and forever on a single tank of a gas just for highway driving? Well, your body has just that!! As with many of my posts, this one could be a full chapter in an Anatomy & Physiology textbook so my goal here is twofold: (1) give you enough information to take some real action against your training plan and/or (2) provide you with enough of a teaser to go dig deeper if you so desire. This topic in particular can really shed some light on the impact (or lack thereof) of changes you’ve made to your training plan in the past and hopefully help guide you as you make future decisions on what you should be doing and why.

Most of what I describe here are my words based on knowledge I gained while getting my Certified Strength and Conditioning Specialist® certification and therefore comes mainly from the book Essentials of Strength Training and Conditioning but trust me you can google any of the terms I use in this post and find an endless number of articles, papers, and blog posts covering these things so I’m only going to provide the one reference I just mentioned.  Personally, I hadn’t explored Anatomy & Physiology much in my other educational endeavours so scratching the surface on cellular respiration, which is really what this is all about, was a real treat for me and I totally plan on learning more. It turns out the human body is incredibly efficient at producing the energy needed for our muscles to do work – our bodies are masters of specialization. We have three distinct systems that produce the fuel that our muscles use to do real work, ATP (Adenosine triphosphate). Knowing a little about how they work and what parameters govern their operation can really help you tune your workout program. Keep in mind as you read these sections, ATP is fuel for the muscles so these systems are all about regenerating that fuel as its used. The human body doesn’t store much ATP, only about 3 ounces at any given time, and when it is used it never depletes completely because you need some ATP for basic cellular functions so how much and how quickly your body can product ATP is important to understand.

The Phosphagen System

Also sometimes called the ATP-CP (Adenosine triphosphate – creatine phosphate), this system can crank out the ATP faster than any of the others. This system produces ATP by using creatine phosphate through what’s called the creatine kinase reaction and since the body stores about 4-6x more CP than it does ATP, this works well but only for short durations. This system is active at the start of all exercise but its real job is to produce the energy needed for short high intensity activity like sprinting, jumping and resistance training – this is your greased lightning engine! You’ll read various ranges in different write-ups but according to Essentials of Strength Training and Conditioning your body is relying solely on the phosphagen system for up to about 6 seconds with complete ATP recovery happening in as short as just a few minutes and complete CP recovery in as little as 8 minutes.  There are lots of takeaways here but I hope one obvious one is that if you are lifting for strength and power, this is your dominant energy system so you should be able to see now why rest is so important between sets!

The Glycolytic System

This system uses carbohydrates to resynthesize ATP via multiple enzymatically catalyzed reactions so it cannot crank out the ATP as fast as the Phosphagen System does – this is your whipping around town engine! Because it uses glycogen and glucose (its carbohydrate sources) to do so and because there is a much larger supply of these on hand, it can produce more ATP than the Phosphagen System can before needing to be replenished. It’s primary range of use is between 30 seconds and 2 minutes where that “gap” between 6 seconds that mentioned in the last paragraph and the 30 seconds here is a time where both systems are working to provide ATP. There is quite a bit of complexity here that I’m skipping to keep this post short and to the point but if you are curious about the lactate threshold, pyruvate, or just where in this process oxygen starts to come into the picture please read up! For now though, just know that if you don’t have sufficient carbohydrates stored in your muscles as glycogen and/or blood as glucose, this mid-range energy system will suffer.

The Oxidative System

Also known as the Aerobic System, you might have guessed by now that this system produces ATP for longer term activities, greater than 3 minutes – this is your highway engine! This system cannot produce ATP very quickly, but it can produce it for very, very, very long periods of time.  It uses carbohydrates and fat as the primary substrates for replenishment of ATP but can also dip into protein with prolonged bouts (>90 minutes). This system is all about low intensity, long duration so think endurance sports like long distance running. This is another very interesting and complex system involving the Krebs cycle and mechanisms such as the Electron Transport Chain – super interesting reading if you want to see what’s going on under the hood.

Summary

Here’s the real takeaway, focus on the energy systems that are primary important for your activity.  All three energy systems can be trained – that is they can become more efficient, physiologically, the more they are used. Below is a summary from my favorite reference for your convenience 🙂