Exercise Physiology: A look at energy

Knowledge is power. Power is watts. Therefore knowledge is watts! Today I am going to try to help you get more watts with a bit of educational reading. I have found myself wondering while training, what the limiters are for different sorts of efforts. If you can strengthen these limiters, then you will be stronger at that level. Also, I find mentally it helps me on the hard training days to know what the biological focus of the workout is. That way I can make sure I put the best quality into those efforts.

Up until this point I have just assumed there were like 6 energy zones, because that is how you train (z1 = recovery up to z6 = sprint). In reality, no energy system is completely isolated (maybe anaerobic for the first instant of a sprint).  Therefore if I want to increase my 5 min power, what would be the best way to do that?

First off you need to know your own limiters. For me, I am a small, lean guy, so muscle and top end sprint power are lacking. For someone that has nice monstrous track legs, they will be short on their threshold ability (in comparison to their sprint, they probably still have a more-than-decent threshold). If you ride your bike a lot, you probably already know where your strengths lay. Taken right from Jones and Carter (2000), “Endurance can be crudely described through the generation of individual ‘velocity-time curves’ which relate a series of velocities (or power outputs) to the time for which these velocities or power outputs can be sustained”. As athletes, our goal is to increase the power we can sustain at any given duration.

You have 3 energy systems that work together to create energy (ATP) for your muscles so they can contract. You have the aerobic and anaerobic system which is split into two. The aerobic system needs oxygen, can produce high amounts of energy, but is relatively slow. The anaerobic system can produce a lot of energy quickly, but is highly unsustainable. This is done by splitting stored molecules for energy (ATP and phosphocreatine) [alactic] and the incomplete breakdown of glucose into energy producing lactate.

Pick your duration of power you want to improve, and see what system needs work. 

The alactic component is thought to produce 20-30% of anaerobic energy in a maximal effort lasting 2-3mins. The replenishment of creatine phosphate can take 5-15mins, so for sprint training or hard gym efforts, you need a few minutes between sets to maximize the output. The lactate component, which then produces the majority of the energy, is limited by the rate of glycolysis. This could be due to inhibition of enzymes that catalyze the reactions, or a lack of activation of the glycolysis process. A driving factor for this could be a decreasing pH due to the lactate (lactic acid if you will) being produced. Endurance training will cause alterations in metabolic demand, mechanical load, neuronal firing pattern, and hormonal levels. This causes the body to adapt to better handle these alterations. It does so by increasing the size and number of mitochondria, increase muscle glucose uptake and insulin sensitivity (which can combat insulin resistance found in type 2 diabetes), increases fat usage for energy, and formation of a greater blood flow network (more capillaries, increase in red blood cell count).

You body functions based on what it is used to. So train for circumstances that you will encounter on race day. From Gastin (2001)

So what are limiters in aerobic work? In sessions longer than 4hrs, increased fatigue is likely caused by psychological, nutritional, thermoregulatory or musculoskeletal factors. If you train frequently and your body has adapted you are able to exercise longer due to increased usage of fatty acids (to preserve glucose) and from increased glycogen storage in muscles. Essentially you have a bigger tank of gas to power your engine. VO2 max is limited by the oxygen supply to the muscle, not the ability of the muscle cell to pull oxygen from the blood.

Your VO2 max is usually considered to be the measure of your aerobic ability. But there are other factors that determine your aerobic performance on top of that.  You also have muscle economy and lactate threshold. Better muscle economy (less oxygen used for the same absolute power/speed) is typically seen in people who do high volume work. This is thought to be related to better muscle firing patterns that are developed through lots of repetition. So while everyone tries to push short HIIT training, there is something to be said about logging in some volume. Increases in lactate threshold could be explained by a reduction in the rate of lactate production or from an increase in the ability to exchange and remove lactate from the blood. Exercise above lactate threshold brings greater levels of fatigue either by using greater amounts of muscle glycogen, or from effects of metabolic acidosis on contractile function. So in a gross summary, VO2 max will tell you how much oxygen you can deliver, muscle economy tells you how efficient your system is with that oxygen, and lactate threshold is how well (or long) you muscles can work at the limit of aerobic ability.
The mitochondria is the powerhouse of the cell, this is where glucose is broken down into energy. Endurance work increases the size and number of mitochondria in the muscle cells. This increases the oxidative capacity of the cells, essentially allowing you to do more work aerobically. Animal studies have found that exercise at 50% of VO2 max can be used to maximize mitochondrial density in type 1 (slow-twitch) muscle types, but much higher intensities are needed for type 2 muscles (fast twitch). Therefore in order to reach your full VO2 potential, you will need to work high and low intensities. It should also be noted that endurance work selectively grows type 1 muscle types. Some research suggests that the conversion from type 2 to type 1 muscle is possible through lots of endurance training.

A study by Spencer and Gastin (2001) looked at anaerobic vs aerobic system usage in runners. They picked well trained runners at 200m, 400m, 800m and the 1500m distances. They found out by testing for accumulated oxygen deficit that the aerobic system plays a big role in even these short sprint workouts. Their data suggested that the crossover to predominately the aerobic system(>50% of energy obtained for function) occurred after 15-30seconds. This result should seem intuitive if you have ever tried to sprint for longer that 30seconds, you just can’t. From this, you can see why track sprinters will benefit from spending more time in the gym to develop the anaerobic, where most battles on the road are much longer than that, so an aerobic engine will help immensely.  *Note, many older studies have suggested that this 50% split of energy systems occurs somewhere between 1-3mins of maximal exercise, which some are suggesting is an underestimation of the aerobic system.

taken from Gastin (2001)

An interesting study by Gomez-Carera et al. (2008) on vitamin C. So vitamin C is an antioxidant, which helps to prevent cellular damage from free radicals. In an exercise study, they found the group that a control group increased their fitness 6-7% compared to a group that took 1 gram of vitamin C/day that didn’t exhibit any increases. So first off, we need to acknowledge that vitC is a vital nutrient that we need (see scurvy), so we aren’t going to cut it out. Secondly, the recommended amount for adults is only 90mg, so this is a fair amount more, but actually quite an easy amount (1 gram) to consume if you eat lots of fruits. Their theory was that the vitC is preventing damage in the cells during exercise. Since there is no damage, the muscles cells see no reason to adapt. With no adaptation to the stresses, there is no increase in ability to handle more stress.

In conclusion, to get stronger is to increase your body’s ability to handle stress. It will only adapt if it constantly is overstressed. These changes are typically metabolically unfavoured, so if you stop exercising, they revert back to normal. So log your long easy miles. Hit your intervals hard. Allow rest to provide time for the body to change so you can hit the intervals harder next time. And give your body the nutrients it needs to make these changes. Without new stresses, there is no need for change. So if you find yourself stuck in a plateau, look at your training and find a way to throw your body a curveball.


References:
Gastin, Paul B. "Energy system interaction and relative contribution during maximal exercise." Sports medicine 31.10 (2001): 725-741.
Gomez-Cabrera, Mari-Carmen, et al. "Oral administration of vitamin C decreases muscle mitochondrial biogenesis and hampers training-induced adaptations in endurance performance." The American journal of clinical nutrition 87.1 (2008): 142-149.
Jones, Andrew M., and Helen Carter. "The effect of endurance training on parameters of aerobic fitness." Sports Medicine 29.6 (2000): 373-386.
Kiens, Bente, et al. "Skeletal muscle substrate utilization during submaximal exercise in man: effect of endurance training." The Journal of Physiology 469 (1993): 459.
Spencer, Matt R., and Paul B. Gastin. "Energy system contribution during 200-to 1500-m running in highly trained athletes." Medicine and science in sports and exercise 33.1 (2001): 157-162.
Westerblad, Håkan, Joseph D. Bruton, and Abram Katz. "Skeletal muscle: energy metabolism, fiber types, fatigue and adaptability." Experimental cell research 316.18 (2010): 3093-3099.