Preventing Muscle Fatigue

I studied exercise physiology as well as nutrition as an undergrad. Now, my IOC program is leading me to a Master’s in Nutrition and Exercise Physiology. It’s nice to be back to ex phys for a little bit. April is still all about carbs as you’ll see from some of my blog posts, but today I wanted to touch base on more of an exercise physiology subject…..muscle fatigue and how to prevent it. And can we?

Before I delve deeper into fatigue, we need to get some basic terms from energy metabolism down. From there I”ll explain more about fatigue. Try to hang in, I won’t bore you with the too technical stuff. However I do have a great chart if you’re interested in seeing all the different pathways of fuel. The profs call it the “dreaded pathway chart.”


ATP: adenosine triphosphate-this is the “energy donor” for our muscle contraction. We use ATP for energy. I’ve heard ATP called the currency of life. Without ATP, we wouldn’t be able to move. ATP is created in the mitochondria and can be generated by aerobic and anaerobic metabolism.

Aerobic Metabolism: the creation of energy (ATP) through the breakdown of fats and carbohydrates in the presence of oxygen. Aerobic metabolism is the main energy creation source for when we are going easy to moderate intensity. For example, walking, jogging or going for an easy bike ride uses aerobic metabolism as our energy source.

Most endurance athletes use aerobic metabolism due to the fact that endurance sports don’t require as much high intensity exercise. This isn’t always the case when doing ITU triathlon, or sprinting to the finish line. When I say, you want to train your body to burn fat as fuel, this is using aerobic metabolism. You can spare your glycogen (carb) stores this way. Slow twitch muscles operates using aerobic metabolism.

ATP is generated for long periods of time, at a fairly decent rate in aerobic metabolism. You and will increase ATP production with endurance training

Anaerobic Metabolism: the creation of energy without oxygen. There are two ways:

  1. Creatine Phosphate (CP) CP uses ADP (Adenosine Diphosphate) and hydrogen ions to create ATP and creatine. The CP system is in play from the first :2-7 seconds mainly. Think of a swimmer sprinting down the pool. If it takes them 12 seconds to get to the other end, they might be using their muscle’s entire CP amounts in only half the time it takes them to get to the other end.
  2. Glycolysisglucose is rapidly broken down during glycolysis, and lactate, hydrogen ions and ATP are produced as byproducts. Glycolysis takes over after the CP system for up to several minutes. Then, the aerobic system comes into play. The hydrogen ions that are given off as a byproduct are what makes you feel like your muscles are burning when sprinting. Coaches and athletes used to think it was lactic acid that caused the burning and fatiguing in muscles…It’s actually the hydrogen ions.

The muscles that operate using anaerobic metabolism are fast twitch muscles. Think, high intensity bursts of speed for up to several minutes. This is more of your track sprinter (running and cycling), weight lifter, cross fitter. They are using carbohydrate as their fuel source, very little will come from aerobic metabolism. For most athletes doing anaerobic sports, 80% of your energy metabolism will come from CP and glycolysis, 20% will come from aerobic sources.

ATP is generated quite fast with anaerobic metabolism. We have small reserves and capacities and we can increase our production of ATP with glycolysis, however not with CP. More on this later.

*Both systems listed above that generate ATP, are activated during sprint exercise. Some athletes think that sprinting is entirely anaerobic, this isn’t the case. When sprinting you will be using both systems, just the shorter and faster the sprint, the more you rely on anaerobic. You can think of this as a carb/fat fuel source debate as well. You will almost never be using 100% carb, or 100% fat during exercise. You will always use a mixture of both. Likewise, with aerobic and anaerobic exercise, will use both energy metabolism pathways.

Carb basics:

  • Both your muscle and liver carb stores can be depleted in 60-90min with high intensity exercise. Think about this as a half marathon and under.
  • Both your muscle and liver carb stores can be depleted in 2-3 hours with moderate intense exercise. Think about this as a marathon, or 50+ mile bike ride.
  • So, its very important for athletes doing endurance exercise/and both aerobic and anaerobic exercise to have fuel on board before your exercise/race, during your exercise/race (if it’s longer) and as your recovery.
  • This is a definite and easy source of muscle fatigue. Athletes that “bonk” don’t have enough carb sources on board, and deplete both their liver and muscle glycogen fuel sources. There are other causes of fatigue too.

Case Study-Causes of Fatigue During Sprinting:

-Imagine that you are a sprint swimmer or track athlete. Your coach gives you 10 sprints in the pool or the track with only a short rest interval. Since I was a swimmer, I’ll use 25y sprints. Let’s say we had to sprint all out, with 5 sec rest. When you start #1, you are using about 80% glycolysis and CP and about 20% from aerobic sources.  By #10, you are 80% aerobic sources and 20% from glycolysis and CP. They have in essence switched. What this means is, once your system has run out, your performance will go down with each subsequent sprint.

By #10, it is harder to turn on glycolysis because the muscle is becoming acidotic with hydrogen ions (byproduct). So you can’t nearly produce what you could in #1 with glycolytic systems. Then, you must rely on aerobic generating ATP systems. Glycolysis needs time to rest in order to generate ATP again. So if you are getting short rest intervals, you won’t be able to do this. CP however can start to be generated again, at least partially. It isn’t bothered by the hydrogen ions.

There are specific exercises that can be done to improve your glycolytic pathways and increase your anaerobic capacity, so when you’re training and racing, you won’t get as fatigued as quickly as you did before. I’ll give some specifics in more detail below.

What are the Causes of Fatigue, and How Can we Decrease Fatigue?

  1. Not enough muscle mass-To fix this, doing more strength training work both in the weight room, and on the road/bike. Think hills. The more muscle, the more mitochondria, thus the more ATP available.
  2. Not enough mitochondria-ATP is produced in the mitochondria, and they are found in muscles. So, like #1, you need to build more muscle to create more mitochondria. Also, to increase mitochondria, you need to do more of both aerobic and anaerobic exercise. This will increase your mitochondrial density. The best way to increase your mitochondrial density is long aerobic exercise 3-6 hours on the bike) and  high intensity anaerobic exercise (20min of Vo2 max intervals).
  3. Fuel stores run out-This one’s easy. Eat more carbs pre, during and post workout.
  4. ATP production low/slow fuel metabolism-Your ATP production will decrease during exercise due to a decreasing rate of substrate delivery, plus byproduct inhibition (hydrogen ions being released, causing the burning in your muscles). You an increase your fuel delivery, and decrease your byproduct (lactate/hydrgen ions) by specific exercise.

Exercises to Increase ATP production and Decrease Hydrogen Ion production:

*I am not your coach (unless I really am), so please speak with your coach before starting any additional training. And if you aren’t working with a coach, getting a coach can really help you to dial in your training and increase your performance. Contact me if you have any questions on coaching.

Start at the first and move down:

  • Increasing your volume of training- this should be the first thing that you focus on. You can use the old adage of 10% a week in miles, or hours. If you’re only running 10mpw, slowly ramp up, 10% each week. If you train in hours per week, slowly increase from say 3, to 3:30, to 4 hour, etc. Just adding volume will do this. My athletes know, that I don’t allow them to move onto the 2nd or third stage until they are running at least 20mpw for injury prevention. Cycling and swimming are slightly different.
  • Tempo/Steady State- This is your pace/effort right at your lactate threshold. You want to start at workouts at 20min at your LT, then increase each week from there. Most athletes can’t do LT for more than 60-90 min (most not over 60min though)-think fast half marathon or 10k or under.
  • Workouts above your LT/Interval training- This would be Vo2 max pacing or above. Workouts in this range will be 6 min or less. This could be doing intervals of 120% FTP on the bike for 4 min on, 4 min off. Also, it could be 200-400m sprints on the track. You could even do up to 800m repeats on the track, but things closer to a mile and above would be in the LT range.

*Remember, you want the majority of your training (75-80%) to be aerobic in nature. Then, 20-25%, depending on your level of training and years of training can be LT/Tempo/Steady State and Vo2 max. You’ll see a vast improvement on your performance just from increasing your volume. Then after you’ve spent time on your aerobic base, move down the list into the anaerobic areas.

There are currently several nutrition supplements being studied for reducing fatigue, increasing your anaerobic metabolism and increasing ATP. Some of these include nitrate, creatine and carnitine. I’m not going to go into those today, but will in a later post down the road.






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