Crash course on Running Physiology: Instructions from an Ultra-Running Doctor
Capillaries. Slow-twitch fibers. Myoglobin. Glycogen. One of the many things I love most about running is that, in spite of its plain simplicity of just putting one foot in front of the other, it is also remarkably complex.
When running is performed correctly, it is a pure scientific endeavour to maximize one’s endurance and speed. Competitive sport and science are two of the fields that validated my career choice in the last couple of years. Many physicians became interested in medicine by venturing hands-on into various areas of scientific study. Here is a breakdown of what I have learned from physiology.
Running physiology is the general study of the physiological effects of running and the specific study of the body’s unique responses to exercise. I will explain the intricacies of how the human body works in relation to running and endurance performance.
Muscle fiber types highly impact running physiology
Peak force, fiber functional properties, actino-myosin ATPase activities, resistance to fatigue, contraction velocity, glycolytic and oxidative capacities all fall across a broad spectrum. Nonetheless, it is possible to divide this continuum into a few clusters.
An Overview of Muscle Fibers
There are three main muscle fiber types in humans:
• Type I (oxidative slow twitch)
• Type II-A (oxidative fast twitch)
• Type II-B (glycolytic fast twitch)
Each muscle type has extremely different functional and physiological characteristics, as their names suggest. Type I fibers have low force/speed/power generation and higher endurance, Type II-B have high force/speed/power generation and lower endurance, and Type II-A fall in between.
On average, humans are born with about 50% slow-twitch muscle fibers and 50% fast-twitch. It depends very much on genetics. So if you plan on being a world-class sprinter, you’d better choose your parents wisely!
As such, there are two cardinal types of runners—those who have superior speed, whose performance increases as the race gets shorter, and those who have superior endurance, whose performance increases as the race gets longer. It is essential to acknowledge differences in muscle fiber types and their correlated metabolic profiles. The types of muscle fibers that make up individual large muscles greatly influence your performance.
Running economy and lactate threshold count more than VO2Max
How can this help you? Well, high mileage, power workouts, and threshold running (tempo workouts) count more for your performance than long intervals. This is especially more important once your VO2max has plateaued.
While VO2max (body’s maximum oxygen intake) has received a lot of attention among coaches and runners, a high aerobic capacity alone is not enough to reach elite-level performances. Yes, it’s mandatory for peak performance, but VO2max is also largely genetically determined.
There are other physiological key players of long distance running that need attention as well, including running economy and lactate threshold. These two are very responsive to training and have significant influence on your performance.
Think about a recipe. You need all the ingredients for success.
Many prominent names in exercise physiology have performed research on this topic and found that lactate threshold is the main physiological predictor of endurance running performance. The lactate threshold marks the moment that running starts to include significant anaerobic (oxygen-independent) metabolism, in addition to the aerobic one. Since this physiological factor represents the fastest sustainable pace, the longer the race, the more vital your lactate threshold.
Running economy is the amount of oxygen consumed at submaximal speeds. It is particularly important in determining running performance because it shows how hard the athlete is working in relation to his or her maximum ability to use oxygen.
More precisely? Consider two runners each having a VO2max of 75 ml of oxygen per kilogram of body weight per minute and a lactate threshold pace of 4 minutes/kilometer (6.5 minutes/mile). If John uses 50 and Andrew uses 60 ml of oxygen while running at a 4:30 pace (or 7:00 in miles), the pace feels easier for John because of his efficient running economy.
Therefore, John can run faster and longer using the same volume of oxygen and will feel the same level of fatigue as Andrew.
How to improve Lactate Threshold, Running Economy, and VO2Max?
• Elevate your lactate threshold by doing continuous runs, short intervals, and long, slow distance runs at threshold pace. Quick tip: you reach the threshold when you are not able to talk anymore.
• Increase your running economy by enhancing the densities of mitochondria and capillaries inside the muscle cell with high mileage. Studies show that runners who run over 100 kilometers per week (or about 65 miles) have an increased running economy.
• Raise your VO2Max by running intervals of 800, 1000 and 1200 meters close to your maximum heart rate.
Metabolism is strongly correlated with enzymes and oxygen (and the secret behind elite running)
In other words, developing your aerobic base through sprint training can improve enzyme activity that enhances your running. Enzymes are biological catalysts that stimulate chemical reactions in the body. Without enzymes, there would not be enough energy needed to run as chemical reactions would not occur. The volume of these biological entities controls which metabolic pathway the body uses. For instance, if you have more aerobic enzymes, your body will steer the metabolism toward a bigger reliance on aerobic metabolism at a certain (submaximal) speed.
As any other biological markers, enzymes can be inhibited or activated (so their effectiveness varies). To sum up, enzymes control your metabolism and hence control the pace at which your body fatigues.
A few studies have demonstrated how enzyme activity varies in response to training. A research paper published in 2006 in the Journal of Applied Physiology discovered that citrate synthase (an important aerobic enzyme) activity increased by 37% in new runners after just 13 weeks of consistent training. The weekly mileage increased from 15 to about 36.
The same goes for the anaerobic enzyme profile of muscles.
A study published in the Journal of Sports Medicine showed that sprint training induces many changes in the anaerobic enzymes such as phosphorylase, lactate dehydrogenase (LDH), phosphofructokinase (PFK), and glyceraldehyde phosphate dehydrogenase GAPDH). Researchers found significantly increased activities in all of these enzymes. It’s a nice finding, given the fact that studies on changes in enzyme activity correlated with sprint running are currently lacking.
Metabolism is also correlated with oxygen. The amount of oxygen also determines which metabolic pathway the body will choose during training.
For instance, toward the end of glycolysis, the metabolic pathway that breaks down carbs, there’s an obstacle on the road. At the point when there is sufficient oxygen to address the muscle’s needs, the product of glycolysis, pyruvate, is quickly converted into a vital metabolic intermediate that goes into the Krebs cycle for oxidation.
The conversion of pyruvate inside the mitochondria (deep into the muscle cell) is an irreversible reaction because the carbs broken down through the process of glycolysis start to be oxidized through the Krebs cycle.
Nonetheless, when there is not enough oxygen to address the muscle’s needs, pyruvate is changed over into lactate. A related result of this last reaction is the vast accumulation of metabolites. It’s the moment when acidosis starts developing, causing the muscles to reach exhaustion and slows down the body.
Now here is the secret behind elite running. Depending on how well your muscles are developed aerobically, by choosing to increase your mileage on a weekly basis and doing LT trainings, you will steer the pyruvate process toward the Krebs cycle and away from acidosis (lactate production) at a certain pace.
But there’s more. The volume of energy you receive from pyruvate entering the Krebs cycle is actually 19 times greater than what you would receive from pyruvate being transformed into lactate. It’s true that pyruvate will always be converted into lactate at a certain fast speed, but the ultimate goal of training should be to elevate the speed at which it occurs.
Glycogen stores play a key role in endurance running
Carbohydrate-loading meals have become a staple of the modern marathon and ultra-marathon pre-race events. Each type of physical activity, running included, depends on the readily available amounts of energy in the form of ATP (adenosine triphosphate) to keep the muscles working efficiently.
During a long run, muscles gain energy from the glycogen within their cells and directly from blood glucose. As the amount of glucose in the blood is converted to energy, the liver converts its glycogen into glucose and releases it into the bloodstream to support and maintain a constant supply of glucose to the muscles.
Therefore, in long distance running, the energy is provided by carbohydrates, and sometimes by a mixture of fat and carbohydrates. While glycogen serves as the main source of these carbs, many runners train without fuel so that they stimulate fat burn. I am one of them. “Teaching” your body to use fat in long distance runs is the key to success, as glycogen depletes quickly and provides only 4.1 kilocalories in one gram. On the other hand, fat provides 9 kilocalories per gram.
Carbs should be your best friend. Carbs are stored in the liver and skeletal muscles as glycogen, yet are also found as glucose in the blood. Running forces the brain to trigger a higher glycogen absorption to regenerate the ATP chemical compound through glycolysis. This is why endurance running is greatly correlated with the volume of pre-exercise muscle glycogen deposits.
Carbs are very important, that’s why ingesting glucose and high-energy gels during ultramarathons usually delays fatigue. Now, since the skeletal muscles prefer carbs as the main type of fuel, why not train to replenish muscle glycogen deposits? The more your glycogen store is emptied, the more it’s actually refilled. Just empty the full glass, and you will receive a refilled larger glass the second time.
Things brings us to…
Glycogen Depletion Explained
Adequate glycogen stores are especially important for runners who train at medium to high intensity for 120 minutes or more. The larger those glycogen reserves, the longer a runner will be able to maintain high blood glucose levels and provide sufficient energy for muscles. Once glycogen stores are depleted, a runner is only able to perform at about 50 percent of maximal capacity. Glycogen depletion is often known by athletes as “bonking” or “hitting the wall,” due to the exhaustion. First time marathoners always hit the wall after about 35 km.
Insulin (yes, that famous hormone) controls glycogen synthesis, along with the uptake and availability of glucose from the circulation. Therefore, the more glycogen in the body, the more insulin will draw glucose to the blood and then into muscles.
There is also a protein involved in transporting glucose through the bloodstream. Furthermore, low levels of glycogen inside muscles have been shown to improve the transcription of genes that facilitate protein synthesis. It’s easy to think of this as a plan to threaten muscles’ survival. The moment the muscle cell is threatened by being deprived of its preferred fuel, the brain receives a powerful signal to create more of that specific fuel in order to fight the threat. Your body will use all of its functions and processes more effectively.
There is a downside to low-glycogen running, as it is quite hard to maintain a decent intensity since carbohydrates are the main fuel. I managed to handle this pretty well by running 30Ks without carbs – only with water.
What conclusion can we draw from here? Studies show that muscles usually replace their glycogen deposits at the fastest rate during the first 2 hours after training. Glycogen resynthesis continues at a much higher than average rate for about 12 hours after a long run (glycogen-depleting). Only after 12 hours the rate of glycogen replenishment goes down to a normal level.
Eat carbohydrates as soon as possible after a race or long run to increase your chances of recovering faster.
Physiology of Hyperthermia
In addition to ensuring the oxygen-rich and carbohydrate-rich blood supply, the heart controls body temperature simply by pumping warm and fresh blood to the extremities, and sweat comes into action. During long runs, heat production and loss can increase dramatically. Dehydration and high humidity usually make heat loss a lot more difficult. Dehydration affects body’s capability of transferring heat from muscles to the skin, while elevated humidity levels affect evaporation. Both situations will highly increase the core body temperature, decreasing performance.
The human body’s ability to control heat through sweat and evaporation is the secret behind endurance running. Without it, the core body temperature would elevate fast enough to generate heat issues after only 10 to 15 minutes of running. It’s not uncommon for endurance runners to finish long runs with body temperatures of 100 degrees Fahrenheit or more.
Sometimes even if they sweat, there are marathon runners who will finish the 42k with body temperatures of 103 degrees Fahrenheit (39,4 degrees Celsius).
Why am I talking about hyperthermia? While studying this piece of research I remembered the moment I perfectly understood what it actually means to tackle hyperthermia like a pro. Earlier this year, in late May, I ran an 80k ultra trail in Cyprus (one of the hottest countries in Europe). I started running at 5am and strived to avoid snakes and handle hyperthermia. The temperature was well above 35 degrees Celsius (95 degrees Fahrenheit), and only after 60k had I understood what it means to cool like a pro. Most of the race I focused on keeping my core temperature low and managed to see a huge difference between certain parts of the race. During checkpoints 6 and 7 I almost sprinted and caught from behind many more experienced runners. That’s because I was cool.
Physiological Benefits of Endurance Running
Long distance running is potentially harmful. Many studies on marathoners demonstrate that the physiological and functional stresses of running 42.2 kilometers sometimes outweigh the benefits.
After completing a marathon, the average runner will have less weight to carry around. In addition to this, when the recovery process is finished, they will have stronger muscles, heart and bones. Other benefits of endurance running are more emotional or psychological in nature.
Even though running long distances is quite hard on the human body, each runner who makes it to the finish line after 42.2 km or more has personally validated the amazing miracle that is the human body.
To sum up, you might say we were born to run. But you also might just as well say we ran to be born. One foot in front of the other, for miles on end. Only a few of us run in this weird age of global urbanization and excessive technology, yet running – be it ultrarunning or sprinting – is deeply written into our DNA.