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. Even though running is considered a simple sport, the inner workings of the body at a skeletal, muscular, and cellular level are both intricate and fascinating. Here, a complete guide to running physiology will be discussed.
Energy: Where Does it Come From?
In order to put one foot in front of the other, a runner must first muster the energy required for simple locomotion. Where does this energy come from? Is the source important? Here, sources of energy are discussed.
In recent years the word “carbohydrate” has been met with scorn among fitness enthusiasts. Proponents of the Paleo and Whole 30 diets have led countless people to believe that carbohydrates are poison. However, for runners carbohydrates are a necessity.
Carbohydrates are stored – in the form of glycogen – in a runner’s muscles and liver. Additionally, carbohydrates exist in the form of glucose in the blood. During glycolysis, which is the breakdown of glucose and glycogen for energy during exercise, the blood, muscle, and liver stores of carbohydrates are depleted. Endurance athletes require adequate carbohydrate stores in order to stave off fatigue and the feeling of “hitting the wall.” Studies have indicated that consuming carbohydrates throughout an endurance event can delay fatigue, thanks to the steady stream of energy that carbohydrates provide.
Following an endurance activity it is important to refill the spent glycogen stores. Insulin controls glycogen synthesis, as well as the availability and uptake of glucose from the blood. This hormone is secreted from the pancreas and serves as the trigger for recruiting glucose from the blood into muscle cells.
Glucose is also utilized for making new glycogen. In general, the more quickly a runner’s muscles can be refilled with glycogen, the more quickly he or she will recover. Therefore, the higher a runner’s blood insulin concentration following exhaustive activity, the faster the glycogen stores can be replenished. The only way to trigger this process is through carbohydrate consumption.
When the human body has fully depleted its glucose stores, it turns to burning fat for fuel. This process is called ketosis, or ketogensis. Here, ketone bodies are produced due to the breakdown of fatty acids and ketogenic amino acids. This process mainly occurs in the mitochondria of liver cells, and only happens when there is an inadequate supply of blood glucose. When ketone bodies are produced, acetyl-CoA is oxidized by the citric acid cycle and then by the mitochondrial electron transport chain in order to release energy.
In the latter stages of an ultra-endurance event (or when too few calories are consumed on a daily basis), the body will break down protein for fuel as a last resort. This process occurs when glycogen stores are completely depleted. Here, the body will breakdown lean muscle mass in order to convert amino acids into glucose. Up to 15% of the required energy can be obtained in this manner.
Energy Metabolism: What are the Different Types?
Besides understanding how the body is fueled, it is also important to understand exactly how the energy systems work. Everyone has three energy systems:
The Phosphagen System
This energy system is important for providing the initial burst of energy required to sprint off the starting line or perform elaborate dynamic form drills. The phosphagen system does not require oxygen (i.e. it is anaerobic) and can only supply enough energy for durations of intense activity up to 5 seconds long.
This energy system is entirely dependent on creatine phosphate, which is found in both the skeletal muscle and the brain. There is a limited amount of creatine phosphate found in the body, and the stores are depleted quickly. Once the short supply of creatine phosphate has been used up, the body must recruit energy from another system in order to sustain activity.
The anaerobic system, also known as the glycolysis or lactate system, is important for activities that do not require oxygen. The anaerobic system provides energy to fuel 1 – 3 minutes of intense activity, such as sprinting. When the anaerobic system is utilized, glucose is broken down via glycolysis. One product of glycolysis is ATP, while another product is hydrogen. If oxygen is present in the body, the aerobic energy system will continually use hydrogen and another product of glycolysis, pyruvate, to produce additional ATP.
If there is not enough hydrogen produced by the aerobic system then pyruvate and hydrogen combine to form lactic acid, which enters the bloodstream and is removed by the liver. If lactic acid is produced faster than the lactate can be removed, the lactate threshold (aka anaerobic threshold) is met. Here, lactic acid will begin to accumulate in the bloodstream and inhibit the body’s ability to utilize fatty acid for energy. Thus, the body will be reliant on glycolysis.
High levels of lactic acid in the blood, in addition to glycogen depletion, leads to muscle fatigue.
The aerobic system is the energy pathway with which distance runners are most familiar. This system is slower, yet more efficient than the anaerobic or phosphagen systems, and can utilize carbohydrate, fat, or protein to produce energy. Additionally, this system is dependent upon the abundance of oxygen to produce fuel. Unlike the other two systems, the aerobic system is able to burn fat for fuel, which is more readily available in the body than carbohydrates, albeit less efficient to burn at high intensities. When training at lower intensity, the aerobic system will primarily burn fat in order to spare the more favorable carbohydrate conversion.
Ultimately, improving the body’s efficiency at utilizing each of the three systems is important for making necessary performance improvements.
Power: How Do We Propel Ourselves Forward?
Next, in order to sustain forward motion, a runner’s physiology involves the usage of different types of muscle fibers. Each individual will have a different composition of muscle fibers, which will dictate his or her strength as an athlete.
Slow twitch muscle fibers are most commonly associated with long distance runners. These muscle fibers are solely recruited for aerobic activity and are also called “red fibers” because of their darker appearance, due to an abundance of myoglobin. In addition to myoglobin, slow twitch muscle fibers also contain mitochondria, which are the organelles that utilize oxygen for adenosine triphosphate (ATP) production.
ATP is responsible for fueling muscle contractions. Unlike fast twitch muscle fibers, slow twitch muscles can sustain force for a long period of time, but are unable to generate large amounts of force. Their low activation threshold means they are the first to be recruited when a muscle contracts. When the amount of force required exceeds the output of a slow twitch fiber, the fast twitch muscles are then recruited.
Fast Twitch B
Fast twitch B muscle fibers are only used for short bursts of exercise that require a high amount of force. For instance, sprinting, hurdling, and jumping all require fast twitch muscle fibers. These muscle fibers are also called fast glycotic, because they rely solely on ATP stored within the muscles for the generation of energy.
This type of muscle fiber has a high activation threshold and is only activated when all other muscle demands have been exhausted. In comparison to slow twitch muscle fibers, they take a shorter amount of time to reach their peak force, but reach fatigue at a quicker rate. Whereas slow twitch muscle fibers are also known as “red fibers,” fast twitch fibers are “white fibers” because they contain less blood and have lighter-colored appearance. When it comes to muscle size and definition in athletes, fast twitch fibers are the ones responsible.
Fast Twitch A
Whereas fast twitch B fibers are solely recruited for anaerobic activity and slow twitch muscles are solely recruited for aerobic activity, fast twitch A fibers are the middle ground between these two fiber types. This muscle type is utilized for high-power, anaerobic activities that are prolonged. For instance, the 400 m and elite-level 800 m dashes mostly require fast twitch A muscle fibers.
Performance: What Factors Play a Role?
When it comes to physical performance, runners are limited by the fitness of their various aerobic, anaerobic, and cardiac systems. The most crucial components are:
VO2 max is one of the most commonly cited values when it comes to predictive measures for athletic performance. This measurement describes the maximum volume of oxygen that a runner’s muscles can consume per minute. While a high VO2 max does not necessarily mean that a runner will be an Olympian, it does mean there is potential for greatness, as an athlete cannot perform at a level greater than his or her current VO2 value.
VO2 max is dependent on a number of factors, most of which can be improved through specific training. The main determinant for VO2 max is cardiac output, which is defined as the amount of blood that is pumped by the left ventricle per minute. In turn, this value relies on stroke volume, which is the volume of blood pumped by the left ventricle per heart beat, as well as heart rate. Stroke volume depends on venous return, heart contraction rate and force, pressures in the left ventricle and aorta, and size of the left ventricle. Therefore, VO2 max is partly dependent upon genetics, since the size of the left ventricle cannot be changed.
The amount of oxygen that muscles can utilize depends on capillary and mitochondrial volumes. If a person has more capillaries in his or her muscle fibers, oxygen diffusion distance to the mitochondria (the organelles that provide energy to cells) will be faster.
The ability of the muscles to extract oxygen is due to the difference in the amount of oxygen going to the muscles via arterial circulation and the amount coming out via venous circulation. This difference is determined by the convection of oxygen through muscle capillaries and mitochondria. Athletes who can naturally shift their blood from inactive to active tissues will be able to utilize more oxygen from their blood.
However, while VO2 max is an important consideration for performance, other factors are just as crucial. By some theories, lactate threshold may be the best physiological distance running performance predictor. As a reminder, the lactate threshold occurs at the point in which a runner is in the sweet spot between anaerobic and aerobic metabolism, i.e. it is the fastest speed that an athlete can maintain while still running at an aerobic level. For long races, this value is especially important. Unlike VO2 max, lactate threshold is less dependent upon genetics and can be improved significantly with training.
An athlete’s running economy is the amount of oxygen that he or she consumes at submaximal speeds. For instance, when two athletes are running the exact same speed, they are likely consuming different amounts of oxygen, which is defined as their running economy. While many elite athletes have similar VO2 max values, running economy values differ significantly. As an example, Kenyan athletes have similar lactate threshold and VO2 max values as American and European runners; however, their running economies are measured to be much higher.
Ultimately, running economy determines how hard an athlete must work relative his or her maximum ability to utilize oxygen. While runners who have high VO2 max values do not necessarily have high lactate thresholds or superior running economies, athletes with exceptional running economy measurements always have a high VO2 max and lactate threshold.
Recovery: What Happens and Why?
The ability of an athlete to gain the effects of training is reliant on his or her ability to recover. There are two main components to recovery, which are degeneration and regeneration.
Degeneration – Regeneration Cycle
Muscles are among the most dynamic components in a runner’s body. They are constantly changing in order to meet the structural and functional constraints placed on them by the athlete. Their ability to continually change based on a runner’s needs is part of the degeneration – regeneration cycle. As a result of hard training, muscle fibers become damaged. When this phenomenon occurs, the cells that comprise the damaged muscle fibers essentially die and are replaced with newer, stronger, healthier muscle cells.
The process that triggers the degeneration – regeneration cycle is known as inflammation, which has recently become a buzzword in the endurance athlete community. While chronic inflammation can be harmful, acute inflammation is a necessary response.
When muscle tissue is damaged the immune system responds by sending macrophages to the area to consume the cellular debris left behind after cellular death. Next, satellite cells form along the borders of the damaged muscle fibers. The satellite cells contain stem cells that will differentiate into new muscle fibers.
In addition to newer, stronger muscle fibers, there are other benefits to the degeneration – regeneration cycle. Intracellular mitochondria increase, as well as the number of capillaries found in muscle tissue. For runners, this cycle is especially prolific because of the sheer number of muscle contractions that are involved in the activity, particularly eccentric muscle contraction. Here, an activated muscle is lengthened, which results in a high amount of force and tissue injury. However, without the subsequent tissue death (and muscle soreness), muscles would not be able to grow back stronger than before.
Battle of the Sexes: Is Physiology Different for Men and Women?
While running performance for both men and women are largely defined by VO2 max, lactate threshold, and running economy, there are special considerations – particularly when it comes to differences in hormones – that changes the physiology of a male versus a female runner.
For instance, research suggests that women adapt to exercise differently than men. When both men and women were trained in the same manner for a year, the training response of women was found to plateau after initial improvements while men continued to improve over time. Overall, this meant that men had a relatively greater increase in aerobic capacity than women at similar training volumes. However, scientists speculate that improper fueling could potentially be holding female athletes back when it comes to greater muscle adaptations. In general, female athletes tend to under-fuel, which limits the body’s ability to build muscle and sustain muscle growth. In comparison, male athletes do a better job in reaching their caloric and macronutrient requirements.
While women may experience a disadvantage relative their male counterparts in training, other research has found that women are superior when it comes to marathon pacing. Out of 2,929 runners, male athletes ran an average of16% slower in the second half of their race than in the first half, while women only slowed their pace by 12%.
Researchers hypothesize that women do not experience glycogen depletion to the same extent as men, and that women are better at utilizing fat for fueling. Additionally, men may be more susceptible to overheating, which could affect second-half times to a greater extent.
Ultimately, running involves a complex number of physiologic responses. Although simple in theory, there are a variety of behind-the-scenes processes that must occur in order for a runner to optimally perform.