Systems of Energy Delivery and Utilization:
Skeletal Muscle and Cardiopulmonary Physiology
The human body requires a continuous supply of oxygen and nutrients to maintain the production of energy for its many complex functions and sustain work during exercise. This process is facilitated by the cardiopulmonary (cardiovascular + respiratory) system of the body that consists of the heart and lungs. These two key organs of the body work together to ensure that blood is carrying oxygen and other nutrients to active tissues (muscle, organs). This becomes especially important during exercise as oxygen and fuel are needed so that the skeletal muscles of the body can continue to contract and perform work. Aerobic and anaerobic training lead to general adaptations of the skeletal muscle system. As a result, the cardiovascular and respiratory systems also adapt. The functional capacity of these systems are not event specific since the improvements that are gained can be used in any mode of activity whether it is running, swimming or cycling.
Skeletal Muscle Physiology
Skeletal muscle contraction is controlled by the brain through nerve cells that originate in its motor cortex. it is this area of the brain that memorizes the training an athlete’s muscles are put through and commits it to memory. Each time an athlete executes a technical skill, this is reinforced to the nervous pathways that connect the brain and muscles that are being trained. This includes the rate at which the muscles were firing, which provides implications for the amount of time an athlete spends training at critical power levels and speeds. Athletes should be aware of these concepts, as poor technical practice of a skill or muscle training at low levels of power or speed can lead to long- term bad habits and compromise in power or speed production. Ultimately, it must be remembered that each time a muscle contracts, it is being programmed for its future functionality.
From the motor cortex of the brain, nerve cells submit impulses to the nerves that connect with motor neurons that directly innervate the skeletal muscle. The force produced from a muscular contraction is a function of the number and size of the motor neurons that are recruited and the frequency with which they are stimulated. Force production is also dependent on the type of muscle fibers stimulated to contract. There are three primary types of muscle fibers, slow-oxidative or Type I (SO), fast oxidative-glycolytic or Type IIA (FOG) and fast-glycolytic or Type IIB (FG). The type of muscle fibers served by a motor neuron are always of the same type, regardless of whether they are next to one another. A muscle is comprised of all muscle fiber types. The distribution of muscle fiber type is dependent on training, genetics and the function a muscle serves. Slow-oxidative muscle fibers are associated with endurance performance and are designed to promote the production of energy through the oxidative metabolic pathway. Fast oxidative-glycolytic and FG muscle fibers are associated with strength and power performance and have an increased capacity for glycolytic metabolism that is generated through glycogen and ATP/PCr that is stored within the muscle cell. In order to be successful, a triathlete requires a significantly higher proportion of SO muscle fibers. It is still critical, however, that they have and develop the capacity of both the fog and fg muscle fibers so that a full spectrum of energy capacity can be utilized. A complete list of characteristics for each type of muscle fiber can be seen in Table 4.2.
Muscle fibers operate based on a concept known as the sliding filament theory and are comprised of many proteins. The two key proteins needed for contraction are actin and myosin. Myosin has a head-like structure, and it binds to actin when a muscle fiber is stimulated to contract to create what is known as a muscle crossbridge. When multiple myosin heads are bound to actin, they serve in an oar type fashion to pull one protein past another. When myosin and actin ‘slide’ over one another, it causes muscle shortening. As a greater amount of force is required to be produced by the muscle, more and more muscle crossbridges must be formed.
The ability of a muscle fiber to contract is dependent on more than just a command from the higher brain centers. In the sport of triathlon, muscle fiber contraction is highly dependent on sustained oxygen delivery and fuel supply. The ability of a muscle to contract is dependent on several key factors, such as the temperature and acidity of the muscle and its environment. The temperature of the muscle is important because increases in muscle temperature (and potentially the body) can inhibit the ability of actin and myosin to bind and contract. In addition, the ability of the body to offload oxygen to the muscle is significantly decreased. Muscle fatigue occurs as a result.
The second and third factors that are important to optimizing muscle function is the rate at which hydrogen ions (H+) and lactate accumulate and the buffering capacity of the skeletal muscle. These two factors interplay as buffering capacity is related to the ability of the body to ‘soak up’ and tolerate hydrogen ions that are produced as a result of glycolytic energy breakdown. The accumulation of H+ is detrimental to the formation of muscle cross bridges as it interferes with the ability of actin and myosin to bind. The buffering capacity of the muscle is defined by the amount of bicarbonate (hCo3-) in skeletal muscle. The hydrogen ions that are produced can be ‘picked up’ by hCo3- and converted to h2Co3 that is immediately broken down by the body into h2o + Co2. Water is then recirculated into the body’s system, and carbon dioxide (Co2) is offloaded at the lungs. Lactate that is produced remains in the blood and either accumulates or is used as a fuel source by other tissues of the body. In addition, an accumulation of H+ results in a lowered pH, which reduces the ability of oxygen to be offloaded to the muscle. More muscle fibers must be recruited to complete any exercise bout as pH continues to decline and the oxygen cost of exercise continues to increase. Together, these factors result in fatigue.
A fourth factor is the electrical charge of the muscle (as indicated by the membrane potential, the potential to contract) and key ‘second messengers’ that signal for muscle contraction to be initiated. The electrical charge of the muscle is maintained by the key electrolytes, sodium and potassium. Calcium serves as the key second messenger that controls the gates, which allows for the influx and efflux of the electrolytes as well as the binding of actin and myosin. Muscle contraction is dependent on the membrane potential that can be maintained through the electrolytes and the muscles own resistance to fatigue. Every time a muscle contracts, it must depolarize and repolarize the membrane potential of the muscle. The ability of the muscle to continue this process is highly dependent on the concentration of electrolytes available to it and the ability of calcium to not become disrupted. Calcium transport can be disrupted through muscle damage, high levels of heat and other instances of extreme muscular stress. Sodium and potassium can be significantly lost in sweat during exercise, which is why it is so important that an athlete maintain fluid and electrolyte balance while in training and competition. Sodium depletion through a decrease in the consumption of sodium containing foods, drinking only water or improperly replacing electrolytes lost in sweat can lead to significant issues (muscle cramps, fatigue) during prolonged exercise, especially in the heat. These electrolytes are also key in assisting with muscle recovery.
The fifth key factor for optimizing muscle function is fuel supply. Muscles require energy in the form of ATP to be available from phosphocreatine, carbohydrate, fat and protein sources. As glycogen reserves become depleted, the ability to sustain muscle contraction begins to diminish unless carbohydrate is provided to sustain the energy needs of the muscle cell. During competition, athletes have been shown to progressively fatigue due to a lack of sufficient energy supply. It is therefore important to ensure that an athlete have a training and competition nutrition plan that sustains the energy needed.
The Cardiopulmonary System
The cardiovascular system consists of the heart and the many blood vessels that together make a continuous circuit that transports blood throughout the body. blood consists of plasma (~55-65%), leukocytes and platelets (~1%) and red blood cells (~38- 45%). Red blood cells (RBCs) are the key components of the blood that are responsible for transporting oxygen throughout the body and providing it to working tissues. There are trillions of RBCs in the human body. Within each rbC, there are approximately 250 million molecules of hemoglobin (hb). hemoglobin is the actual protein that serves to transport oxygen. it is a key component of providing oxygen enriched blood to the body’s tissues. Increases in rbC and hb are achieved through endurance training and an increased production of the hormone erythropoietin (EPO). This hormone is naturally increased when the body experiences
hypoxemia (oxygen supply is not meeting the demand of the body’s tissues), as is the case with altitude training. Professional endurance athletes travel intentionally or live at altitude in order to increase the number of rbCs and hb that are available to transport oxygen to the exercising muscles.
The pulmonary (respiratory) system is responsible for providing gas exchange through the lungs. oxygen transfers from the alveoli of the lungs into the blood with each breath, which can be seen in figure 4.2. Carbon dioxide, a by-product of metabolism, is simultaneously offloaded by the alveoli and into the ambient (room) air. The response of the pulmonary system to exercise is characterized by the rate of breathing (ventilation, VE) and the volume of oxygen consumed (VO2) and carbon dioxide (VCO2) produced.
The Effects Of Training on the Skeletal Muscle and Cardiopulmonary Systems
Adaptations to training require a repeated stimulus for approximately 3-4 weeks before the body has fully taken on the stress that has been applied to the skeletal and cardiopulmonary systems. Success in the sport of triathlon requires strong aerobic and anaerobic adaptations in these systems. Training adaptations for triathlon involve both local muscular endurance and ‘whole body’ cardiopulmonary endurance. it is the training performed by the skeletal muscles, however, that stimulates and drives adaptations of the heart and lungs. As a result, it is important that the effects of training be understood first and foremost with regards to the skeletal muscle.
Aerobic endurance training encompasses those adaptations that result from training at intensities that are at or below the anaerobic threshold (which is defined by the 4mmol/L-1 mark in the blood lactate response to exercise and serves as the highest intensity that a steady state of exercise can be maintained without significant rises in blood lactate). The intensity of training is intended to be only so high as to ensure that it can be sustained for periods of time that are similar to or greater than the actual competition duration. The goal of aerobic endurance training is to enhance the ability of the muscles to perform more efficiently through increasing and improving structural adaptations that promote the use of oxygen. This is accomplished by increasing the capacity of SO muscle fibers and potentially converting FG fibers to FOG fibers that can improve their use of oxygen.
There are four key structural adaptations that can occur as a result of aerobic endurance training. These include an increase in the number of capillaries supplying the muscle fibers, the number and size of the mitochondria in skeletal muscle, and concentrations of oxidative enzymes and muscle myoglobin content. Capillaries are the very small blood vessels that are embedded deep within the skeletal muscle. They directly transport oxygen and nutrients (carbohydrate, etc.) and remove carbon dioxide and metabolic by- products, such as lactate and hydrogen ions. An increase in the number of capillaries that surrounds the muscle promotes oxygen delivery. it is the increase in myoglobin that improves the ability of the muscle to utilize oxygen by accepting it for transport to the needed areas of the muscle, primarily the oxidative pathways that exist in the mitochondria. Mitochondria utilize the oxygen that is delivered to create ATP through the oxidative metabolic pathways of the krebs cycle and through beta-oxidation (Figure 4.3). These pathways are further enhanced by the increase in oxidative enzymes that occur. As a result, the body is able to increase the utilization of fat as a fuel source during exercise. This allows for an increase in the amount of energy derived through aerobic metabolism and also serves to spare muscle glycogen, both of which are critical to sustaining performance in endurance events.
Anaerobic interval training for endurance events serves to increase the amount of energy that can be efficiently produced through anaerobic glycolysis and the ATP-CP energy systems. Interval training also serves to increase skeletal muscle buffering capacity and should be designed to improve power, strength and anaerobic capacity. This type of training is frequently referred to as high intensity interval (hiT) training and is comprised of repeated bouts that are short to moderate in duration (30 seconds to 5 minutes). The training intensities associated with this form of endurance training are those above the anaerobic threshold and predominantly based on critical power outputs and paces that are necessary to sustain during competition. Following a period of HIT, it has been seen that athletes can perform the same workload with lower levels of lactate and a decreased rate of perceived effort or exertion (RPE). In addition, higher work intensities can be sustained for longer while also tolerating greater levels of lactate.
The improvements in work capacity that have been reported with HIT are a result of three key adaptations. The first key adaptation is an increase in the enzymes associated with the production of ATP through anaerobic glycolysis and the ATP-CP energy system. This allows for an increase in the utilization and oxidation (generating energy through oxygen based metabolism) of carbohydrate as a fuel source. high intensity interval training increases the use of the oxidative energy pathways and decreases the amount of lactate that is spilled into the blood at a specific workload. This is a result of carbohydrate continuing through the Kreb’s cycle to further generate ATP. The ability to utilize more carbohydrate is a result of increasing the number of muscle fibers that are recruited to do work and thus increasing work capacity. As a result, higher levels of lactate are produced at the end of a maximal effort, which indicates an increased anaerobic capacity. Secondly, the accumulation of high lactate levels stimulates an increased development of bicarbonate, hCo3-. As was discussed previously, hCo3- is used to soak up h+ that are produced during the breakdown of carbohydrate through non-oxidative energy production. As a result of increased HCO3- levels, an increased number of h+ can be removed. This allows for a greater number of muscle crossbridges to continuously be formed and more forceful muscle contractions to be sustained, resulting in improved performance. Lastly, there is a significant decrease in the core body temperature that is reached following hiT. As a result, the body is not accumulating heat and impairing muscle function. This creates a higher sustainable work output.
The development of neuromuscular patterns has also been suggested to be one of the benefits of HIT. These type of training activities facilitate adaptations in the neuromuscular patterns recruited during race pace activity. As was mentioned earlier, the brain has a region known as the motor cortex. Within this region of the brain, muscular patterns and the number of motor units required to perform them are stored. During competition, these patterns are called upon to facilitate performance. Those muscular patterns that have been utilized the most predominate during this time of physical stress.
Another means of improving performance through adaptations in the skeletal muscle is through resistance training. There are three key goals with this type of training. The first is to improve muscular strength as defined by the maximum force that can be generated by a muscle or group of muscles. A second goal is to improve muscular power, which is the explosive aspect of strength, by incorporating a specific movement at a given speed. The third goal of resistance training for endurance athletes is to improve muscular endurance. This is defined as the ability to sustain repeated muscular contractions at a fixed workload for an extended period of time. Increasing the amount of force that can be produced at a given speed and improving the ability to sustain that force over a distance produces an improvement in performance as a result of lessening fatigue.