The Skeletal 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.


figure-44_med Adaptations in the cardiopulmonary system are a direct result of the skeletal muscles adjusting to the work they are performing. This in turn stimulates the heart and lungs to adapt. The effects of training on the cardiorespiratory system have traditionally been defined by VO2max and the concept of cardiac output. VO2max is the maximal amount of oxygen that the body can consume. it is defined as cardiac output X a-v O2 difference. Cardiac output can be defined as heart rate X stroke volume, where heart rate (HR) refers to the frequency with which the heart contracts and stroke volume is the volume of blood that is ejected from the heart with each contraction. The a-v O2 difference is the average difference between the oxygen content of the arterial and mixed venous blood (figure 4.4).

Endurance training can increase VO2max and cardiac output. This is a function of three key adaptations occurring in the body:

1. Total blood volume is increased.

2. The heart becomes stronger as a result of the work it is doing.

3. There is an improved delivery of oxygen to the muscles of the body.

As a result of these adaptations, the heart is capable of pumping larger amounts of blood more efficiently to the working muscles with every contraction. Oxygen is then delivered, and carbon dioxide, along with other by-products of metabolism, is removed more efficiently.

The increase in total blood volume that occurs with aerobic endurance training is a result of a two phase process. in the first phase, hormones stimulate an increase in total body water retention over a ten day period. The second phase is characterized by an increased production of red blood cells that occurs over an approximate four week period. The improvement in total blood volume benefits athletes through three different mechanisms:

1. An improved ability to regulate body temperature as the increased water content allows for improved heat dissipation and thus an increase in sweat rate.

2. An improved efficiency and functionality of the heart muscle.

3. An increased oxygen carrying capacity as a result of the increased number of red blood cells.


The heart is a muscle that responds to training much like that of the skeletal muscle. With training, a load can be imposed on the heart through increasing the number of times it must contract or the strength with which it contracts. With repeated contraction of the heart, this muscle becomes more efficient and stronger with each contraction. As a result, the heart does not have to contract as frequently to perform the same amount of work. In addition, prolonged engagement in aerobic distance training results in an enhanced stroke volume and therefore a reduction in resting and exercising heart rates at a specific workload. Using the formula for cardiac output (stroke volume x heart rate), it can be understood how heart rate is reduced at submaximal exercise intensities as a result of an increase in stroke volume. during maximal exercise, the increase in blood volume results in an increased maximal cardiac output and subsequently VO2max. Another benefit of a stronger heart and larger stroke volume is the ability for recovery to occur faster following a hard or near maximal bout of exercise.

The improvement in total blood volume and heart efficiency also results in an improved delivery of oxygen to the working muscles. In addition, the concentration of hemoglobin is increased because there is a growth in red blood cell volume. This increases the oxygen carrying capacity of the blood. The increase in blood volume results in an improved transit time for supplying oxygen to working muscles. Together, these produce an improved endurance performance.

Adaptations in the respiratory system also facilitate the improvements in VO2max and cardiac output. With training, the respiratory system becomes more efficient and can increase the amount of oxygen that is supplied with each breath. Ventilation is decreased as a result. Ventilation is a function of tidal volume and the frequency of breathing (tidal volume x frequency). Tidal volume is the volume of air that is inspired or expired with each breath. The primary means for a decrease in ventilation is the increase in tidal volume that allows for a lower frequency of breathing. The improved capacity of the lungs during exercise has been shown to be an important factor in improving endurance performance.