Muscular Adaptations resulting from Endurance and Aerobic Training
Muscle tissue adapts to endurance/aerobic level training in many unique and useful ways. On the physiological side, trained muscle tissue is able to extract more oxygen from the blood supply and, combined with changes in the control of energy metabolism, has increased capacity for work. Other endurance/aerobic training adaptations like increased cardiac output, tolerance to lactic acid and neurological adaptations combine with specific muscle adaptations to improve performance. As with all training adaptations, muscular adaptations are substantially influenced by the intensity, duration and frequency of training. In almost every case, within the population of healthy athletes, optimal muscle adaptations and enhanced performance will continue to occur with regular and appropriate training.
The ability to sustain continuous, maximal aerobic capacity (MAC) cycling effort requires a functional balance between energy use and energy supply. In the muscle cells, energy production occurs in specialized cells called mitochondria. The mitochondria break down a compound called ATP (adenosine triphosphate) and produce the energy needed for muscle contraction and as long as these energy needs are met, the muscle will not fatigue.
During moderate to high intensity work lasting longer than 8-10 minutes, energy needed to break down ATP is supplied primarily by oxidative (aerobic) metabolism. The fuels needed in this energy equation include oxygen, fatty acids and carbohydrates. Oxygen comes from the exchange of blood gases in the lungs (carbon dioxide for oxygen) and is carried by the red blood cells.
Carbohydrates and fatty acids are derived from the diet or broken down from stores in the body (liver and muscle glycogen and body fat respectively). These fuels are transported in the blood to the capillaries surrounding the muscle cells where they are released from and taken up by the mitochondria.
Aerobic training stimulates muscular adaptations that influence all the processes controlling energy production and use providing the foundation for improved physical performance for endurance athletic activities.
Muscle Fiber Type and Function
Skeletal muscle is comprised primarily of slow-twitch (Type I) and fast-twitch (Type II) muscle fibers. The proportion and function of these fiber types is critical to both athletic performance and event specialty. The type and distribution of muscle fibers is a major factor in determining which type of athletic activity an individual is best suited for. Understanding the differences in fiber type and function leads to a better approach to training these different fiber types for their intended work.
Slow-twitch (Type I) muscle fibers are fueled by oxidative (aerobic) metabolism. They have relatively high blood flow capacity and high capillary density as well as high mitochondria content. These fibers are incredibly resistant to fatigue as long as they have adequate blood flow. Endurance athletes, in all sports, have a very high percentage of Type I fibers giving these athletes a high potential for improved performance.
Fast-twitch (Type II) muscle fibers are commonly divided into two primary subtypes-Type IIa and Type IIb. Type IIa fibers are similar to Type I fibers in that they have fairly high capillary density and blood flow capacity and mitochondria content. They function well under endurance (aerobic) loads with a high capacity for oxidative metabolism and are relatively resistant to fatigue. Type IIb, on the other hand, are very different. They have comparatively low capillary density and blood flow capacity and low mitochondria content and, while they produce the greatest force, they fatigue rapidly when recruited for maximal contraction.
At low intensity, Type I fibers are doing the majority of the work with some Type IIa fibers assisting. At moderate intensities you will continue to recruit more Type I fibers but will also engage a higher percentage of Type IIa fibers to produce more power. The highest intensity will see the optimal recruitment of both Type I and Type IIa fibers and the addition of Type IIb fibers for maximal efforts. All three of these fiber types are critical to high-level cycling performance.
While there are very measurable adaptations in skeletal muscle as a result of training, is has not been demonstrated that training causes a substantial change in the characteristics and function of specific fiber types or the distribution of these fibers in the muscle groups effected by training. For elite level endurance athletes, the very high percentage of Type I fibers (from 70-90%) can be accounted for genetically and are not considered to be an adaptation to training. The same is true for power athletes and their high percentage of Type II fibers. Of the 3 primary fiber types, Type I fibers are the most trainable and have the greatest potential for improving performance in cycling by developing their ability to produce power aerobically.
The Effects of Aerobic Training on Mitochondria Content
A critical adaptation to endurance/aerobic training is an increase in the size and number of mitochondria throughout the trained muscle fiber resulting in an increased capacity for aerobic metabolism from oxidation of both fatty acids and carbohydrate for endurance level work. This adaptation occurs in both Slow-twitch (Type I) and Fast-twitch (Type IIa) fibers when they are exposed to regular training at appropriate duration and intensity.
The Effects of Aerobic Training on Muscle Capillarization
Another useful training adaptation involves the increase in the number of capillaries (very small blood vessels) surrounding individual muscle fibers, known as capillary density. As the demand for blood flow by working muscle increases the body adapts by increasing the number of capillaries surrounding the individual muscle cells. The number of capillaries surrounding a muscle fiber improves the oxygen exchange capacity between capillary and fiber by providing a greater surface area for the gas exchange to occur and by reducing the distance oxygen molecules must travel after they are released from the red blood cells. Increased capillarity density and uptake of oxygen by active muscle tissue accounts for much of the increase in maximal oxygen consumption and improvements in Vo2 observed in endurance athletes.
The Effects of Aerobic Training on Blood Flow Capacity
Even as the muscle fibers develop new capillaries and become more efficient at extracting and using oxygen for energy this capacity is still limited by the amount of blood that can flow past the muscles, known as “blood flow capacity”. The normal blood flow capacity of skeletal muscle is exceptionally high. However, maximum cardiac output (the amount of blood moved by the heart) would be unable to fill all the blood vessels and capillaries in your muscle mass if they were maximally dilated. So, even during intense exercise that requires maximal oxygen consumption and blood flow, the limitations of cardiac output mean that only a portion, albeit a large one, of an individual’s entire muscle mass can be active at any time. And even then, the muscle being recruited is limited by blood flow capacity and the availability of oxygen.
Even with the limitations to cardiac output the peak flow capacity of muscle does increase as a result of endurance training. Research suggests several causes for this improvement. It is likely that the vascular adaptations and increased capillarization, combined with increases in cardiac output help to optimize blood flow and provide more nutrients and fuels for exchange between capillaries and muscle fibers. These adaptations, combined with efficiencies in metabolic processes, contribute greatly to the noticeable increased performance in endurance-trained athletes.
The Effects of Duration, Intensity and Frequency of Training
It is clear that muscle adapts to training in a variety of ways. The quality of these adaptations is determined by several influences. Setting aside discussion of diet, rest, recovery and competition influences, the three factors that most greatly effect muscular adaptations are the duration of a workout, the intensity of the efforts and how often the workout is repeated over a given training cycle. Manipulation of these factors has a dramatic influence on the biomechanical and physiological adaptations that occur in skeletal muscle.
Mitochondrial content is most affected by the duration of the workout session and prolonged stress on the aerobic system. The longer the workout the more potential there is for increases in mitochondrial content in the muscle fibers effected by training. However, there is a limit to the effective gains in any particular training cycle. Another factor effecting increases in mitochondrial content is level of fitness. Untrained individuals will see more rapid increases while trained individuals, already very fit, will see improvements but in smaller increments and at slower rates.
It is important to remember that many different training applications are needed to produce noticeable gains in performance. The proper mix of duration, intensity and frequency of workouts are needed to produce measurable improvements. This often involves a complex combination of training stresses intended to stimulate positive changes in the areas already mentioned; neuromuscular control, cardiovascular efficiency, cardiac output, muscle fiber recruitment, energy metabolism, capillarization and increased blood flow.
The Effects of Detraining on Muscle Adaptations and Aerobic Conditioning
If increased duration, intensity and frequency of training can increase fitness than it stands to reason that a decrease in these elements will, over time, cause an athlete to loose some of the gains made in training. This condition is called “detraining”. More specifically, a restriction in just one of these training elements, resulting from injury, illness, schedule conflicts or an extended absence from competition can cause a decrease in progress and/or performance. The good news is that, in healthy individuals, the detraining process can be fully reversed and previous levels surpassed with the resumption of training.
When designing a training plan it is important to remember that adaptations to training stresses take time to manifest into actual performance on the bike. The detraining/retraining cycle is evidence of this principle. The complete list of adaptations resulting from endurance training is complex and covers many areas of biology, biomechanics, anatomy and physiology. The specific adaptations in skeletal muscle are more basic and closely linked to metabolic changes, increased blood flow and increased fiber recruitment. Event specific training, at varying intensities and under varied conditions, will stimulate the adaptations you need to increase your performance and maintain steady progress over several seasons of riding or competition.