Top Endurance

The following sections examine the effects that each of these methods of training have on skeletal muscle.

Aerobic or Endurance Training
Endurance training stresses and challenges Type I (slow oxidative) muscle fibers more than Type IIX (fast glycolytic) muscle fibers. As a result, Type I fibers tend to enlarge with endurance training. Although the percentage of Type I and Type IIX fibers does not appear to change, endurance training may cause Type IIX fibers to take on more of the characteristics of Type IIA (fast oxidative glycolytic) fibers if they are regularly recruited during exercise.

The number of capillaries supplying each muscle fiber increases with endurance training. Recall that the capillary bed is a microscopic, meshlike structure that is embedded deep in the muscle. The capillaries serve as a “transfer point” through which oxygen and nutrients (e.g., glucose) are delivered to the exercising muscles via arterial blood, while carbon dioxide and metabolic by-products (e.g., lactate and positively charged hydrogen ions) are removed via venous blood. This delivery-and-pickup process is enhanced if the number of capillaries is increased, thereby allowing the exercising muscles to perform more efficiently.

Endurance training increases both the number and size of the mitochondria in skeletal muscle. This is particularly true for Type I (slow oxidative) muscle fibers. As described earlier, the mitochondria are microscopic, capsule-shaped units located in the muscle cell (see figures 1.7 on page 5 and 1.13 on page 18) that are essential for the production of ATP via the oxidative phosphorylation energy system (figure 1.6 on page 5). By increasing both the size and number of mitochondria, endurance training enhances oxidative energy production.

The activity of many oxidative enzymes is increased with endurance training. Figure 1.6 is a simplified representation of the oxidative energy system. It shows how 2 molecules of acetyl CoA enter a mitochondrion and move into the Krebs cycle, then on to the electron transport system (ETS) to produce 32 molecules of ATP. The Krebs cycle is a series of biochemical reactions essential to the production of the 32 molecules of ATP, which are ultimately synthesized in the ETS. Most of the oxidative enzymes that are enhanced via endurance training are located in the Krebs cycle phase of the oxidative phosphorylation energy system. Similar to the increase in mitochondria, an increase in the activity of oxidative enzymes serves to enhance oxidative energy production.

Finally, endurance training increases muscle myoglobin content by 75 to 80 percent. Myoglobin is the “smaller brother” of hemoglobin and has many similar structural characteristics. Like hemoglobin, myoglobin’s primary physiological function is to transport oxygen. Whereas hemoglobin carries oxygen from the lungs to the exercising muscles via the bloodstream, myoglobin picks up oxygen after it has been dropped off in the capillary bed by hemoglobin. Next, myoglobin transports oxygen to the mitochondria, where the oxygen is used to produce ATP. By increasing myoglobin content, endurance training enhances oxygen delivery within the exercising muscle.

Anaerobic or Sprint Training

Anaerobic training increases ATP-CP and glycolytic enzymes in skeletal muscle. Some of these enzymes are shown in figure 1.4 (immediate energy system; on page 3) and 1.5 (short-term energy system; on page 4). Similar to the increase in the oxidative enzymes, increases in the ATP-CP and glycolytic enzymes will serve to enhance the production of ATP by those two energy systems.

Another benefit of anaerobic training is that it can increase the buffering capacity of skeletal muscle. As described earlier, lactate and positively charged hydrogen ions (H+) are metabolites that are produced in the glycolysis energy system. High concentrations of H+ can slow down the release of calcium (Ca2+) in the excitation phase of skeletal muscle contraction, thereby contributing to premature fatigue of the muscle. As a result of anaerobic training, the amount of bicarbonate (HCO3–) in skeletal muscle is increased. As shown in figure 1.16, bicarbonate acts as a very effective buffer for reducing acidosis in the exercising muscle. Bicarbonate essentially picks up the potentially detrimental H+ and subsequently removes it safely from the body in the form of H2O and CO2.

Resistance Training

Athletes may use various types of resistance training programs (e.g., heavy weight and low reps versus moderate weight and high reps), and many physiological adaptations occur as a result of regular resistance training. Regular resistance training means a minimum of 3 to 5 training sessions per week for at least 8 weeks.

An increase in the actual size of the skeletal muscle fiber is known as hypertrophy. Most research studies have shown that regular resistance training in combination with an adequate diet will produce skeletal muscle hypertrophy. The degree of hypertrophy will vary depending on the specific resistance training program (weight, reps, number of training sessions per week, and so on).

Regular resistance training also increases the number of muscle motor units that are active. A single muscle motor unit is made up of several muscle fibers along with the nerve that innervates those muscle fibers and stimulates them to contract in unison. To better understand how muscle motor units work, just think of a group of 10 athletes pulling on a rope tied to a car. Each individual athlete can be compared to an individual muscle motor unit. (In this case, the term motor refers to movement, not the motor of the car.) If all 10 athletes are pulling hard on the rope, this represents a much stronger “muscle” than if only 5 of the 10 athletes are pulling on it because there are twice as many “active muscle motor units.” The same is true in relation to resistance training. After several weeks of resistance training, the skeletal muscle has produced more active motor units (or people pulling on the rope) than there were before the person started the resistance training program.

Muscular strength is increased as a result of resistance training. Muscular strength is defined as the maximum force that is generated by a muscle or muscle group. Muscular strength is usually measured using a one-repetition maximum (1RM) lift, or the maximum amount of weight that an individual can lift just once. An athlete who can bench press 300 pounds in a 1RM has twice the muscular strength as an athlete who can bench press 150 pounds in a 1RM.

Resistance training also increases muscular power, which is not the same as muscular strength. Muscular power is the explosive aspect of strength and is the product of muscular strength and the speed of a specific movement. For example, athlete A and athlete B can both bench press 150 pounds in a 1RM. However, athlete A completes the lift in 1 second whereas athlete B can complete the lift in 1/2 of a second. Although athlete B has the same muscular strength as athlete A, he has twice the muscular power because he can lift the same weight in half the time.

Muscular endurance is another important performance characteristic for endurance athletes, and this characteristic is also enhanced via regular resistance training. Muscular endurance refers to the capacity to sustain repeated muscular actions, such as when running for an extended period of time. It also refers to the ability to sustain fixed or static muscular actions for an extended period of time, such as when attempting to pin an opponent in wrestling. Muscular endurance is usually measured by counting the number of repetitions that an athlete can perform at a fixed percentage of the athlete’s 1RM. For example, if the athlete bench presses 200 pounds in a 1RM, the athlete’s muscular endurance can be measured by counting how many repetitions the athlete can complete at 75 percent of the 1RM (150 pounds).