Several bundles of muscle fibers, called fascicles, comprise a skeletal muscle. A muscle fiber is composed of several myofibrils bundled together. Myofibrils contain a series of sarcomeres arranged end-to-end (20).

Sarcomeres are the functional and contractile component of skeletal muscle through a dynamic interaction between the proteins actin and myosin.

According to the sliding filament theory, actin and myosin slide past each other to produce sarcomere shortening. Ca⁺⁺ is released in the sarcomere in response to an action potential that exposes myosin crossbridge binding sites on actin. Myosin crossbridges bind to actin and pull actin filaments closer to the center of each sarcomere, producing force and stiffness within the skeletal muscle (20).

Force production by a skeletal muscle can be voluntarily graded. However, the muscle fibers innervated by one motor neuron (i.e., a motor unit) act in an all-or-none fashion. As more motor units are recruited in a particular skeletal muscle, the muscle produces more force (20).

There are several different muscle fiber types based on structure and function. These include Type I, Type IIA, and Type IIB.

Type I muscle fibers (slow-twitch, oxidative fibers) have high mitochondria content and a rich blood supply. These fibers are generally smaller in size (diameter) and innervated by smaller diameter axons. Type I fibers contract slowly and are resistant to fatigue because their method of energy metabolism is aerobic. However, Type I fibers produce less tension, resulting in less force production.

Type IIA muscle fibers (fast-twitch, oxidative-glycolytic fibers) also have high mitochondria content and are moderately capable of performing aerobic and anaerobic metabolism. This fiber type exhibits performance and metabolic characteristics of both Type I and IIB fibers.

Type IIB muscle fibers (fast-twitch, glycolytic fibers) have sparse mitochondria content and blood supply. They fatigue easily but are capable of producing higher force and tension. Type IIB fibers act in an anaerobic capacity during activity.

All muscle fibers within a motor unit have the same metabolic characteristics (fiber type). Therefore, motor units can be classified as slow contracting/fatigue resistant (Type I), fast contracting/fatigue resistant (Type IIA), and fast contracting/fatigable (Type IIB). In general, fast-twitch/fatigable motor units are largest (contain the greatest number of innervated fibers) and are highest threshold, whereas slow-twitch/fatigue-resistant motor units are smallest (contain the fewest number of innervated fibers) and are lowest threshold.

Muscle fiber type composition in a particular human skeletal muscle is genetically determined (20). There is little information regarding muscle fiber type transformations in response to training or exercise.

Specific training programs have been shown to be effective in targeting improvements in performance of specific fiber types. Heavy resistance training has been shown to cause increased Type IIA fibers and decreased Type IIB fibers, whereas Type I fiber composition in human skeletal muscle was unchanged (1). Low-resistance, high-volume training caused improved muscular endurance, which is thought to indicate an improvement in Type I fiber performance (2,8).

Structural and genetic characteristics of muscle fiber types have been modulated with fiber-specific stimulation in vitro (19). However, it is unknown whether such changes occur in all muscle fiber types or if the transformation will be sustained over time in vivo.

Because muscle fiber composition is genetically determined, athletes may participate in sports or activities that involve muscle contractions that are more “natural.” Whether a distance runner can train to be a successful power lifter or vice versa is an issue that has not yet been clearly elucidated.

Skeletal muscle can produce joint movements through concentric and eccentric contractions. Skeletal muscle contractions produce muscle tension and control body and joint movement.

Concentric contractions describe a movement that involves shortening of a muscle against a load, whereas eccentric contractions involve controlled lengthening of a muscle against a load.

Eccentric muscle contractions produce greater muscle force and more myofibrillar disruption than concentric exercise (9,10).

Eccentric muscle contractions (often referred to as “negatives” in weightlifting) are more effective in producing strength gains and hypertrophy than concentric contractions, but they are more likely to cause delayed-onset muscle soreness (7,10,11). However, both eccentric (negative) and concentric (positive) contractions elicit gains in skeletal muscle strength and size (11).

Isometric muscle contractions produce muscle tension without joint movement — for example, pushing against a wall or contracting the quadriceps muscle while holding the knee motionless at a particular point in the knee range of motion.

Isotonic muscle contractions produce muscle tension and joint movement against a constant load where rate of movement is variable. For example, a dumbbell curl is a contraction against a constant load that can be voluntarily moved at a self-selected rate. This is the most typical contraction in weightlifting.

Isokinetic muscle contractions involve a constant rate of joint displacement that is maintained by varying amounts of resistance based on muscle effort. This is uncommon in weightlifting or athletic settings. Isokinetic exercise requires expensive machinery and is usually most applicable in the rehabilitation setting.

Isotonic and isokinetic muscle movements can be performed through concentric or eccentric muscle contractions.

Improvements in muscle strength, power, or endurance are best achieved by overloading the muscle(s) being trained.

The overload principle states that when a muscle is exposed to a stress or load that is greater than what it usually experiences, it will adapt so that it is able to handle the greater load (17,20,29).

Similarly, the SAID principle (specific adaptations to imposed demands) states that a muscle or body tissue will adapt to the specific demands imposed on it. For example, if a muscle is overloaded, its fibers will grow in size so it is able to produce enough force to overcome the imposed load (20,29).

Observed strength gains within the first few weeks of a weightlifting program are mostly due to neuromuscular adaptations (6). As exercise intensity increases and muscles begin to fatigue, the nervous system recruits larger motor units with faster contraction rates and higher tension-production capabilities to provide the force necessary to overcome the imposed resistance (20).

Muscular strength has been defined as the maximum force or tension output generated by a muscle or muscles during a specific task. Related to muscle strength, muscular power refers to not only the ability of a muscle or muscles to generate force, but also the rate at which force can be developed.

Early strength gains and increased muscle tension production from training result from a more efficient neural recruitment process, as well as more densely packed protein filaments within the skeletal muscle (24).

Human skeletal muscle hypertrophy occurs when the cross-sectional area of a muscle fiber increases (5,24). As a skeletal muscle hypertrophies, contractile proteins are synthesized (6), and the muscle is therefore capable of producing more tension.

Type IIA fibers exhibit the greatest growth, whereas Type IIB and Type I fibers exhibit the least amount of growth in response to heavy resistance training (6). Muscle hypertrophy is more common in fast-twitch than slow-twitch muscles.

Strength training leads to muscle hypertrophy, which increases muscle mass (24).

Muscle hypertrophy is typically observed with resistance training after 6-7 weeks of strength training (6,17).

There appears to be a gender difference in the rate at which muscles hypertrophy favoring males (12). Additionally, females lose muscle mass quicker than males when detrained (12).

Resistance-trained muscles hypertrophy in order to adapt to greater imposedloads (15). Hypertrophy of individual muscle fibers contributes to changes in muscle cross-sectional area (22). Muscle fiber hyperplasia does not appear to play a role in increased muscle cross-sectional area or strength gains in resistance-trained men (22).

In general, it is thought that strengthening exercises using a resistance that is greater than the 6-repetition maximum (RM) are best for muscle hypertrophy, whereas those at a resistance of less than the 20-RM are best for endurance training (16). Simply stated, fewer repetitions using higher weights are best for strength gains, whereas more repetitions using lower weights are best for endurance gains.

The DeLorme method is a progressive resistance exercise program based on the overload principle (6,26). This method is based on the 10-RM. First, a weight that the athlete is able to lift 10 times (with the desired muscle group[s]) is determined. A total of three sets of 10 repetitions are performed per session for each muscle at 50%, 75%, and
100% of the 10-RM. The athlete is encouraged to perform more than 10 repetitions during the third set to serve as an overload to the muscle group being trained. As the athlete’s 10-RM increases, so does the resistance in each set (see Table 110.1).