5.12: Exercise and Muscle Performance

Physical training alters skeletal muscle by increasing endurance or strength through cellular adaptation, while inactivity and aging lead to muscle atrophy. 

Master this section and you'll be able to

• Describe and compare the structural and cellular adaptations that occur in skeletal muscle in response to endurance versus resistance training how resistance exercise builds muscle.
• Distinguish between muscle hypertrophy and atrophy, and sarcopenia and analyze how lifestyle factors and age influence changes in muscle mass and performance.
• Explain how performance-enhancing substances affect muscle.

Muscle Training and Changes in Muscle Size

Physical training can change both the appearance of skeletal muscles and their performance. With regular exercise, muscles become stronger and often look larger and more defined. On the other hand, lack of activity causes muscles to weaken and shrink. Key terms:

• Muscle hypertrophy: growing without new cells.
  When muscles grow, they do not make brand-new muscle cells. Instead, existing muscle fibers add more structural proteins (like actin and myosin). This makes each fiber thicker, which increases the overall diameter of the muscle. This growth process is called hypertrophy. 
   
• Muscle atrophy: use it or lose it.
  This is the opposite of muscle hypertrophy. In atrophy, structural proteins inside the muscle fibers are lost, so the fibers get smaller. This leads to a visible decrease in muscle mass and weaker performance.
   
• Sarcopenia: Age-Related Atrophy
  When atrophy happens as a natural part of aging, it is called sarcopenia. This condition often starts slowly but can have big impacts on strength, balance, and independence if not addressed with regular physical activity.

Muscle fibers are highly adaptable. Their cellular components (aspecifically mitochondria, enzymes, and protein content) can change in response to how much, and in what way, the muscle is used. For example, endurance training increases mitochondria for better energy production, while resistance training boosts contractile proteins for strength.

 

Cellular Adaptations to Endurance Training

Slow-twitch oxidative (SO) fibers — also known as Type 1 fibers — are the “marathoners” of the muscle world. These muscle cells are not built for heavy lifting, but for long-lasting, repeated contractions. They rely mainly on aerobic metabolism, which allows them to keep working without fatiguing quickly.

Regular endurance training enhances stamina rather than bulk and makes the slow-twitch fibers even more efficient:

• Mitochondria multiply → more sites for aerobic metabolism, so more ATP can be made.

• Myoglobin increases → this oxygen-binding protein, stored in the sarcoplasm, acts like an oxygen reserve tank for mitochondria.

• Angiogenesis occurs → new capillaries form around the fibers, improving oxygen delivery and removal of waste.

Because oxygen and nutrients need to diffuse into the fibers, muscle mass does not increase dramatically with endurance training. This keeps the diffusion distance short and efficient.

The percentage of SO fibers in a muscle helps determine its role:

• Postural muscles (like those in the back) have many SO fibers to keep us upright.
• Endurance athletes (such as marathon runners) benefit from a higher proportion of SO fibers.

While endurance training has many benefits, it can also lead to overuse injuries, such as stress fractures and tendon and joint inflammation.

Figure \(\PageIndex{1}\): Elite marathon runners. Marathon runners and top athletes in other endurance sports, often show muscle biopsies with up to 80% slow-twitch fibers in key leg muscles (such as the soleus), compared to a more balanced mix in the general population. Whether this distribution is primarily genetically determined or the result of years of endurance training-induced adaptations remains a debated topic in exercise physiology. 

Cellular Adaptations to Strength (Resistance) Training

Resistance exercises, unlike endurance training, rely heavily on fast-twitch glycolytic (FG) — also known as Type 2 —  fibers. These muscle cells are designed for short bursts of powerful movements rather than long, repeated activity. Type 2 fibers contract strongly because they hydrolyze ATP rapidly and form many cross-bridges in a short amount of time.

Analogous to the marathon runners containing a much higher proportion of type 1 fibers, muscles specialized for power contain a higher proportion of glycolytic type 2 fibers.  

Regular resistance training enhances muscle bulk through hypertrophy.  Hypertrophy occurs when additional myofibrils and sarcomeres are added to existing fibers, and as a consequence, each muscle fiber gets thicker. Unlike endurance training, resistance training usually does not increase the number of mitochondria or capillary networks. Instead, it primarily builds structural proteins and connective tissue. Tendons also strengthen, so they can safely transmit the powerful force produced by muscles to bones.

 Figure \(\PageIndex{2}\): Visible muscle enlargement in Bodybuilder. Top resistance training athletes generally have a large number of type 2 fibers and relatively few type 1 fibers. Because this muscular enlargement is achieved by the addition of structural proteins, people trying to build muscle mass often ingest large amounts of protein.

Progressive Overload: Why Lifting Gets Harder

For muscles to continue growing, the training load must increase over time — a principle known as progressive overload. Lifting the same weight over and over will only maintain existing muscle mass. To stimulate further growth, the load must become heavier, forcing muscles to adapt to the new challenge. This is why strength athletes gradually add weight to their workouts.

Risks of Improper Resistance Training

While resistance training can be highly beneficial, it carries risks if done incorrectly.

• Overuse injuries can affect muscles, tendons, or bones.

• Damage often occurs if the load is too heavy, if recovery time is too short, or if joint alignment is poor during exercise.

• At the cellular level, intense training can damage the sarcolemma and myofibrils, contributing to the soreness felt after workouts.

Interestingly, this very repair process — where new structural proteins replace damaged ones — leads to stronger, larger muscles over time. However, overtraining without rest can result in tendon injuries or even skeletal damage.

 

Comparison of Endurance Training and Resistance Training Effects on Skeletal Muscle

Feature	Endurance Training (e.g., marathon running, cycling)	Resistance Training (e.g., weightlifting, sprinting)
Main Fiber Type Used	Slow Oxidative (SO) fibers	Fast Glycolytic (FG) fibers
Contraction Style	Low force, sustained over long periods	High force, short bursts
Energy Pathway	Aerobic metabolism (lots of O₂, many mitochondria)	Anaerobic glycolysis (less O₂, quick ATP use)
Adaptations in Muscle	↑ mitochondria, ↑ myoglobin, ↑ capillaries (angiogenesis)	↑ myofibrils and sarcomeres (hypertrophy)
Effect on Muscle Size	Little change in size (fibers stay lean for oxygen diffusion)	Significant increase in fiber thickness (bulky muscles)
Effect on Connective Tissue	Small changes	↑ connective tissue and tendon strength
Oxygen Storage & Delivery	Improved (more myoglobin + capillary networks)	No major increase
Performance Outcome	Better endurance and fatigue resistance	Greater strength and power
Injury Risks	Usually low (unless extreme overuse)	Higher risk if load is too heavy or technique poor (muscle tears, tendon damage, joint stress)

 

---

Performance-Enhancing Substances in Sports

Some athletes look for ways to boost their performance beyond what training and nutrition alone can provide. These performance-enhancing substances can affect muscle strength, recovery, and endurance, but they also carry major risks.

1) Anabolic Steroids: Muscle Builders

• What they are: Synthetic forms of testosterone, the primary male sex hormone.

• How they work: Stimulate muscle protein synthesis, leading to bigger and stronger muscles.

• Why athletes use them: To increase muscle mass and power output.

• Risks: Infertility, aggressive behavior (“roid rage”), cardiovascular disease, and even brain cancer. Many side effects are irreversible.

2) Erythropoietin (EPO): The Oxygen Booster

• What it is: A hormone normally produced by the kidneys that increases red blood cell production.

• How it helps: More red blood cells = more oxygen carried to working muscles = more aerobic respiration.

• Why endurance athletes use it: To delay fatigue and improve stamina in sports like cycling and distance running.

• Risks: Thickened blood, which increases the risk of dangerous blood clots, strokes, or heart attacks.

3) Human Growth Hormone (hGH): The Recovery Aid

• What it is: A hormone naturally made by the pituitary gland that promotes growth and tissue repair.

• How athletes misuse it: To speed up healing of muscle and connective tissues after intense training, so they can return to competition faster.

• Risks: Abnormal bone growth, joint pain, swelling, and increased risk of certain cancers.

4) Creatine: The Legal Edge?

• What it is: A compound stored in muscles as creatine phosphate, used to quickly regenerate ATP in the first seconds of contraction.

• How it may help: Provides a quick energy boost for short, explosive activities like sprinting or weightlifting.

• Controversy: Some studies suggest modest benefits, while others find little to no effect.

• Risks: Generally considered safer than steroids or hormones, but high doses may stress the kidneys.

The Bigger Picture: Ethics and Safety

• Most of these substances are banned in competitive sports because they give an unfair advantage. Using them without medical supervision is illegal and can cause serious, sometimes permanent health damage. The bottom line: training, nutrition, and rest remain the safest and most effective ways to improve performance.

 

Everyday Connection

Aging and Muscle Tissue

Sarcopenia (muscle atrophy with age) is inevitable and the primary reason why even highly trained athletes succumb to declining performance with age. This decline is especially noticeable in athletes whose sports require strength and powerful movements, such as sprinting, whereas the effects of age are less noticeable in endurance athletes such as marathon runners or long-distance cyclists.

The decline in muscle mass is significantly accelerated through a sedentary lifestyle and causes a loss of strength, including the strength required for posture and mobility. 

The good news is that sarcopenia can be significantly slowed, halted, and, in many aspects, reversed through lifestyle interventions:

• Physical activity is the most effective treatment. Progressive resistance training, which includes lifting weights, using resistance bands, or doing bodyweight exercises, has been shown to increase muscle mass and strength in older adults. Multimodal exercise programs that combine resistance, aerobic, and balance training are especially effective.
• A protein-rich diet supports muscle growth. Experts recommend that older adults, especially those with sarcopenia, increase their daily protein intake. The goal is to provide the body with the amino acids needed to build and repair muscle tissue, particularly when combined with strength training.

Figure \(\PageIndex{3}\): Muscle Atrophy. Muscle mass is reduced with disuse!