How Your Muscles Change With Exercise

Muscle Adaptation: A Deep Dive

Key Concepts: Skeletal muscle, muscle fibers (cells), slow twitch fibers (Type I, slow oxidative), fast twitch fibers (Type IIB, fast glycolytic), ATP (Adenosine Triphosphate), capillaries, myoglobin, mitochondria, glycolysis, motor units, hypertrophy, muscular endurance.

1. Muscle Structure and Function

• Skeletal muscles are attached to the skeleton via tendons and facilitate movement through contraction.
• Muscles are composed of hundreds to thousands of muscle cells, also known as muscle fibers.
• Muscle fibers contract, causing movement at the joints where the muscles are attached.

2. Types of Muscle Fibers: Slow Twitch vs. Fast Twitch

• Slow Twitch (Type I, Slow Oxidative):
  • High aerobic capacity, utilizing oxygen to break down carbohydrates, fats, and proteins to synthesize ATP.
  • Characterized by a high number of capillaries for oxygen and nutrient delivery.
  • High myoglobin content for binding and storing oxygen within the muscle fibers.
  • Packed with a high number of mitochondria, the organelles responsible for ATP synthesis using oxygen.
  • Produce approximately 36 ATP per glucose molecule when oxygen is present, compared to 2 ATP without oxygen.
  • Smaller diameter, contract with less force, and have a slower contraction speed compared to fast twitch fibers.
  • Highly resistant to fatigue, excelling in muscular endurance.
  • Well-suited for activities like long-distance running, yoga, low-weight high-repetition weight training, and maintaining posture.
• Fast Twitch (Type IIB, Fast Glycolytic):
  • Fewer capillaries, lower amounts of myoglobin, and fewer mitochondria compared to slow twitch fibers.
  • Larger in diameter and store more glycogen (storage form of glucose).
  • Contract with more force and velocity.
  • Primarily utilize glycolysis to synthesize ATP, breaking down glucose anaerobically (without oxygen).
  • Glycolysis produces only 2 ATP and lactic acid as a byproduct.
  • Fatigue quickly due to limited ATP production and lactic acid buildup.
  • Well-suited for short bursts of high-intensity activity like sprinting or explosive jumps.

3. Muscle Fiber Adaptation to Exercise

• Slow Twitch Fiber Adaptation:
  • Increase in the number of capillaries to deliver more oxygen and nutrients.
  • Increase in myoglobin content to bind more oxygen within the muscle fiber.
  • Increase in the number and size of mitochondria to process oxygen and make more ATP.
  • Increase in the number of transporters on the muscle fiber and mitochondrial membranes to facilitate nutrient and oxygen exchange.
  • These adaptations lead to improved muscular endurance.
• Fast Twitch Fiber Adaptation:
  • Adapt by getting larger, a process called hypertrophy.
  • Muscle fibers increase in size due to the addition of protein contractile units.
  • Requires adequate protein intake to support muscle growth.
  • A muscle with a larger cross-sectional area produces more force.
  • Tend to be able to store more glycogen, providing more glucose for glycolysis.

4. Motor Units and Muscle Fiber Recruitment

• Muscle fibers are organized into groups or units called motor units.
• A motor unit consists of a motor neuron (nerve cell) and the muscle fibers it controls.
• Motor units contain only one type of muscle fiber: either slow twitch or fast twitch.
• During weak contractions or when lifting light weights, the body recruits motor units containing slow twitch fibers first.
• As the weight or intensity increases, the body recruits more motor units, including those containing fast twitch fibers.
• Reaching a certain level of intensity is necessary to engage fast twitch fibers and stimulate adaptation.
• Intensity can be increased by increasing the weight or increasing the speed of the movement.

5. The Interplay of Fiber Types

• Most athletic activities engage both slow and fast twitch fibers, although one type may be dominant depending on the activity.
• Endurance runners primarily engage slow twitch fibers but may recruit fast twitch fibers during sprints or surges in speed.
• Sprinters primarily engage fast twitch fibers but may use slow twitch fibers during recovery runs.
• The percentage of fast versus slow twitch fibers varies between individuals and muscles.
• Postural muscles generally have a higher percentage of slow twitch fibers for endurance.

6. AG1 by Athletic Greens

• AG1 is a daily supplement containing 75 ingredients, including vitamins, minerals, superfoods, probiotics, and adaptogens.
• It can aid in muscle recovery and provide a boost in energy.
• NSF certified, ensuring the accuracy of the label.
• Athletic Greens offers a free one-year supply of immune-supporting vitamin D and five free travel packets with the first purchase.

7. Conclusion

Skeletal muscles are highly adaptable structures that respond to exercise by altering their composition and function. Slow twitch fibers adapt to improve endurance, while fast twitch fibers adapt to increase strength and power. The type of adaptation depends on the type of activity performed and the recruitment of specific motor units. Understanding the characteristics and adaptations of different muscle fiber types can help individuals optimize their training and achieve their fitness goals.