Skeletal muscle adaption to exercise

Question 1

Fiber types 1

Answer 1

• Slow twitch fibers (red, type 1) and fast twitch fibers (white, type 2a and 2b)
• Everyone has 45-55% type 1 fibers, no gender differences exist but large variation from person to person
• The aMN innervating a muscle fiber dictates which fiber type the fiber will become
• Endurance athletes have mostly type 1 fibers, where as sprint athletes/weight lifters have mostly type 2

Question 2

Fiber types 2

Answer 2

• Type 1 is slow to fatigue, much more myoglobin (aerobic)
• Type 2 is fast to fatigue, less myoglobin (2b: anaerobic, 2a: anaerobic and aerobic)
• Weight lifters/power athletes have enlargement of both fiber types
• Both endurance athletes and weight lifters demonstrate a conversion from 2b to 2a
• Larger muscle fibers and total muscle mass are the principle gender differences

Question 3

Muscles and aging

Answer 3

• With aging there is a progressive decrease in the number and area of type 2 fibers
• Reduction of muscle mass and actual loss of motor units (denervation muscle atrophy), particularly in lower extremities
• But age presents no barrier to training adaptations of muscles
• Concentric strength declines sooner than eccentric strength, and arm strength declines more slowly than leg strength
• Highest strength levels: 20-40

Question 4

Methods to enhance force production

Answer 4

• Neuromusclular adaptations
• Changes in muscle fiber type proportions and size
• Muscle hypertrophy
• Muscle hyperplasia

Question 5

Neuromuscular adaptations

Answer 5

• Initial (first 4-6 weeks) results of resistance training are primarily due to neuromuscular adaptations
• Later (by 6 wks) muscle hypertrophy plays a larger role in increasing force production
• 5 primary neuromuscular adaptations: 1) increased motor unit recruitment, 2) increased firing frequency of motor units, 3) increased motor unit synchronization, 4) increased activation of synergistic muscles, 5) increased inhibition of antagonistic muscles
• CNS and psychologic factors are important contributors to muscle strength

Question 6

Changes in fiber type, hypertrophy, and hyperplasia 1

Answer 6

• Resistance training causes significant increase in fiber area of both type 1 and 2 (but mostly 2) fibers
• Hypertrophy of muscle predominantly caused by increased protein synthesis that results in larger muscle size
• Hypertrophy increases the nuclei in muscle cells and the synthesis of protein filaments that constitute the sarcomeres
• Eccentric muscle contraction results in more hypertrophy than concentric contractions

Question 7

Changes in fiber type, hypertrophy, and hyperplasia 2

Answer 7

• There is no increase of mitos the ratio of mito to myofibrillar protein volume decreases (only for resistance training)
• Aerobic training improves cardiovascular fitness by increasing mito volume, # of mito, mito nzs, and capillary density
• No evidence yet of hyperplasia in humans

Question 8

Exercise-induced muscle damage

Answer 8

• Delayed-onset muscle soreness (DOMS) due to:
• Microscopic tears in muscle tissue (causes release of creatine kinase, myoglobin, and troponin I)
• Osmotic press changes (edema)
• Muscle spasms
• Over stretching and tearing of connective tissue,
• Acute inflammation

Question 9

Exercise-induced calcium entry

Answer 9

• When there is damage to the muscle fiber, large amounts of Ca may enter the cell
• The Ca may cause an autolytic process that degrades both contractile and non contractile structures

Question 10

Factors contributing to development of muscle

Answer 10

• Genetics
• NS activation
• Physical activity
• Nutrition
• Endocrine
• Environment
• Adrogens (steroids): increases protein synthesis and inhibits protein degradation
• Inhibition of myostatin: myostatin is a negative regulator of muscle mass (levels are elevated in wasting diseases), thus inhibiting it can increase muscle mass