Muscle Structure and Function

Question 1

Provide an overview of muscle structure

Answer 1

• Sarcomere, myofibril, muscle fibre, fascicle, muscle
• Sarcomere: Smallest contractile unit, contain protein filaments (A and M) , between 2 transverse Z discs and H zone in middle
• Myosin: Thick filaments with globular heads (attach to actin)
• Actin: Thin filaments, twisted helix, tropomyosin and troponin
• Postural (higher type I)
• Power muscles (higher type II)

Question 2

What are the mechanisms of muscle contraction at rest vs contracting

Answer 2

• Rest: No connection between A and M, high affinity, unable to bind due to tropomyosin (covering bind site on A) and troponin (holds tropomyosin in place)
• Ca: Facilitates removal of T and T
• ATP: M head binds ATP and produces ADP, leads to cross bridge and power stroke

Question 3

Describe the process of sliding filament model

Answer 3

• AP triggers voltage sensitive proteins / Ca channels in sarcoplasmic reticulum to open
• Ca floods into cell, troponin + Ca bind → movement of tropomyosin away from active sites
• M heads + ADP + P + energy → head extends = binds to A
• Releases stored energy and M changes shape → pull on strands and shrink sarcomere
• Generate force (power stroke)
• ADP + P unbind → new ATP binds in place on M head → release from A, breakdown ATP to ADP + P and repeat process on another A
• Cross bridges detach, Ca unbinds and M cannot bind to A anymore

Question 4

What is excitation contraction coupling

Answer 4

• Sequence of events that begin with a neural impulse and end with contraction, excitation of a motor nerve, propagation of an action potential
• Events at the neuromuscular junction, calcium release from sarcoplasmic reticulum
• Sliding filament theory and muscle contraction
• Power production is the key determinant of recruitment

Question 5

What are type I muscle fibres

Answer 5

• Slow twitch (slow contractile speed)
• Small in diameter
• Low force production
• High oxidative (aerobic) capacity
• Many mitochondria / capillaries / myoglobin
• High aerobic enzyme activity and highly resistant to fatigue
• Dominant muscle fibre during exercise below CP
• Linear relationship between number of slow twitch and VO2max

Question 6

What are type II muscle fibres

Answer 6

• Fast twitch (fast contractile speed)
• Large in diameter
• High force production and glycolytic capacity
• Many glycolytic enzymes, greater glycogen and PCr stores
• Highly fatiguable, recruited during high-intensity exercise
• A: Few mitochondria, highly fatiguable, anaerobic, low efficiency, exercise higher than CP
• X: Many mitochondria, slightly fatiguable, combination, moderate efficiency, short high intensity exercise (<1min)

Question 7

How do fibres respond to training

Answer 7

• Aerobic training, conversion of type II X to type II A to type I
• Distance runners 70-80% type I
• Track sprinters 25-30% type I
• Non-athletes 47-53% type I

Question 8

How are muscle fibre types assessed

Answer 8

• Muscle Biopsy
• Site is anaesthetised, small incision is made (1-3cm)
• Needle inserted into muscle
• 10-100mg sample is extracted (grain of rice)
• From metabolically active sites
• Sample stained to identify fibre type

Question 9

What is the purpose of training

Answer 9

• Improvements in health and performance
• Cardiorespiratory fitness
• Muscular strength and endurance
• Body composition, flexibility, speed, agility, power, adaptation

Question 10

What is overloading

Answer 10

• Principle of training
• Training load beyond level athlete is accustomed to
• Progressive overloading is required for continued improvement
• Adequate recover and sufficient stress prevent overtraining

Question 11

What is adaptation and reversibility

Answer 11

• Principle of training
• Training adaptation is not permanent, decay once stimulus is removed
• Improvement only possible if stimulus increases over time
• Same load = adaptation and then plateau
• Excessive stimulus = inability to adapt, decrease in performance

Question 12

What is the FITT principle

Answer 12

• Training loads can be monitored / modified, assessing
• Frequency (how often)
• Intensity (how hard, energy expenditure, RPE)
• Time (how long)
• Type (training method(s))
• Higher relative training intensity increases max VO2

Question 13

What are the principles of training / their purpose

Answer 13

• Specificity
• Overload
• Reversibility
• Applying these principles promotes improvement through adaptation

Question 14

What is specificity

Answer 14

• Principle of training
• Responses to training are specific to type of exercise, muscle groups and energy systems used
• SAID Principle: Adaptations are specific to stimulus

Question 15

What is karvonens HR reserve

Answer 15

• HRR = difference between resting and maximal HR
• HRR = HR max – HR rest
• Training HR (THR) should be 80-90% of HR max for endurance athletes