Muscular Contraction (Lec 8) Flashcards
Contractile Proteins
Myosin (Thick filaments):
− Globular heads
Actin (Thin filaments):
− Actin
− Tropomyosin
− Troponin
Mechanisms of Muscle Contraction
- Myosin head binds to an active site on actin (‘cross bridge’)
- Myosin head moves pulling actin filaments towards the centre of the sarcomere (‘power stroke’)
- Sacromere shortens, muscle shortens, force is generated
- Cross bridges detach
- Dependent upon sufficient Ca2+ and ATP
Role of Ca2+
- At rest, myosin and actin are unable to bind d/t tropomyosin and troponin
- Ca2+ binds to troponin → moves tropomyosin away to expose myosin binding site on actin
Role of ATP
- Myosin head contains a binding site for ATP
- ATP → ADP + P = cross bridge + power stroke
- A new ATP binds to myosin to release it from actin
Excitation-Contraction Coupling
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
- Muscle contraction
Muscle Fibre Types
- Type I fibres (slow twitch)
- Type II fibres (fast twitch)
- Type IIa
- Type IIx (IIb)
Differences in speed of contraction, maximum force production, oxidative capacity, fatigability…
Type I Muscle Fibres
• Slow contractile speed, Low force production
• High oxidative (aerobic) capacity, Many mitochondria
• Many capillaries
• Lots of myoglobin
• Great aerobic enzyme activity
• Highly resistant to fatigue
Dominant muscle fibre during exercise below CP
Type II Muscle Fibres
• Relatively large in diameter
• Fast contractile speed, High force production
• High glycolytic capacity (many glycolytic enzymes)
• Greater glycogen and PCr stores
• Highly fatigable
Recruited during high-intensity exercise
Type II Muscle Subtypes: Type IIa
Type IIa
• Moderately well-developed oxidative capacity
• Many mitochondria
• Moderate number of capillaries
Gradually recruited during exercise bouts >CP
• i.e. High intensity aerobic exercise <40min – 1hr
• Typical anaerobic exercise bouts
Type II Muscle Subtypes: Type IIx
Type IIx • Largely anaerobic • High contractile speed, force, power • Great glycolytic capacity • High concentration of CP • Slight oxidative capacity • Highly fatigable Recruited during short (<1min), very high-intensity anaerobic exercise, when requirement for force and/or speed is high
Distribution of Fibre Types
- Skeletal muscle contain both type I and type II fibres however the proportion of these fibres differ
- Postural muscles – higher proportion of type I
- Power muscles – higher proportion of type II
Principles of Training: Specificity
Responses to training are specific to the type of exercise, the muscle groups involved and the energy systems used
SAID Principle: Specific Adaptation to Imposed Demands
• Adaptations are specific to the stimulus
• No adaptation = no improvement
• Lack of specificity results in a reduced training stimulus
Principles of Training: Overload
Training load must be at a level beyond which the athlete is accustomed for improvements in performance
- For continued improvements to occur the athlete must be progressively overloaded
- As fitness increases the body becomes more resilient to stress, recovery is faster, and initial training loads are highly achievable
- To ensure continued improvement, the stimulus must increase (or vary)
- Progressive overload is the key to achieving improvements without overtraining
Principles of Training: Adaptation
Improvement is only possible if this sequence is followed:
- Increasing stimulus (load) → adaptation → performance → improvement
If the load is always the same adaptation occurs in the initial stages followed by a plateau:
- Same stimulus → plateau → lack of improvement
If the stimulus is excessive or overly varied the athlete will be unable to adapt (maladaptation):
- Excessive stimulus → maladaptation → decrease in performance
Principles of Training: Reversibility
- Training adaptation is not permanent
* Training adaptations will decay once the stimulus has been removed
