Muscle size and number of fibers
- All muscles have the same structural units, but larger muscles contain many more fibers
- Small muscles may have only a few hundred fibers
- Large muscles like those in the legs can have hundreds of thousands
Muscle twitch
- a single contraction of one muscle fibre
- A single twitch doesn’t cause the whole muscle to contract
2 ways for multiple fibres to twitch
- Motor unit recruitment
- Frequency of stimulation
Motor unit recruitment
Greater number of fibres recruited to contract results in greater muscle tension
Definition of a motor unit
- When a motor neuron enters a muscle, it branches to innervate multiple fibres
- When the neuron fires, all its fibres contract together
How motor unit recruitment increases strength
- Motor unit fibres are spread throughout the muscle, not clustered together
- A single unit causes a weak contraction, so stronger contractions require recruiting multiple motor units
Preventing fatigue during sustained contractions
- To avoid fatigue, the body alternates active and resting motor units
- Allows continuous contraction for longer periods
Frequency of stimulation - Single Twitch
If a muscle fibre is restimulated after it has relaxed, the second twitch is the same magnitude as the first
Frequency of stimulation - Twitch Summation
If a muscle fibre is restimulated before it has completely relaxed, the second twitch is added on to the first one
Frequency of stimulation - Tetanic contraction
- If a muscle fibre is stimulated so rapidly that the twitches overlap
Unfused tetanus
fibres don’t fully relax between stimuli
Fused tetanus
- There is no relaxation at all
- Produces the strongest single-unit contraction known as tetanus
Length-tension relationship
- The tension that can be generated at tetanus depends on the length of the muscle at the start of contraction
- Relationship is explained by the sliding filament mechanism
- Each fibre has an optimal length (L₀) where maximal force is produced
Less than optimal length
- When a fibre shortens too much, thin filaments overlap thick filaments in areas without cross-bridges
- As shortening continues, thin filaments from opposite sides of the sarcomere overlap each other, and thick filaments come in contact with the z-lines
- Occurs when the fiber length is about 70% of its optimal length (L₀)
Optimal length
- The maximal number of cross-bridge binding sites are available to the cross-bridge for binding
- Thin filaments do not overlap the central region of the thick filaments
- At rest, muscle fibres are generally at optimal length
Greater than optimal length
- During passive stretch the distance between Z-lines increases and overlap between thick and thin filaments decreases
- With fewer cross-bridges available, less tension can be produced
What happens at 30% longer than Lo?
only 50% of maximal tension is generated
What happens if the sarcomere is stretched 70% beyond Lo?
- There is no filament overlap
- Contraction cannot occur at all
Muscle tension and bone
- Most muscles are attached to 2 bones around a joint
- Allows movement of the bones by contraction or relaxation of the muscles
Tendons
- Connective tissues surrounding the muscles form tendons
- This physically attaches muscles to bones
Muscle tension with a load
- For a muscle to shorten, its generated force must exceed opposing forces
- When an external load is added, the biceps must generate additional tension to overcome both the triceps’ resistance and the weight of the load to move it
Muscle soreness
- Also called myalgia
- Often experienced because of overexertion and improper rest
Contusion
Occurs when a muscle is subject to sudden, heavy extrinsic compressive force
Strain
- Occurs when muscle fibres are exposed to an excessive force caused by intrinsic tension
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Module 1: Section 1 + 256
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Module 1: Section 316
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Module 1: Section 413
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Module 1: Section 531
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Module 2: Section 120
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Module 2: Section 216
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Module 2: Section 332
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Module 2: Section 451
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Module 2: Section 511
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Module 4: Section 127
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Module 4: Section 235
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Module 4: Section 333
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Module 4: Section 426
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Module 4: Section 59
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Module 4: Section 628
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Module 5: Section 115
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Module 5: Section 234
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Module 5: Section 352
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Module 5: Section 47
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Module 5: Section 533
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Module 5: Section 67
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Module 6: Section 121
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Module 6: Section 223
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Module 6: Section 344
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Module 6: Section 428
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Module 6: Section 512
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Module 6: Section 613
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Module 6: Section 743
