Skeletal Muscle Physiology
Muscle Mechanics:
Most natural movements are neither strictly isometric or isotonic
Two types of muscle contractions are recognized and defined by whether a muscle shortens when contracted
Isometric Contraction (“same length”)
Isotonic Contraction (“same tension”)
Skeletal Muscle Physiology
Muscle Mechanics:
How does a muscle regulate the amount of tension developed?
Time
Twitch :
Single contraction-relaxation sequence
Not a useful contraction
1) Frequency of stimulation
Time
Maximum
Tension
Incomplete Tetanus :
Rapid cycles of contraction-relaxation
Summation :
Addition of twitches to produce a more powerful contraction
Time
Maximum
Tension
Complete Tetanus :
Rapid stimulation erases relaxation phase
Most normal muscle contraction involves complete tetanus
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Skeletal Muscle Physiology
Muscle Mechanics:
WHY do we see increased tension with increased stimulation frequency?
Active State :
Condition in which myosin cross-bridges are attached to actin, causing the fibers to resist a force that would pull them apart
Simplified muscle model
1) Contractile component (myofibrils)
2) Parallel elastic components :
• Plasma membrane / connective tissues
3) Series elastic components (elements) :
• Tendons / connective tissues
• Z disks / myosin cross-bridge links
Skeletal Muscle Physiology
Muscle Mechanics:
WHY do we see increased tension with increased stimulation frequency?
Active State :
Condition in which myosin cross-bridges are attached to actin, causing the fibers to resist a force that would pull them apart
• During a twitch , much of active state spent
taking up slack in series elastic elements
• In tetanus , series elastic elements full stretch
allowing for external tension to mirror internal
tension
2

Skeletal Muscle Physiology
Muscle Mechanics:
How does a muscle regulate the amount of tension developed?
2) # of muscle fibers activated
Motor Unit :
A single motor neuron and all the muscle fibers innervated by it
(motor unit = all-or-none)
Fine Control / Rapid Reaction :
1-10 fibers / MU (e.g., ocular muscles)
Gross Control / Slow Reaction :
1000’s fibers / MU (e.g., quadraceps)
Recruitment :
Addition of motor units to produce smooth, steady muscle tension
• Small  large motor units activated…
• Motor units overlap; provides coordination
Skeletal Muscle Physiology
Muscle Mechanics:
When a muscle undergoes a contraction, there is an inverse relationship between force generated and shortening velocity of the muscle
For humans to move, the muscles must generate mechanical power…
Power =
Work
Time
=
(force) x ( Δ length)
Time
Power = (force) x (shortening velocity)
Optimal power in muscle appears
At V / V max
~ 0.4
( true for most muscles )
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