motor units with smallest muscle fibers
activated first
motor units with largest muscle fibers
only activated when the most powerful contractions are necessary
what does muscle tension vary with
1. changes in stimulus voltage
2. changes in stimulus frequency
muscle twitch
single contraction followed by complete relaxation
latent period
muscle tension begins to increase, but no response seen on myogram
period of contraction
cross-bridges are active and muscle reaches peak tension
period of relaxation
reentry of calcium into SR as muscle tension decreases to zero
why do muscle twitch contractions vary
depends on fiber types contained in each muscle
what increases the number of motor units that are stimulated
strength of the stimulus (voltage)
recruitment
multiple motor unit summation
subthreshold stimulus
stimuli that produce no observable contractions
threshold stimulus
stimulus at which the first observable contraction occurs
maximal stimulus
strongest stimulus that produces increased contractile force
asynchonrous
usually occurs when all motor units are being stimulated
-helps to prevent or delay fatigue
increased stimulus frequency
can result in stronger muscle twitches
stimulus frequency between 10-20 stimuli per second
1. muscle twitches undergo complete relaxation
2. each subsequent twitch is stronger than the one before due to increased sarcoplasmic calcium levels
***referred to as TREPPE --> "Staircase effect
stimulus frequency between 20-40 stimuli per second
1. period of relaxation of each muscle twitch is interrupted by subsequent stimuli
2. causes successive contractions "piggy back" on one another, increasing tension
3. absolute refractory periods is always honored
***temporal wave summation which produces
stimulus frequency between 40-50 stimuli per second
1. period of relaxation of each muscle twitch is completely eliminated
2. produces one long, sustained contraction called Complete (fused) tetanus
3. rarely occurs in human body due to asynchronous nature of muscle fiber contraction
muscle twitches only produce what
internal tension
internal tension
when muscle fibers, contract, they pull on connective tissue sheaths (noncontractile components)
temporal summation produces what followed by what
produces internal tension followed by external tension
external tension
as sheaths become taut, they pull on the load (insertion); it takes longer to generate external tension than internal tension
**results in movement
length- tension relationship
amount of tension generated by muscle and force of its contraction depend on how stretched or contracted it was before stimulated
muscles between 60% and 175% of their resting length are able to generate the greatest muscle tension
true
a contracting muscle does not always shorten
true
muscle tension
force exerted by a contracting muscle on an object
load
weight of the object to be moved
isometric contractions
same measure"
1. muscle stays at a fixed length during contraction
2. muscle tension develops- doesn't overcome the weight of the load
3. cross-bridges generate force but filaments don't slide
passive force
generated by stretching the muscle and results from elastic recoil of the tissue itself
-due to Titin
active fore
generated during cross-bridge cycle
Total force
sum of passive and active forces
isotonic contractiosn
same tension
1. muscle tension sufficient to overcome the weight of the load
2. muscle will shorten and the load will be moved
3. filaments are sliding
2 types of isotonic contractions
1. concentric
2. eccentic
concentric
muscle shortens while doing work
eccentric
muscle is forced to elongate while contracting
what is the velocity and the duration of a contraction variable due to
1. more motor units
2. fiber type
3. weight of the load
more motor units results in what
faster, more prolonged contractions
-large muscles have more motor units and can generate a longer duration of contraction
fiber type
slow oxidative fibers can contract for longer periods, but fast glycolytic can often contract with a higher velocity
weight of the load
1. muscles contract fastest and most forcefully when there is no added load on them
2. increase load = increase latent period
3. increased load results in delayed contraction, and short contraction duration