Muscle Tissue: Force Generation

load

resistance to a muscle (i.e., what a muscle is pushing, pulling, lifting, etc.)

muscle tension

force generated by contraction of muscle, may or may not result in change in muscle length

muscle tone

partial state of contraction of resting muscles

length-tension relationship

tension generated by a muscle depends on how stretched out or contracted it was at rest

contracted

when a muscle fiber is extremely __________, there is no room for filaments to slide

stretched

when a muscle fiber is completely _________, it is difficult for myosin heads to “get a grip” on actin, few cross bridges able to form, very little overlap between myosin and actin

partially

having _________ contracted state of muscle means you are more ready to generate tension or lift load

twitch

one contraction-relaxation cycle to a SINGLE stimulus, lasts < 0.1 s

latent period

delay between stimulus and visible contraction, APs generated, Ca²⁺ released from SR and binds to troponin, tropomyosin moves, exposing active sites on thin filament

contraction period

muscle acts on resisting load, tension develops and increases, sarcomeres contract, creating tension

relaxation period

return to baseline tension, Ca⁺² is pumped back into SR, actin active sites re-covered by tropomyosin

slow twitch fibers (type I)

slow to twitch, SR slow to release and reabsorb Ca⁺², myosin hydrolyzes ATP slowly, fatigue-resistant

fast twitch fibers (type II)

fast to twitch, extensive SR releases and reabsorbs Ca²⁺ quickly, myosin rapidly hydrolyzes ATP, fatigue quickly

intensity and frequency

2 factors that affect twitch strength

intensity

number of motor units

frequency

how rapidly stimuli are coming from nervous system

wave summation

addition of one twitch to previous one, new stimulus arrives before previous twitch ended, 2nd twitch “piggybacks” on previous twitch → increased tension

isotonic

type of muscle contraction, generates tension, muscle length changes

concentric

type of isotonic contraction, muscle shortens

eccentric

type of isotonic contraction, muscle lengthens (but still contracting), responsible for DOMS

delayed onset muscle soreness

soreness post-exercise caused by eccentric contraction, theories: lactic acid buildup, connective tissue damage, microtears in myofibers, inflammation, enzyme efflux

isometric

type of muscle contraction, generates tension, but no change in muscle length (e.g., holding weight mid-air), important in posture and stability

strength

the ________ of a muscle depends on muscle size, size of active motor units, number of motor units recruited, stimulus frequency, and length-tension relationship

hypertrophy

growth in muscle size

atrophy

lack of use of muscle leads to loss of size

small motor units

eye muscles, not a lot of strength, not many muscle fibers activated with each stimulation

large motor units

leg muscles, lots of muscle fibers activated and contracting at one time, which can produce a lot of strength and tension