EXSS 376- WK6L1 (Whole Muscle Contractions)

static muscle contraction

• isometric

• muscle produces force but does not change length

• joint angle does not change

• myosin cross-bridges for and recycle (no sliding)

dynamic muscle contraction

• muscle produces force and changes length

• joint movement is produced

what are the two types of dynamic contractions?

• concentric

• eccentric

concentric

• muscle shortens while producing force

• most familiar contraction type

• sarcomere shortens; filaments slide toward center

• think lifting weight

eccentric

• muscle lengthens while producing force

• cross-bridges form, but sarcomere lengthens

• think lowering weight

what are the two mechanisms that effect force of muscle action?

• increased number of motor units recruited

• increased frequency of motor unit discharge

explain increased number of motor units recruited

the muscle generates considerable force when all of its motor units are activiate

explain increased frequency of motor unit discharge

repetitive stimuli that reach a muscle before it relaxes and increases the total tension

motor unit recruitment

adding motor units to increase muscle force

size principle

• with increased muscle force, motor neurons with larger axons are recruited

• low-threshold motor units are recruited before larger threshold motor units

explain size principle simply

• small motor units turn on first

• big ones turn on later when you need more power

precise movements require smaller contractions, meaning…

less motor units and small motor units

large movements that require increased force production, meaning…

more motor units and large motor units

what are the four factors that determine tension developed by each muscle fiber?

• action potential frequency (frequency-tension relation)

• fiber length (length-tension relation)

• fiber diamete

• fatigue

what are the two factors that determine the number of active fibers?

• number of fibers per motor unit

• number of active motor units