force generation, muscle tone, motor units, types of contraction

twitch contraction

a single contraction and relaxation of a muscle fiber produced by a single stimulatory action potential from the motor neuron

tetanus

smoothly sustained muscle contraction caused by high frequency action potentials from the motor neuron

fusion

the maintenance of muscle contractions between stimulating action potentials that occurs during tetanus

summation

increase in tetanus contraction strength above that of a twitch contraction

recruitment

an increase in the number of motor units that are stimulated in order to increase contraction strength

threshold stimulus

a small stimulus that is the first to activate one of the smallest motor units

length tension in a fully shortened muscle

no more room to shorten, reliance less on sarcomeres, less force produced due to the overlap of actin and myosin filaments

length tension in a fully lengthened muscle

not all myosin heads are in contact with actin, less force produced due to the reduced number of cross-bridges formed.

length tension in the middle of contraction

optimal overlap of actin and myosin, resulting in maximal force production due to the greatest number of cross-bridges formed and optimal sarcomere length

concentric, fast contraction

no added load or light load

isometric contraction, no movement

load matches force production

eccentric contraction, lengthening contraction

heavier load than the muscle can carry

central fatigue

a decrease in muscle performance due to factors originating in the central nervous system (brain)— body is set to reserve energy supply and not push itself to the maximum

peripheral fatigue

a decrease in muscle performance due to factors originating in the peripheral nervous system, such as muscle fiber depletion or metabolic disturbances

causes of fatigue

depletion of glycogen (has to be long intense workout), metabolite build up (H+, K+, Ca 2+), muscle temperature too high, depletion of ATP (very rare)

muscle tone

a constant low level of contraction of muscle (posture, joint stability, readiness to move)

concentric contraction

contracting in which the muscle shortens

eccentric contraction

a type of contraction where the muscle lengthens while under tension, creates 50% more force than the others (elastic), strains are most often here, soreness

isometric contraction

a type of muscle contraction where the muscle generates force without changing length, maintaining a constant position

muscle soreness

tiny tears in the muscle

motor unit

all of the muscle fibers that are innervated by a single motor neuron, all or none contract

small number of muscle cells in a motor unit

motor unit controls fine movements and low force like in the eyes or fingers

large number of muscle cells in a motor unit

motor unit generates powerful contractions and gross, course movements, high force like in the soleus

unfused/incomplete tetanus (more tension results, less force)

if another tension is applied before the muscle relaxes completely ____________

fused/complete tetanus (less tenstion, more force)

at high stimulus frequencies, there is no relaxation between stimuli

number of motor units contracting, size of muscle cells in units, & length tension relationship in each fiber

force generation is dependent on…

stimulus strength

the number of motor units contracted depends on

motor unit size principle

the smallest fiber motor units are recruited first, as the stimulus increases, the larger motor units become stimulated

the number or motor units being stimulated & number of muscle cells being stimulated (also their size and strength)

how does the body control how hard a muscle contracts?