isometric: contraction when the length is held constant
isotonic
: contraction when the load is held constant
auxotonic
: contraction when neither length nor load is constant
Types of Contractions
twitch: the force response of a muscle cell to a single stimulus
summation
: addition of the force responses
if a 2nd stimulus is given after the 1st has relaxed, the magnitude of the 2nd is the same
if the 2nd stimulus occurs before the 1st has relaxed, the magnitude of the 2nd is higher
tetanus
: repetitive stimulation of a muscle at intervals short enough to provide summation
unfused
tetanus: at a low stimulus frequency, the force at the plateau oscillates
fused
tetanus: at high stimulus frequencies, the force plateau is smooth
due to summation, the force a muscle generates ranges from a single twitch to a fused tetanus
Isometric Contractions: length-tension relation
experimental arrangement
the magnitude of isometric force a skeletal muscle can generate depends on its length
stretch an isolated, relaxed muscle to some constant length and stimulate it to tetanus
stretch it further, fix the length and tetanize it again
passive and active force
passive force is the force exerted by the muscle as it is stretched out before stimulation
due to the elastic elements in the fibers (connective tissue)
below a certain length the passive force is zero, and
Ý as the muscle is stretched
active force is the additional force one observes when the muscle is stimulated
as the muscle is stretched, the active force 1st
Ý and then ß as the muscle lengthens
bell shaped relation between active force and length for isometric, tetanic stimulation
total force
total force is the sum of passive and active force at any length
structural basis of active force-length relation
sarcomere lengths between 2 - 2.25
mm have optimal overlap producing peak active force
sarcomere lengths > 2.25
mm: overlap ß, thus active force ß with Ý length
sarcomere lengths > 3.65
mm: no overlap thus no active force
sarcomere lengths < 2
mm: active force ß due to interference between thin filaments with themselves, and at very short lengths due to collision of the thick filaments with Z-lines
also, less Ca++ is released from the SR at short sarcomere lengths
Isometric Contractions: force-velocity relation
experimental arrangement
the velocity of shortening in an isotonic contraction depends on the load
attach a weight to an isolated muscle
tetanically stimulate the muscle, if the weight is heavier than the maximum tetanic force the muscle can generate, the weight will not move
Þ isometric contraction
if the muscle can lift the load, the contraction will be isotonic
maximum velocity of shortening
velocity of shortening vs the load is hyperbolic
max velocity of shortening is proportional to the max rate at which myosin hydrolyzes ATP
power output
power = force X velocity
power is zero in isometric case (vel = 0) and at maximal velocity of shortening (load = 0)
power output is maximal when load is 1/3 the maximal isometric force
overload
if the load on a muscle is greater than the max force, the muscle will not shorten
if the table supporting the load is removed, the muscle will lengthen
velocity of lengthening for overloaded isotonic contrac is slow over a range of loads up to 2x maximal isometric force
thus the crossbridges are capable of resisting a forced extension to some degree
above this range the muscle lengthens rapidly or "gives"