Skeletal Muscle Mechanics

what is the relationship between muscle length and contraction strength. Is this relationship true in all circumstances?

(length tension relationship) with increased length/preload, the force of contraction increases until the sarcomere hits a pivotal length of 2.2 microns, at which point increased preload will yield a decreased tension in response to the stimulus

T/F for contraction of muscle: summation increases the amount of peak calcium release in the muscle from the Sarcoplasmic Reticulum summation increases the force development by increasing the exposure of the muscle cell to the calcium levels there is a maximum force that can be developed by increasing action potential frequency it is possible to increase the force production of a muscle by increasing the frequency of action potentials produced or electrical stimuli

F ( a single stimulus releases all of the available calcium from the sarcoplasmic reticulum) T T (tetanus) T

T/F for an isotonic contraction: the afterload cannot be sensed initially by the muscle at rest as the tension in the muscle increases, the size decreases the first period of the isotonic contraction could be considered to be isometric the convention to show shortening is an upward deflection as the length decreases the tension stays the same for an increased afterload, more force will need to be generated for an increased afterload, the latency until shortening occurs for an increased afterload, the velocity of shortening will be increased

T F (as tension increases the muscle stays at the same length until the force equals the weight or force needed to overcome the weight of the afterload) T T T T T F (decreased)

T/F for muscle mechanical relationships: increasing length will increase tension the maximum sarcomere length in both cardiac and skeletal muscle is 2.2 microns skeletal muscle and cardiac muscle have both an ascending limb and descending limb on the length tension curve cardiac sarcomeres can be stretched past 2.2 microns if the sarcomeres are stretched to the point where there is no longer overlap between the heavy and light chains in the sarcomere, there cannot be force through stimulus

T F (skeletal sarcomeres can stretch past this point) F (cardiac *does not* have the descending limb because the sarcomeres in cardiac muscle cannot be stretched past 2.2 microns, due to titin being more taut in cardiac sarcomeres) F (cannot be stretched past 2.2 microns, due to titin) T

consider a skeletal muscle whose preload is 24mm. The afterload is 25g. When the tension produced by the muscle equals to the afterload, what will happen to the length of the sarcomeres in the muscle fibers (increase decrease stay the same)

decrease (shorten)

as the muscle shortens it must become wider or "more fat" in accordance with conservation of mass. What affect would this have on crossbridge cycling?

decreased because the myosin heads and F-actin are farther apart and the cross bridges are unable to form

in an isometrci contraction the total length of the muscle ________ change

does not

how is it that the force increases in a isometric contraction?

increase in the motor units that are recruited

what happens to the length of the series elastic element as the force is increased up to the point of being equal to the load to be lifted

increases

why is it that in vivo, the main regulatory factor of muscle contraction strength is the number of motor units recruited and not the length or preload of the sarcomeres

most muscles in our body are stretched from their origin to their insertion and cannot vary their length to a considerable amount, therefore they rely on motor unit recruitment in order to increase the strength of contraction

the initial length of the muscle before contraction

preload

what happens to the length of the series elastic elements once the force is enough to lift the desirable load

stays approximately the same length (for our intents and purposes it is the same but it reality would fluctuate slightly)

what is the difference in strength regulation in cardiac muscle versus skeletal muscle

there are no motor units in cardiac muscle

what is the sarcomere length that provides the absolute maximum *developed* force

2.2microns

the preload is the force applied to the muscle in order to lengthen it before or after the formation of crossbridges between the light and heavy chains?

before