11/25/2014 Medical Physiology Muscle Mechanics Eric Olson Flashcards
What happens when skeletal muscle contracts?
It develops a force or tension
Define isometric in the context of muscle mechanics
When a muscle develops tension, but stays the same length because its ends are attached to immovable objects, the contraction is said to be isometric
Does the the length of the muscle stay constant during an isometric contraction?
Yes, though the tension increases
Is external work performed during an isometric contraction?
No, since the muscle length is constant, and the load is not moved (W = F x ∆d)
What is the power in an isometric contraction?
Zero (Р = dW/dt)
Is the energy of ATP hydrolysis utilized whenever the muscle is exerting tension?
Yes, even though no external work is performed during an isometric contraction
Cite a function of isometric contractions
Maintaining posture
Describe what happens in an isometric contraction
During the development of tension, the degree of shortening of the contractile element (CE) equals the degree of elongation of the series elastic elements (SE), thereby keeping the overall length of the muscle fiber constant
How does moving a book involve both isometric and isotonic contractions?
Combination of bicep undergoing isotonic contraction (production of movement) and back muscle undergoing isometric contraction (postural rigidity)
Do isotonic contractions do all the work?
Yes
Define isotonic twitch
When the force of contraction of the muscle is at least equal to the load so that the muscle shortens, the muscle is said to contract isotonically
What does the tension curve for an isotonic twitch show?
A plateau during which the force or tension is constant (iso-same; tonic- tension)
The ___ the load the ___ the plateau and the ___ the time lag between stimuli and the start of muscle shortening
greater; higher; greater
What happens if the load exceeds the amount of force the muscle can generate?
An isometric twitch results
Describe the four phases of an isotonic twitch
(1) isometric tension development
* tension develops without shortening until the force developed equals the load
(2) isotonic shortening
* shortening of the muscle at constant tension
(3) isotonic relaxation
* elongation to the initial length at constant tension
(4) isometric relaxation
* reduction of tension at the initial length
Is external work performed during an isotonic contraction?
Yes. W=Fxd > 0
In the tension-time curve, what does the slope represent?
The rate of development of tension (dF/dt)
*dF/dt is a function of both the active properties of the muscle and the passive properties of the tissue
When is the velocity of shortening maximal?
At the beginning of shortening
For a series of single isotonic twitches at a given initial length, as the weight (afterload) increases…
(1) the initial velocity of shortening decreases:
- “the muscle lifts a heavier load more slowly than a lighter load”
- more of the cross-bridges are needed to maintain tension against a heavier load, so less
are available for shortening
(2) the amount of shortening decreases:
- “the muscle lifts a heavier load less high than a lighter load”
- at a slower velocity with a heavier weight, the muscle will shorten less before relaxation occurs
(3) the duration of shortening decreases, i.e. the muscle drops the weight sooner
- “the muscle holds a heavier weight for less time than a lighter load”
- re-accumulation of calcium into the SR causes the tension to be less than the load sooner; hence, the weight is dropped sooner
What happens at the point where the muscle cannot lift the weight at all?
The maximal amount of tension for the particular initial length is being generated
When a muscle contracts isotonically under increasing loads the contractions display the following effects in the Distance shortened vs. time plot:
- The latent period (time lag between stimulation and shortening) increases
- The duration of shortening decreases
- The velocity of shortening decreases.
The Force-Velocity curve has two distinguishing points:
Vmax –> maximal velocity of contraction, extrapolated to zero load
* Vmax is a function of myosin ATPase in varying muscle types (slow, fast)
Po –> maximal isometric tension that can be generated at a given initial length, and a given state of contractility
* Po is a function of the number of force-generating cross-bridges that can be formed
The larger the muscle, the ___ the number of cross bridges and the ___ is Po
larger; larger
In skeletal muscle Po can ___ with frequency modulation, recruitment of additional motor units, and with initial lengths closer to the optimal initial length Lmax , but not by changes in ___
increase; contractility
Is contractility defined for skeletal muscle?
No
In cardiac muscle, Po can ___ with an increased initial length, and by increased contractility, but not by recruitment (since the heart contracts as a syncytium)
increase
When a muscle is contracted isotonically under successively increasing loads the contractions display the following effects:
- The latent period (time lag between stimulation and shortening) increases
- The duration of shortening decreases
- The velocity of shortening decreases
Do all types of fibers have the same velocity of shortening?
No. Certain types of muscle fibers can shorten faster than others
Define preload
Load required to stretch a muscle from its resting length
For skeletal muscle, the afterload is equivalent to the ___
total tension
