Skeletal Muscle Mechanics |
2 |
OVERVIEW OF MUSCLE MECHANICS
Preload
Preload is the load on a muscle in a relaxed state, i.e., before it contracts. Applying preload to muscle does 2 things:
•Stretches the muscle: This in turn, stretches the sarcomere. The greater the preload, the greater the stretch ofthe sarcomere.
•Generates passive tension in the muscle: Muscle is elastic (see titin, pre vious chapter) and thus "resists" the stretch applied to it. Think of the "snap-back" that occurs when one stretches a rubber band. The force of this resistance is measured as passive tension. The greater the preload, the greaterthe passive tension in the muscle.
Afterload
Afterload is the load the muscle works against. If one wants to lift a 10 Kg weight, then this weight represents the afterload. Using the I0 kg weight example, 2 pos sibilities exist:
•If the muscle generates more than 10 kg of force, then the weight moves as the muscle shortens. This is an isotonic contraction.
•If the muscle is unable to generate more than 10 kg of force, then the muscle won't shorten. This is an isometric contraction.
•Types of tension
-Passive: Produced by the preload
-Active: Produced by cross-bridge cycling (described in previous chapter)
-Total: The sum of active and passive tension
LENGTH-TENSION CURVES
Length-tension curves are important for understandingboth skeletal and cardiac muscle function. The graphs that follow are all generatedfrom skeletal muscle in vitro, but the information can be applied to both skeletal muscle and heart muscle in vivo.
Passive Tension Curve
The green line in Figure IIl-2-1 shows that muscle behaves like a rubber band. The elastic properties ofthe muscle resist this stretch and the resulting tension is recorded. There is a direct (non-linear) relationship between the degree ofstretch and the passive tension created that resists this stretch.
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Chapter 2 • Skeletal Muscle Mechanics
•Preload ofA: When there is no preload, the evoked muscle contraction develops -2 g of active tension.
•Preload of B: At this preload, the active tension produced by stimulation ofthe muscle is greater, -4 g.
•Preload of C: This preload results in less active tension than the previous preload. Thus, active tension increases as the muscle is stretched, up to a point. Ifstretched beyond this point, then active tension begins to fall.
•Optimal length (L0): L0 represents the muscle length (preload) that produces the greatest active tension. In Figure III-2-1, this occurs at the preload designated by B.
Figure IIl-2-1 shows asimplified picture ofa sarcomere. Actin is thethinbrownline, while myosin is depicted in purple. The magnitude ofactive tension depends on the number ofactin-myosin cross-bridges that can form (directly related).
•PreloadA: actin filaments overlap
-Thus, the force that can be exerted by myosin tugging the actin is compromised and the active tension is less.
•Preload B (L0): all myosin heads can bind to actin, and there is separa tion of actin filaments
-Thus, active tension generated is greatest here because there is optimal overlap of actin and myosin.
•Preload C: the stretch is so great that actin has been pulled away from some of the myosin filament, and thus fewer actin-myosin interactions are available, resulting in diminished active tension.
-Iftaken to the extreme, greater stretch could pull actin such that no actin-myosininteractions can occur, and thus no active tension results (active tension curve intersects the X-axis). This is an experi mental, rather than physiologic phenomenon.
•Total tension: sum ofpassive and activetension (bottom ofFigure III-2-1)
RELATIONSHIP BETWEEN VELOCITY AND LOAD
Figure IIl-2-2 shows that the maximum velocity of shortening (Vmax) occurs when there is no afterload on the muscle. Increasing afterload decreases velocity, and when afterload exceeds the maximum force generated by the muscle, short ening does not occur (isometric contraction).
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Section Ill • Skeletal Muscle
In Figure 111-2-2:
Afterload
Figure 111-2-2. Force-Velocity Curve
*Maximum velocity (Vmax) is determined by the muscle's ATPase activ ity. It is the ATPase activity that determines a fast versus a slow muscle.
**Maximum force generated by a muscle. This occurs when summation is maximal (complete summation) and all motor units for the given mus cle are fully recruited. The absolute amount of force is directly related to muscle mass and preload, with the greatest force occurring when the preload is at L0•
Muscle A: a smaller, slower muscle (red muscle)
Muscle B: a larger, faster muscle (white muscle)
As load increases, the distance shortened during a single contraction decreases. So, with increased afterload, both the velocity of contraction and the distance decrease.
PROPERTIES OF WHITEVS. RED MUSCLE
White Muscle
Generally, this isthelarge (powerful) muscle that is utilized short-term, e.g., ocular muscles, leg muscles of a sprinter. Major characteristics are as follows:
•Large mass per motor unit
•High ATPase activity (fast muscle)
•High capacity for anaerobic glycolysis
•Low myoglobin
Red Muscle
Generally, this is the smaller (less powerful) muscle utilized long-term (endur ance muscle), e.g., postural muscle. Major characteristics are as follows:
•Small mass per motor unit
•Lower ATPase activity (slower muscle)
•High capacity for aerobic metabolism (mitochondria)
•High myoglobin (imparts red color)
68 MEDICAL
SECTION
Cardiac Muscle
Mechanics