Muscle Physiology II

How does the number of sarcomeres in series within the muscle fiber determine the amount of contraction?

The longer the tendon to tendon length, the more sarcomeres in series.

• The greater the physiological cross sectional area, the greater number of sarcomeres in parallel

contractility

ability to shorter, and limited by range of motion

excitability

sometimes called irritability. Responds to stimulation by a chemical neurotransmitter (Ach)

Extensbility

sometimes called distensiblity. Ability to stretch/lengthen and corresponds to stretching of the perimysium, epimysium, and fascia

Elasticity

Ability to return to normal state after lengthening or contracting

internal twitch tension

Force generated inside the muscle fibers by the contractile element (actin–myosin cross-bridges) during a twitch

• Contractile element (CE) → produces internal tension

external twitch tension

orce measured outside the muscle, usually at the tendon or bone

• At the start of a twitch, internal tension rises first, but external tension lags because some of that force is initially used to stretch the series elastic element rather than move the load.

• External tension = force transmitted through the SE to the load

load

Load is the external resistance applied to the muscle–tendon unit that determines whether the muscle shortens, lengthens, or remains isometric.

concentric contraction

• muscle shortens under load, weakest contraction of the three types, best at producing force slowly

• Internal tension > external tension early in contraction

isometric contraction

• muscle length does not change under load

• very strong contraction

eccentric contraction

• muscle lengthens under load, strongest of the three contraction types, best at producing force fast under high load

the frequency-tension relation

the frequency of stimulation determines muscle fiber tension

• with just one stimulus, Ca2+ lels and tension generated increase slightly, then completely fall

• wave summation results in greater tension

tetanus force

Tetanus force is the maximum, sustained force a muscle produces when it is stimulated at a high enough frequency that individual twitches fuse together.

what does the force-velocity curve show?

• the speed at which a muscle changes length also affects the force it can generate

• ideal overlap of thick and thin filaments

length-tension relation

• resting fiber length determines the tension generated

• obtimal resting length is mid-length, after which active tension decreases and passive tension increases

factors affecting force production

• fiber type

• % fiber recruitment

• fiber architecture

• number of muscle fibers

• angle of pull

• temperature

• rate of stimulation

• elastic properties

• force-velocity relationship

why does work output depend largely on muscle fiber length?

• work is the product of distance x force

• long, large muscles do large amounts of work when moving a heavy load a great disatnce

effect of posture

posture adjusts the body’s center of gravity to reduce the need for muscle action

power

shortening velocity x force of load

• power output depends on fiber type, length, and load

• power is maximized when a sizable (intermediate) load is still able to be moved at a moderate velocity

How do energetic factors affect contractile performance?

• ATP is quickly replenished (anaerobically) from creatine phosphate and glycolosis

• if ATP demand is moderate, pyruvate and fatty acids are converted to acetyl-CoA and enter the Krebs

• Slow twitch fibers rely more on this slower aerobic process, where ATP yield is larger but slower to deliver

why are motor units recruited in order based upon their size?

• with increased effort, motor units are recruited in order from slowest and weakest to fastest and strongest, due to differences in the sensitivity of their motor neurons to stimulation (SO and motor neurons are most sensitive)

• SO —> FOG —> FG