Muscle Mechanics and Physiology

Question 1

What is the relationship between force generated and attached crossbridges?

Answer 1

the more crossbridges that are attached, the more force will be generated

Question 2

How does the CNS control muscle force?

Answer 2

activates motor units, not whole muscles or individual muscle fibers

Question 3

Why do muscles inherently want to shorten?

Answer 3

due to the structure of the sarcomere and the orientation of the myosin heads

generating force when the muscle is lengthening is the easiest way to get injuring

Question 4

What is the process of muscle activation?

Answer 4

nerve impulse goes down from the brain to the spinal chord

motor neuron signal reaches muscle in about 10 ms

in adult stage, it is 1 neuron to 1 muscle cell

depolarization travels down into the T-tubules

release of calcium and activates the actin-myosin apparatus

Question 5

What are the two components of the S1 myosin head?

Answer 5

one for binding to actin and hydrolysing ATP

another acts as a “lever” arm that makes the actual movement

Question 6

Describe the main stages of the crossbridge cycle.

Answer 6

1) ATP binding allows actin-myosin disassociation
2) hydrolysis of ATP swings the head to the next attachment site
4) the power stroke is associated with the release of ADP+Pi

Question 7

What is the relationship between a motor neuron AP and the muscle it innervates?

Answer 7

on emotor neuron innervates one motor unit and all of the muscle fibers in that unit

a single action potential produces a single, simultaneous twitch in each of its muscle fibers

Question 8

How does the speed of movement of filaments affact for generation?

Answer 8

the higher the speed of sliding the mor likely that attached cross-bridges will be broken by this movement

consequently, the faster the movement, the fewer attached cross-bridges and the less force produced

Question 9

How does the speed of the cross-bridge cycle interact with the speed of filament sliding in movement?

Answer 9

the faster the myosin can make an attachment, the more cross bridges attached at a given movement speed, which increases force generated

cycle speed mainly depends on the various isozymes of myosin

Question 10

What are the factors tht determine the number of attached cross bridges?

Answer 10

force-stimulus frequency (F-f)

force-length (F-L)

force-velocity (F-V)

Question 11

How does muscle stimulus frequency affect the force generated?

Answer 11

average force increases as the number of motorneuron APs per time increases, resulting in a function that increases to a maximum force

Question 12

How does the length of a muscle affect the force generated?

Answer 12

peak optimal overlap to generate the most force and a drop off on either side due to the number of cross-bridges that can be formed

alteration in filament overlap is a consequence of different limb positions

ex. an elastic spring - stiffness is an increase in force for an increase in length

Question 13

How does the speed of filament movement affect the force generated?

Answer 13

the number of attached cross bridges depend on the speed of the movement versus the speed of the myosin ATPase

force is decreased with shortening and increased with lengthening

alteration in speed of filament movement is due to the speed of limb movements

ex. a dashpot or shock absorber - viscosity is a decrease in force for an increase in velosity, viscosity gives damping or smoothing of movement

Question 14

passive stiffness

Answer 14

stiffness generated by the tinin molecules that join the m lines and z lines together

with with the connective tissue that surrounds muscle fibers and muscles, these structures generate a passive elastic force

Question 15

series elasticity

Answer 15

stiffness contributed by the tendons

Question 16

total stifness in a muscle system

Answer 16

the sum of the contractile element (whose stiffness comes from the force-length function), the passive elasticity, and the series elasticity

Question 17

What are the energy sources that muscles use? When is each source drawn upon?

Answer 17

pure ATP - immediate to 5s

creatinine phosphate (CP) - 1s to ~30s

anaerobic glycolysis (glycogen) - ~5s to ~120s

aerobic glycolysis (glycogen) - ~30s onwards

fat - 20-60 minutes onwards

Question 18

creatinine phosphate

Answer 18

baseic pathway is CP + ADP -> ATP + C

very fast mobilization < 1s

very little stored, but mroe than ATP - about 10-15s worth

Question 19

glycogen as an energy source

Answer 19

basic pathway (anaerobic glycolysis) -> ATP + pyruvate

mobilization time is 5-30s

enough stored for 30mins to 2hrs

basic pathway (aerobic glycolysis) -> pyruvate -> citric acid cycle -> O2 phosphorylation -> ATP

mobilization time is 30-120s (limited by ability of mitochondria to take up pyruvate)

amount stored is enough for 30 min to 2hrs because the source is still glycogen

Question 20

fat as a an energy source

Answer 20

basic pathway - free fatty acids -> citric acid cycle, O2 phosphorylation -> ATP

mobilization time is 20-60 minutes

amount stored is very large, up to weeks of energy

1lb of fat is sufficenty energy to walk or run about 35 miles

Question 21

What is a motor unit?

Answer 21

one motor neuron and the muscle fibers it innervates

Question 22

What are the basic variations in motor unit properties?

Answer 22

contraction speed - relation to myosin cycle speed

factor resistance - relation to energy utilization

maximum force - relation to number and size of muscle fibers/motor unit, it depends on the total number of bridges

Question 23

What kinds of muscle fibers are in a muscle unit?

Answer 23

slow fibers - can contract for long periods of time, but can’t generate much force

fast fatigue resistant - can generate medium force and go on for a medium amount of time

fast fatigable - can generate a lot of force but can’t contract for long