Determinants of Muscle Force Production (week 6)

Question 1

the amount of force produced in a sarcomere is…

Answer 1

the amount of force produced in a sarcomere is directly proportional to the number of cross bridges formed

Question 2

the number of cross bridges formed at any point in time depends upon what 4 parameters?

Answer 2

1. length of the muscle (force-length relationship)
2. contraction velocity (force-velocity relationship)
3. level of activation
4. time since the onset of activation

Question 3

state what it is meant by the key term - force-length relationship of a sarcomere

Answer 3

the force-length relationship of sarcomere describes how the force a sarcomere can produce changes with the length of the sarcomere

Question 4

describe how the experiment was set up in order to calculate the force-length relationship of a sarcomere (4 things)

Answer 4

1. isolated a single muscle fibre from a frog
2. had a stimulator to artificially send an AP to the muscle cell
3. had a force transducer to measure the force produced by the muscle fibre
4. micrometer set a certain length of sarcomere

Question 5

who did the experiment to determine of the force-length relationship of a sarcomere can be calculated

Answer 5

Gorden, et al, 1966

Question 6

describe the PROCESS of Gordon’s experiment to determining the force-length relationship of a sarcomere

Answer 6

1. experiment began with muscle at resting length
2. measured force produced by muscle at it’s resting length in an isometric contraction
3. changed length of muscle + repeated
4. found most force of an isometric contraction occurs when the muscle is at it’s resting length

Question 7

explain Gordon’s findings to his experiment to calculate the force-length relationship of a sarcomere

Answer 7

1. at resting sarcomere length, there was optimal cross over of actin and myosin for cross bridges to form
2. at short muscle lengths, there was too much cross over + actin-actin cross over also blocked myosin binding sites
3. at long muscle lengths, not enough cross over to form lots of actin-myosin cross bridges

Question 8

state 2 facts about muscle force production

Answer 8

1. the amount of force in a muscle is a sum of the force in the sarcomere as well as the force in the passive structures of the muscle
2. Fmuscle = Fsarcomere + Ftendon

Question 9

state 3 general facts about ‘passive tension in elastic tissue’

Answer 9

1. connective tissue/fascia made of elastic tissue called collagen
2. collagen provides resistance to stretch like an elastic band
3. the amount of resistance is proportional to the force applied once the structure is stretched beyond it’s resting length

Question 10

too much stretch in a tendon…

Answer 10

too much stretch in a tendon will level off/plateau it’s force production ability and if it continues to stretch it can cause a complete rupture of the collagen tendon

Question 11

describe the force-length relationship of a muscle (4 things)

(remember this is for an isometric muscle contraction)

Answer 11

1. max force in active (contractile) component (sarcomere) occurs when max number of cross bridges are formed
2. an inc^/dec^ in sarcomere length means fewer cross bridges formed and less force produced
3. if muscle is stretched while actively contracting, it’s able to generate more force due to the passive (elastic) components of the muscle
4. total muscle force is the sum of active and passive components of force

Question 12

state what it is meant by the key term - force-velocity relationship of a muscle

Answer 12

the force-velocity relationship of a muscle can produce changes accordingly to the velocity with which it is changing length

Question 13

state the set-up to the experiment to determine the force-velocity relationship of a muscle

Answer 13

1. isolated muscle fibre from a frog
2. muscle fibre, stimulator, force transducer, a mass attached to a lever allowing muscle to contract at different velocities (higher mass = slower velocities)

Question 14

explain the 5 key landmarks on the force-velocity relationship of a muscle graph

Answer 14

1. max contraction velocity occurs when muscle generates zero force (i.e. there is no resistance to contraction)
2. as the resistance to shortening (force required) increases, the speed of shortening decreases
3. a muscle can generate it’s almost max force when performing an isometric contraction
4. if the load exceeds the force generated by the muscle it will lengthen and more force will be produced
5. if the force continues to increase beyond the max force the muscle can produce it will tear

Question 15

explain isometric contractions

why does a muscle produce less force when shortening and more force when lengthening?

Answer 15

a constant number of cross bridges are formed in isometric contractions

Question 16

state 3 facts about concentric contractions

why does a muscle produce less force when shortening and more force when lengthening?

Answer 16

1. cross bridges cycling as muscle shortens
2. the faster a muscle shortens, the less time for the cross bridges to form and un-form
3. therefore, there are fewer cross bridges attached at any point in time and a muscle generates less force

Question 17

state 4 facts about eccentric contractions

why does a muscle produce less force when shortening and more force when lengthening?

Answer 17

1. passive structures in the muscle are stretched
2. this inc^ force in the muscle due to passive tension
3. passive tension isn’t effected by velocity, but active tension is, therefore force will dec^ at fast lengthening velocities
4. damage to a muscle occurs if these contractions are fast as active components of force is reduced

Question 18

state what it is meant by the key term - force-length-velocity relationship of a muscle

Answer 18

as soon as a muscle contraction is not isometric, you must think of the force-length relationship and the force-velocity relationship in parallel

Question 19

state 3 facts about motor units

Answer 19

1. the neurone and the muscle fibres represent the nervous and motor systems respectively
2. combine together to form the neuromuscular system
3. the motor unit (MU) is the functional unit of the neuromuscular system

Question 20

state what it is meant by the key term - force-activation level relationship of a muscle

Answer 20

as activation increases, the level of force generated for a given length and given velocity increases (can be shown on the F-L and F-V graphs)

Question 21

state what it is meant by the key term - force-time relationship of a muscle

Answer 21

the force-time relationship of a muscle describes how force is developed over time in a muscle

Question 22

state what it is meant by the key term - twitch contraction

Answer 22

a twitch contraction is the mechanical response to a single action potential (consists of the latent period, contraction time, and the relaxation time)

Question 23

state what it is meant by the key term - latent period (of a twitch contraction)

Answer 23

the period of time from the AP to the onset of contraction. the time delay is due to the excitation-contraction coupling reaction

Question 24

state what it is meant by the key term - contraction phase (of a twitch contraction)

Answer 24

the time when tension is developing due to the cross bridge cycling

Question 25

state what it is meant by the key term - relaxation phase (of a twitch contraction)

Answer 25

the time that tension is decreasing (i.e. relaxing) and is longer than the contraction phase. this is due to the time it takes to get all the Ca2+ sequestered

Question 26

explain the force-time relationship graph of a muscle (3 points)

Answer 26

1. the muscle does not produce maximum force as soon as it is activated
2. when a muscle is activated, the force increases over time until it reaches the maximum force for that level of activation
3. the greater the level of activation, the larger the maximum force is reached

Question 27

time to develop force in a muscle is dependent upon what 2 things?

Answer 27

1. latent period - transmission delay, excitation-contraction coupling delays
2. contraction time - influenced by which muscle fibres are recruited

remember, not all skeletal muscle fibres have the same twitch contraction time

Question 28

state what it is meant by the key term - force-time relationship of a muscle (think about the force-time graph)

Answer 28

the ramping up to peak force happens more quickly in fast twitch muscle fibres than in slow switch muscle fibres (hits their peak force faster)

Question 29

state, and explain, 2 ways of determining muscle fibre distribution

Answer 29

1. muscle biopsies (different fibres stain different colours, percentage of each can be determined)
2. non-invasive procedures (fibre type distribution from EMG signal)

Question 30

explain the force-time relationship graph of a muscle (5 points)

Answer 30

1. which muscle fibres are recruited is a function of the level of activation of the muscle (size principle of MU recruitment)
2. small + allow fibres recruited first
3. if more force is required, larger + faster ones will be recruited
4. this progresses until all fibres in a muscle are recruited
5. for a greater level of activation, the force generated will be larger and the force may ramp up more quickly for a mixed fibre muscle

Question 31

muscle comes in many forms. state the names of the 3 main forms (not types of muscle)

Answer 31

1. axial muscle - stabilisers
2. distal limb muscles - force production + fine control
3. proximal limb muscles - movement producers

Question 32

describe axial muscles

Answer 32

1. stabilisers

2. short fibres, minimal free tendon, slow/fatigue resistant

Question 33

describe distal limb muscles (2 points)

Answer 33

1. force producers + fine control

2. moderate fibres, long free tendons, mixed fibre types

Question 34

describe proximal limb muscles

Answer 34

1. movement producers

2. long fibres, moderate free tendon, mixed fibre type

Question 35

state the 2 types of muscle architecture

Answer 35

1. parallel/fusiform (eg - bicep brachii)

2. pennate (eg - gastrocnemius)

Question 36

state 3 facts about parallel/fusiform muscle architecture

Answer 36

1. muscle fibre/fascicles attached directly to bone or tendon
2. long muscle fibres, capable of large length change
3. Fmuscle = Ftendon

Question 37

state 4 facts about pennate muscle architecture

Answer 37

1. muscle fibres/fascicles attached to tendon vie aponeurosis
2. shorter muscle fibres, more fibres per unit area
3. Ftendon = Fmuscle x Cos(pennation angle)
4. inc^ pennation angle will dec^ force along the line of action

Question 38

state what it is meant by the key term - physiological cross sectional area (PCSA)

Answer 38

the PCSA represents the sum total of all the cross section of fibres in the muscle in the plane perpendicular to the direction of the fibres (determines how much force a muscle can produce)

Question 39

state what it is meant by the key term - ACSA

Answer 39

ACSA = anatomical cross sectional area

Question 40

the isokinetic dynamometer can: (2 things)

Answer 40

1. measures force produced by groups of muscles that move a joint in a given plane
2. measures how much force you can produce when you are trying to move your knee either in: an isometric contraction, or a concentric + eccentric at a constant velocity (isokinetic contraction)

Question 41

what piece of equipment can be used to measure force production in humans

Answer 41

the isokinetic dynamometer

Question 42

muscle force vs joint torque (2 things)

Answer 42

1. Fmuscle doesn’t equal Tjoint
2. Tjoint = Fmuscle x r

(r = moment arm)

Question 43

state what it is meant by the key term - moment arm

Answer 43

the moment arm is the distance between the lines of action of the muscle and the centre of muscle of a joint