8 - Muscle Mechanics Flashcards
What is muscle force production?
amount of force developed in a sarcomere is directly proportional to the number of cross bridges formed
What does the number of cross bridges formed depend upon?
- length of the muscle
- contraction velocity
- level of activation
- time since onset of activation
What does the force-length relationship of a sarcomere show?
how the force a sarcomere can produce changes with the length of the sarcomere
How was the force-length relationship calculated?
In vitro:
- single muscle fibre
- fibre held at different lengths
- stimulated to contract
- force measured with a load cell
Describe the force-length relationship of a sarcomere
as the force increases so does the sarcomere length up till an optimum point before it rapidly declines in length
Describe passive tension in elastic tissues
- connective tissue of fascia within muscle is made of an elastic material called collagen
- collagen provides resistance to stretch like an elastic band
- the amount of resistance is proportional to the force applied once the structure is stretched beyond its resting length
When does the maximum force in the active component occur in the MTU?
when the maximum number of cross bridges are attached
What does a decrease or increase in length of the sarcomere in the MTU mean?
that fewer cross bridges are attached and less force in developed
What happens if a muscle is stretched while actively contracting?
it is able to generate more force due to the passive components of the muscle
What is the activity and type of contraction related to the quads?
- countermovement phase of countermovement jump (eccentric)
- propulsive phase (concentric)
What is the activity and type of contraction related to the hamstrings?
- stance phase of running (concentric)
- swing phase of running (eccentric)
What is the activity and type of contraction related to the gastrocnemius?
- moving from flat foot to top toe (concentric)
- standing on tip toe (isometric)
What does the force generated by a muscle depend upon?
its contraction velocity
Describe the force velocity relationship of a muscle
- fibre bundle attached to a load
- fibres stimulated to contract and allowed to shorten, moving the load
- velocity of the shortening cycle measured
- repeated for a number of different loads
- the larger the load, the slower the muscle will shorten
What does the force velocity diagram describe?
how the force a muscle can produce changes according to the velocity with which it is changing length
When does maximal contraction velocity occur in a muscle?
when the muscle generates zero force
What happens if the load exceeds the force generated by a muscle?
it will lengthen and more force will be produced
What happens if force continues beyond maximum force that the muscle can produce?
it will continue to increase and will eventually tear
Why does muscle produce less force when shortening and more force when lengthening?
- isometric = a constant number of cross bridges are formed
- concentric = cross bridges are cycling as the muscle shortens
- eccentric = passive elastic structures in the muscle are stretched
Explain the relationship between concentric contraction and cross bridges
the faster a muscle shortens the less time there is for the cross bridges to form and uniform, therefore there are fewer cross bridges attached
What needs to be considered if the muscle is not under isometric contraction?
force length + force velocity relationship
What does the force length relationship of a muscle describe?
how the force of a muscle can produce changes according to the length of the contractile and elastic components of the muscle
What does the force activation relationship of a muscle fibre show?
as activation increases the level of force generated for a given length and velocity increases
What does the force time relationship diagram show?
when muscle is activated the force increases over a period of time until it reaches the peak force for that level of activation
What is the mechanical response of a muscle fibre to a single action potential known as?
a twitch
What are the three major phases of a twitch?
1 - latent period
2 - contraction period
3 - relaxation period
What is the latent period?
the period of time from the action potential to the onset of contraction
What is the contraction phase?
the time that tension is developing due to the cross bridge cycling
What is the relaxation phase?
the time that the tension is decreasing and is longer than the contraction phase due to the amount of time it takes to get all the calcium sequestered
What are the three types of muscle fibres and giver their characteristics
type 1:
- slow
- small
- oxidative
- aerobic
- fatigue resistant
type 2a:
- fast
- larger
- oxidative
- aerobic
- fatigue quicker due to size
type 2b:
- fast
- rapid response
- anaerobic
- fatigue easily
What does fibre type influence?
the amount of force produced and the velocity of the contraction
What is contraction velocity determined by?
- calcium release and uptake
- cross bridge kinetics which are dependant on the type of myosin in the fibre
Does ramping up the peak force happen quicker in fast or slow twitch muscle fibres?
fast
How can we determine muscle fibre type and their distribution?
muscle biopsies, fibre type distribution from EMG signal
Which type of muscle would you expect to be predominantly slow twitch muscle fibres?
those related to endurance and continuous force production
Which type of muscle would you expect to have a high proportion of fast twitch muscle fibres?
those related to power production and fast but transient force development
What is the order of fibres recruited for the motor unit?
slow
