module 11 Flashcards
what are tendons? what are thier stuctures like? what are they made of?
not rigid structures that attach muscles to bones
- they have the right amount of compliance to affect muscle function in a positive way
structure: CT around muscles coverage to form the tendon, white, glossy, smooth, some are enclosed in a CT sheath, others slide with respect to adjacent tissues and skin, some wrap around pulleys and others pull in a direct line from muscle to bone
- comprised mostly of collagen, arranged in staggered patterns, creating a fibril - fibrils are bundles into tendons
- wavy appearance - with load wavy appearance disappears (crimp pattern)
tendon mechanical properties?
stress and strain
passive structures, unlike muscles they don’t produce active tension
tensons and the stress-strain relationship
low load: relatively compliant - toe region (develops little internal force until the fibres line up together)
load increases: stiffness of tendon increases
*behaves non-linearly
can use Youngs Modulus to compare the s-s properties of tissues
- during muscle contraction, the tendon strains bc muscle applies a load on the tendon
- the amount of tenson elongation depends on the stiffness of the tendon (load deformaiton of the tendon must be well adjusted to +ly affect mm function) - YM of tendon is 100x larger than passive mm
*on average, tendon strain is about 3% at max isometric muscle tension
*need the correct amt of stiffness to transfer the force from the muscle to the tendon to the bone to move object
explain how the tendon behaves with isometric actions.
sarcomere can shorten, tendon can lengthen - but net length of the muscle stays the same
- force internal = mm internal force = isometric tension
*active muscle force generation depends mainly on 2 imp properties: force-length and force-velocity relationships
*when muscle is active and develops force according to the FL and FV relationships, the tendon will strain - allows muscle to shorten even further
*magnitude of tendon elongation depends on its stress-strain properties
* consequence: mm activation at a fixed joint angle will not lead only to a truly isometric contraction
how can we apply tendons to balance?
control of balance predicted to occur through reflexive control - reflexes generate forces to maintain eq in reaction to movement of the body
*forces required to maintain balance would be generated through the spring-like stiffness of active muscles
*when swaying forwards, the calf muscles would be stretched while active (ecc action), eliciting reflexes leading to increased muscle activity and restoration of body position towards a neural position
what are the important findings of the Lorams paper regarding balance?
SOL and Gas mm exhibit paradoxical contractions when maintaining standing balance
* as we sway forwards, SOL and Gas muscle fibers shorten while active (con action); they lengthen passively when swaying backwards
*contrast with the idea of a spring-like stiffness regulation of balance - stiffness of the tendon is insufficent for static stabiliazation
*control of balance thorugh repeated impulses (transient shifts in muscle lengths)
if the muscle-tendon unit shortens at a constant velocity what happens with the muscle and tendon separately?
when a joint is allowed to move, the muscle fibers and tendon may not shorten at a constant velocity even if unit does
- this limits the use of our simple biomechanical model to estimate muscle fiber length/velocity during varioud muscle actions
*if the interest is to estimate m-t unit length, these models work but for mm fiber length/velocity, we need to measure it directly - or model the tendon
explain the force-time relationship
development of active muscle tension is delayed with respect to the activation of the muscle - rpresents another important mechanical characteristic of muscle
definition: time from muscle fiber action potential to the onset of muscle tension development
*delay is referred to as the electro-mechanical delay: excitation and contraction dynamics
explain the two parts of the force-time relationship
- excitation dynamics: delay associated with muscle stimulation: conduction delay in T-tubues, release of Ca+, Cb formation
- contraction dynamics: delay associated with the actual build-up of muscle tension: tension development in contractile elements (type of MU recruited), stretching SEC by contractile elements - will be affected by the type of muscle action and contractoin history
force-time relationship and EMG singaling - why is EMG not a good indicator of mm force 1
*the duration of the EMG signal may differ form the duration of the force signal
*typically, the EMG signal will terminae before the force signal - this delay depends on the type of muscle action, together the EM delay will lead to a temporal dissociation between EMG adn force - maintaining isometric action avoids partly this problem
*timing of force decay also affects movement coordination
why is EMG not a good indicator of mm force 2?
f-t relationship:
- problem gets amplified for faster movements and rapidly changing forces
- for constant forces applied at static joint angle, the EM delay is only a problem at the onset and offset of the action
*mm force is expressed through Ms system as joint moment that causes joint angular acceleration - acceleration will be detected before velocity and displacement; be careful when using displacement
how does tendon compliance result in a systematic change in the force-length relationship?
- changes the width and shape
- mt unit fl relationship is not simple the muscles fl relationship with added tendon length
tendon compliance and muscle mechanics. what would happen if the tendon is perfectly stiff? what does compliance result in?
- perfectly stiff: just acts as a rigid linkage between muscle and bone - results in offset of FL relationship
- need right amount of compliance (3% strain at Po will be beneficial to muscle/skeletal system)
*compliance = rightward shift of the FL relationship due to added elasticity in series
how do sarcomeres explain the tendon and muscle mechanic changes in FL relationship?
- sarcomeres are allowed to shorten at the expense of tendon elongation
- sarcomere configuration does not change - remains ocnstant
- longer tendon in series result in a greater rightward shift of FL relationship (less stiff) - differential degree of sarcomere shorteing depending on its initial length à
*no gain too stiff, no gain too long = need right amount of compliance
tendon and muscle mechanics in humans? what is the ideal tendon strain, what range does it increase, how does the shift occur?
humans: tendons strain about 3% at max isometric contraction
*increse the range over while the muscle-tension unit curve operates by 50%
*shift in the mm optimal length due to shift in optimal sarcomere length
tendon = active tension over a bigger range, shift in peak tension (right)
*result of sarcomere shortening at the expense of tendon lengthening - tendon is perfectly matched to the muslce (too stiff = no benefits, too compliant = mm would shorten to nothing)
