Respiratory Muscle Physiology Flashcards
Relationship Pdi and lung volume?
Increasing lung vol decreases potential diaphragm to generate force, as fibres have shortened and follow length-tension relationship
Define fatigue
Loss of capacity to generate force/velocity as a consequence of muscle load which is reversible by rest
Central fatigue vs peripheral fatigue?
Central = within CNS Peripheral = within contractile apparatus
What is central fatigue?
Muscle force generated during sustained/repeitive contractions gradually decline due to a decrease in motorneuronal output. MVC < max contraction via electrical stimulation
Boring or repetitive tasks produce more CF
Is central fatigue seen in diaphragm?
McKenzie 1992
Diaphragm Pimax fell to 79%, elbow flexros to 58%. Measured using MVCs and twitch interpolation. Decline particularly seen in expulsive manoeuvres.
Mechanisms of central fatigue?
Reduced activity from motorneuron pool
Supraspinal and spinal mechanisms - recurrent inhibition, reduced motor cortical activity, intrinsic motorneuron membrane properties (fall in activity with sustained activation)
What is muscle wisdom?
Fall in central firing frequency with prolonged contractions. Matches the motor unti discharge rate with the fatigue related contractile properties of the muscle, optimising force generation.
What is peripheral fatigue?
Failure at NMJ, propagation of AP along t-tubulues, changes in EC-coupling, changes within muscle cell.
Can be HFF or LFF.
What is HFF? LFF?
> 50Hz, occurs alongside LFF, resolves quickly
Occurs at 1-20Hz, occurs in isolation, long lasting
Is HFF of the diaphragm observed?
Not under normal physiological conditions. It would be catastrophic.
Is LFF of the diaphragm observed?
Yes. In healthy subjects following inspiratory resistive loading/max voluntary ventilation. Not generally seen following whole body exercise.
Not seen in clinical cases, eg COPD. ? prevented by higher command.
What is the effect of exercise on relaxation?
Relaxation is an active process. It consumes energy. Rate slows early in response to excessive loads that cannot be maintained.
What do elite athletes show?
Able to drive respiratory system hard enough to generate small but significant amounts of LFF, Twitch Pdi is seen to be reduced.
Mechanical constraints of lung play a role. EIAH. Competition for blood flow with locomotor muscles.
What did Babcock 2002 show?
No reduction in Pdi if using PAV
What does competition for blood flow have to do with diaphragmatic fatigue in elite athletes?
Override central protective mechanisms to provide blood flow to locomotor muscles at the expense of diaphragm, allowing fatigue to develop.
