Chapter 5 Flashcards
Difference between supramaximal and submaximal stimulation.
Supramaximal Stimulation (total capability, max): M-wave goes unchanged/slightly decreases at fatigue. The area increases-result of decreased conduction velocity of the action potential along muscle fiber membranes. Does not always result in increased force because cells are not complete.
Submaximal stimulation: EMG increases gradually, additional units are recruited to maintain force. Recruitment sustained by increase in excitability of motoneuron pool. Majority of recruited motor units demonstrate decline in firing frequency. Decrease in firing rates once target force was attained.
Motoneurons during late adaptation.
When current injection is used to excite alpha-moto neurons above their current threshold for rhythmic firing, frequency decreases gradually (secs/mins)
Motoneuron excitability plays a role in decreased muscle activation that occurs with stimulation.
Late adaptaion is not fatigue of the motoneuron, can be overcame with increased current injection.
Central factors in neuromuscular fatigue what occurs in muscle spindle excitation and why?
Spindle activity increases with increase drive, new spindles are recruited and those already firing increase with increase in strength and contraction.
An excitatory influence, there is decrease in excitation innervating a fatiguing muscle to accompany a decline.
Presynaptic inhibition
The depolarization of Ia afferent terminals to motoneurons by axoaxonal synapses which reduce the size of the excitatory postsynaptic potential.
Presynaptic interneurons are subject to excitatory and inhibitory control.
Decreases the motoneuron pool at beginning of contraction and increases in the muscles not involved in contraction. This ensures that sensory information from spindles is received in full.
MVC as a result of presynaptic inhibition
During repeated isometric MVCs continued to fatigue contratcion of agonist and antagonist muscles, evidence by EMG decrease in agonist and no change in antagonist.
Tug-of-war between cortical inhibition and spinal excitation
Lack of major change in MEP of fatigued muscle during TMS is due to an equal and opposite change in cortical and spinal excitation.
6 (theories) results of expriments using cortical and subcortical stimulation during a sustained MVC to fatigue.
1) The level of cortical inhibition increases
2) level of cortical excitation increases
3) cortex appears to drive motoneurons at sufficient intensity to elicit maximal contractile responses in the control condition but not during fatigue
4) central fatigue related to afferent info from periphery
5) decreases in corticospinal stimulation of motoneurons may also occur.
6) TMS results in a superimposed contractile response during fatigue, central failure is in the sites driving cortex limit full expression of contractile force during sustained MVC.
Evidence of fatigue during prolonged low-force contractions.
During low-force sustained contractions there is gradual development of an inability to drive the motor cortex optimally during brief MVCs. Evidence of fatigue evoked twitch responses fell.
Central fatigue occurs.
Isometric v.s. anisometric tasks regarding fatigue.
Recent studies show, motor unit discharge rates decreased when contractions were isometric but stimulated the torque changes that occur during anisometric task.
Blood flow and afferent info may be different thus their effects on motoneuron firing rates.
Rotation of motor units during contractions maintained at very low intensities
Recruitment of motor units are heterogeneous. They found that subjects who lasted longer before reaching fatigue were also those who showed evidence of motor unti rotation consistently.
Rotation occurs among the synergists during later task to fatigue.
At 50% MVC they saw motor units ceased being activated then reactivated later in the session while whole muscle activity and other units remained activated at a constant. Made possible by PICs and activation and inactivation of ion currents.
