contraction types and electromechanical delay

Question 1

electromyography to understand the control of movement

Answer 1

EMG is a tool used widely to advance our understanding of movement control.
Findings from a plethora of studies inform current clinical practice.
Some physiotherapists use EMG in the clinic for biofeedback.

Question 2

The connection between the CNS and muscle – the single motor unit

Answer 2

Motor units range in size allows graded, sustained and controlled force
Input from descending pathways, spinal interneurons & afferent fibres
One-to-one relationship between action potential in motoneuron & action potential in muscle fibres > muscle force
Measuring electrical activity generated by the muscle action potentials provides information about the neural drive to muscles

Question 3

same shaped AP in nerve and muscle cell

Answer 3

Movement of ions over membrane.
Time 0ms: Na+ in via few gates 
Depolarisation: rise
Many Na+ gates open, influx of ++ 
Repolarisation: descend
K+ move out Na gates closed
Hyperpolarisation:
Na+ out, K+ in via Na+/K+ pumps – influences maximal discharge rate

Question 4

Neuromuscular Junction NMJ

Answer 4

he connection between the nervous system and the muscle is the neuromuscular junction (motor point, motor end plate, synaptic cleft)

Question 5

AP

Answer 5

Every discharge of an action potential in the motoneuron = action potential in the muscle fibres that belong to that motor unit
***
Myoelectric action potentials (red arrows) are initiated by the Ach released by the motor neuron. >
The action potential sweeps across the muscle fibre and into the transverse (T) tubules. >
This initiates the release of calcium that regulate muscle activity.

Question 6

Myoelectric AP

Answer 6

A myoelectric action potential propagates at ~2-6 m/s along all muscle fibres from same motor unit
It propagates in both directions from the motor point to the end of the muscle fibre

Question 7

Electromyography records

Answer 7

Electromyography records a complex summation action potentials as well as
a complex summation of overlapping asynchronous action potentials

Question 8

often use bipolar elctrodes because

Answer 8

cleanliness of the signal

- reports muscle not environment

Question 9

Biological determinants of EMG signal:

Answer 9

Fibre angle (relative to electrode), fibre type
- More muscle fibres in a larger unit – more myoelectric action potentials.
Slow twitch (Type 1, postural) have a slower myoelectric action potential conduction velocity

Question 10

Biological determinant of EMG signal 2

Answer 10

Change in muscle position, cross talk and other muscles
- An electrodes recording zone is the region or muscle from which the myoelectric signal can be recorded.
Consider if the electrodes will be above the muscle belly throughout motor task
Cross talk - The recording zone of an electrode may be bigger than the muscle being measured. Cross talk can make a muscle appear active while it is not

Question 11

Technical determinant of EMG signal

Answer 11

skin preparation, other electrode noise, electrode placement

• relative to fibre direction and to motor point(s)
• get best reading when Place electrodes between motor points and myotendinous junction

Question 12

Does EMG differ between contraction types?

Answer 12

concentric: single motor units discharging at a higher rate than eccentric therefore need to drive muscles
- increase during concentric to maintain same amount of torque
SIGNIFICANTLY MORE EMG NEEDED DURING CONCENTRIC - DRIVING MOTOR UNITS HARDER

Question 13

Mechanisms for higher muscle force for same neural drive

Answer 13

Active force is generated through the actin and myosin cross-bridge
In eccentric movements actin and myosin ‘hold on’, more stretch of the elastic components (eg titin) > more force for same energy (# action potentials)
Decreased rate of cross-bridge detachments&raquo_space; greater force production on the eccentric bout.

Question 14

EMG

Answer 14

EMG provides a good indication of neural activation of a muscle.
EMG cannot provide any information about passive forces.
EMG does not = force
Consider how muscle length and contraction type may change during the motor task.

• Measuring onset of EMG does not tell you when the muscle force is produced.

Question 15

Electromechanical delay

Answer 15

is the time lag between onsets of muscle activation and muscle force production and reflects both electro-chemical processes and mechanical processes

Question 16

Electromechanical delay PROCESS

Answer 16

ONSET OF NEURAL DRIVE > SYNAPTIC TRANSMISSION, EXCITATION, ACTIVATION > ONSET OF MUSCLE FASICLE MOTION > TRANSMISSION OF MUSCLE FORCES THROUGH APONEUROSIS > ONSET OF MYOTENDINOUS JUNCTION MOTION > TRANSMISSION OF MUSCLE FORCE THROUGH TENDON > ONSENT OD FORCE PRODUCTION AROUND JOINT