17. EMG 2

Question 1

EMG amplitudes are generally less during eccentric muscle actions than concentric muscle actions. Why?

Answer 1

EMG amplitudes are generally lower during eccentric muscle actions because fewer motor units are required to produce the same or greater force,

as the passive elastic components and mechanical properties of the muscle contribute more to force generation during lengthening.

Question 2

What Intrinsic or Physiological Factors affect the EMG signal?

Answer 2

Muscle fiber diameter

• As muscle cell diameter increases, EMG amplitude increases
• Type II fibers tend to have a large diameter than Type I fibers

Muscle temperature

• As muscle temperature increases, conduction velocity increases
• This leads to higher frequency and amplitude of the EMG signal

Blood flow to the muscle

• As muscle blood flow increases, the amplitude of the EMG increases
• For the same level of force more blood flows to a muscle during dynamic exercise than during isometric exercise

Question 3

What happens to EMG amplitude during maximal contactions?

Answer 3

EMG amplitude decreases during maximal contractions

Why does this happen?
● All fibers in the muscle are active
● As the muscle fatigues, active fibers drop out

Question 4

What happens to EMG amplitude during submaximal contraction?

Answer 4

EMG amplitude increases during submaximal contraction

Why does this happen?
● During a submaximal action typically type I fibers are active
● As fatigue sets in, type II fibers are activated to prolong activity

Question 5

What Technical Factors affect the EMG signal?

Answer 5

The Electrodes

• Best location for the electrodes is on the belly of the muscle
• Larger electrodes have a larger recording surface
• Electrodes aren’t selective and can pick up activity from nearby muscles (crosstalk)
• Electrodes can pick up electrical signals that do not originate from the muscle (ambient noise, electrical activity associated with any given environment)

Question 6

What Extrinsic Factors affect the EMG signal?

Answer 6

The study participants themselves

• Skin and subcutaneous fat acts as a filter
• As the distance between active fibers and the electrodes, the frequency and the amplitude of the EMG signal decreases

The equipment

• The connection between the electrode and skin
• The location of the equipment (relative to one another) and the quality of the equipment will affect the quality of the EMG signal

Question 7

So what does EMG actually measure?

Answer 7

EMG provides information about muscle electrical activity

• EMG can be reflective of muscle strength (indirect info)
• But EMG is not a measurement of force production
• EMG is an indication of muscle activity