Muscle Activation

Concepts in Electromyography

EMG

The recording of muscle action potentials (MAPs)

Recorded with surface electrodes as the MAPs are conducted along the sarcolemma

Reflects muscle activation

Motor Unit

The motor neuron and all the muscle fibers that it innervates

THE STRUCTURE OF A MOTOR NEURON

The junction is a site where a motor neuron communicates with a muscle fiber.

Motor axon terminal releases neurotransmitters (such as acetylcholine or epinephrine) which travel across a synaptic cleft and bind to receptors on a muscle fiber.

This binding causes depolarization, thus possibly causing an action potential.

The Neuromuscular Junction

The action potential spreads across the sarcolemma causing the muscle fiber to contract.

THE NEUROMUSCULAR JUNCTION

EVENTS LEADING TO MUSCLE ACTION

Difference between the electrical charges inside and outside a cell, caused by separation of charges across a membrane

High concentration of K+ inside the neuron and Na+ outside the neuron

K+ ions can move freely, even outside the cell to help maintain imbalance

Resting Membrane Potential (RMP)

Sodium-potassium pump actively transports K+ and Na+ ions to maintain imbalance

The constant imbalance keeps the RMP at –70mV

Depolarization—inside of cell becomes less negative relative to outside (> –70 mV)

Hyperpolarization—inside of cell becomes more negative relative to outside (< –70 mV)

Graded potentials—localized changes in membrane potential (either depolarization or hyperpolarization)

Changes in Membrane Potential

Action potentials—rapid, substantial depolarization of the membrane (–70 mV to +30 mV to –70 mV all in 1 ms)

Requires depolarization greater than the threshold value of 15 mV to 20 mV

Once threshold is met or exceeded, the all-or-none principle applies

What Is an Action Potential?

1. The resting state

2. Depolarization

3. Propagation of an action potential

Events During an Action Potential

4. Repolarization

5. Return to the resting state with the help of the sodium-potassium pump

AN ACTION POTENTIAL

EMG

Surface EMG reflects the whole muscle rather than isolated motor units

The firing of many motor units is observed simultaneously

Results in a wave form that is the summation of all the activity in the range of detection of the electrodes.

Recording of the Signal Surface Electrodes Acquired at 1000 points per second via

a data acquisition system (Biopac) Amplified Data stored on a computer Filtered at 10-500 Hz

Unit of the Signal

Expressed in V as an amplitude value– the amount of EMG

Expressed in Hz as a frequency value– the rate at which the action potentials are

conducted

0.0000 2.0000 4.0000 6.0000 8.0000 10.000 12.000seconds

-2.00000-1.50000-1.00000-0.500000.000000.500001.000001.50000

Volts

emg v

l

-0.500000.000000.500001.000001.500002.000002.500003.000003.50000

Volts

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EMG Uses

To quantify training adaptation The increases in strength are generally

due to two things– 1. Neural adaptations– 2. Hypertrophy

MODEL OF NEURAL AND HYPERTROPHIC FACTORS

Training Adaptations (cont.)

Reciprocal Inhibition - inhibition of the antagonistic muscles to allow for a greater expression of strength

Cross-training - get stronger in the untrained limb even though not training it

EMG Uses

To monitor Fatigue– Increase in the amplitude over time at

submaximal levels• recruiting more and more fibers as fatigue

progresses

– Shift to lower frequencies with fatigue• decease in motor unit firing rate• decreased conduction velocity

EMG Uses

Fatigue Threshold– theoretically, is the level at which you could

continue without fatigue– EMG has been used to determine this level

Electromechanical Delay– time lag between stimulation to a muscle

and force production

5800.0 5850.0 5900.0 5950.0 6000.0 6050.0 6100.0 6150.0milliseconds

-0.50000

0.00000

0.50000

1.00000

Voltsemg

-6.00000-4.00000-2.000000.000002.000004.000006.000008.0000010.0000

Volts

mmg

EMG Uses

To determine the best lifts for recruiting a specific muscle during strength training

EMG Uses

Linearly Related to Submaximal Force Production

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0 20 40 60 80 100

% MVC

EMG

Ampl

itude

EMG

EMG Uses

To determine recruitment order– Normal sequence when falling

• rotation of ankle - contraction of the tibialis anterior - quadriceps - hip - abdominal muscles

– Sequence in the elderly• activate the hip then the quadriceps• the quadriceps are slow to contract

– The elderly recruit muscles in a different order when they begin to fall

Recruitment (cont.)

In addition, the elderly have a greater amount of co-contraction which results in a stiffer response (the elderly tend to recruit muscles that don’t need possibly due to lack of confidence)

EMG Uses

Fiber Typing– muscles made of predominately slow twitch

muscle fibers (ie., soleus) have a lower frequency signal then muscles with predominately fast twitch fibers (quads)

EMG Uses

Clinically– diagnose muscle diseases (cerebral palsy)– emotional assessment (more muscle tone

and resting EMG when stressed)– biofeedback either auditory or visual

(relaxation training or pain control)• can decrease your EMG simply by watching the

signal