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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.
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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 Neuromuscular Junction w The junction is a site where a motor neuron communicates with a muscle fiber. w Motor axon terminal releases neurotransmitters (such as acetylcholine or epinephrine) which travel across a synaptic cleft and bind to receptors on a muscle fiber. w This binding causes depolarization, thus possibly causing an action potential. w The action potential spreads across the sarcolemma causing the muscle fiber to contract.
Resting Membrane Potential (RMP) w Difference between the electrical charges inside and outside a cell, caused by separation of charges across a membrane w High concentration of K+ inside the neuron and Na+ outside the neuron w K+ ions can move freely, even outside the cell to help maintain imbalance w Sodium-potassium pump actively transports K+ and Na+ ions to maintain imbalance w The constant imbalance keeps the RMP at –70mV
Changes in Membrane Potential 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) Actionpotentials—rapid, substantial depolarization of the membrane (–70 mV to +30 mV to –70 mV all in 1 ms)
What Is an Action Potential? w Requires depolarization greater than the threshold value of 15 mV to 20 mV w Once threshold is met or exceeded, the all-or-none principle applies
Events During an Action Potential 1. The resting state 2. Depolarization 3. Propagation of an action potential 4. Repolarization 5. Return to the resting state with the help of the sodium- potassium pump
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 mV as an amplitude value • the amount of EMG • Expressed in Hz as a frequency value • the rate at which the action potentials are conducted
EMG Uses • To quantify training adaptation • The increases in strength are generally due to two things • 1. Neural adaptations • 2. Hypertrophy
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
EMG Uses • To determine the best lifts for recruiting a specific muscle during strength training
EMG Uses • Linearly Related to Submaximal Force Production
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