Muscle Activation Concepts in Electromyography

EMG n The recording of muscle action potentials (MAPs) n Recorded with surface electrodes as the MAPs are conducted along the sarcolemma n Reflects muscle activation

Motor Unit n 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 n Surface EMG reflects the whole muscle rather than isolated motor units n The firing of many motor units is observed simultaneously n 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 n Surface Electrodes n Acquired at 1000 points per second via a data acquisition system (Biopac) n Amplified n Data stored on a computer n Filtered at Hz

Unit of the Signal Expressed in  V as an amplitude value –the amount of EMG n Expressed in Hz as a frequency value –the rate at which the action potentials are conducted

EMG Uses n To quantify training adaptation n The increases in strength are generally due to two things –1. Neural adaptations –2. Hypertrophy

MODEL OF NEURAL AND HYPERTROPHIC FACTORS

Training Adaptations (cont.) n Reciprocal Inhibition - inhibition of the antagonistic muscles to allow for a greater expression of strength n Cross-training - get stronger in the untrained limb even though not training it

EMG Uses n 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 n Fatigue Threshold –theoretically, is the level at which you could continue without fatigue –EMG has been used to determine this level n Electromechanical Delay –time lag between stimulation to a muscle and force production

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

EMG Uses n Linearly Related to Submaximal Force Production

EMG Uses n 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.) n 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 n 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 n 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