Question

1. Electromyography (EMG)

(a)  Consider the technique of electromyography (EMG) to examine muscle function about joints during movement. Explain where the captured, high frequency raw signal originates, and describe the physiological basis with reference to the relevant parts of the neuromuscular system.                                                                                         [8 marks]

(b) When processing EMG signals, the following terms are frequently encountered in the literature:

(i) Band Pass filtering

(ii) Full wave rectification

(iii) Normalisation to maximum voluntary contraction (MVC)

(iv) Median frequency

Please explain what each of these terms mean, and describe their relevance/role in the context of interpreting EMG.                

Answer 1

a)Electromyography or EMG is a technique that records the movement of muscles. It is based on the fact that when our muscle contracts an electric activity is generated and this is recorded from the areas of neighboring skin. Electrodes are placed close to the muscle groups to record the activity during EMG. When muscle is activated then the electrical activity in this muscles are propogated as action potentials along the sarcolemma and are picked up by the electrodes.

Physiological basis of EMG

The neuromuscular system includes motor neurons in spinal cord, sensory neurons in dorsal root ganglion and skeletal muscle fibers that links Central nervous system to Peripheral Nervous system. This system controls the movements of body, posture control and breathing. Electrical impulses are triggered from motor neuron , it propagates along motor axon. At the neuromuscular junction this impulse results in the release of acetylcholine a neurotransmitter from motor nerve terminal. The muscle fibers has receptors that bind acetylcholine and it opens ion channels changing the membrane potential of muscle. This leads to a muscle contraction after a chain of events. In skeletal muscles organs such as spindles and Golgi tendon senses changes in the muscle length and tension. This change causes electric impulses that is carried along the fibers of dorsal root ganglion neurons that activates the motor neuron.This activation in turn generates electrical impulses along the motor axon.Any dysfunction in this system is detected using the Electromyography technique.Any abnormalities in this electrical impulses is detected by EMG.Activities are measured in microvolts and if there is stronger contractions the recorded voltage amplitude is high or if no activity it shows electrical activity at rest.

b)i)Band pass filtering: The EMG signals from the surface of skin is contaminated by noise signals. All low and high frequency signals are removed by a filtering method called band pass filtering. Band pass filter removes baseline drift and any DC offsets. Cut off is at 5 to 20Hz.Band pass also removes high frequency noise so that the original signal can be identified.High frequency values are 200Hz to 1KHz.

ii)Full wave rectification: It is a rectification process to obtain the absolute value of raw signal.In order to ensure that the average value of signal is not zero this rectification is essential as the signals have positive and negative components. In full wave rectification the EMG signal from below baseline is added to signal above baseline to make a positive conditioned signal.

iii)Normalization of MVC: Normalization is converting the EMG signal to a scale that is relative to a known and repeatable value. MVC normalization uses a root mean square value from a recording .The common method is using a maximal voluntary isometric contraction value as the reference. EMG is used to determine force of an isometric contraction after it is normalized to the Maximum voluntary contraction of a particular muscle using an isometric contraction.

iv)Median frequency: It is a value of frequency at which the EMG spectrum of power is divided to two regions with equal amplitude. It is the frequency standard that indicates fatigue.