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ENGG*3150 Lab Experiments

Experiment #3 - Surface Electromyography

In Lab #3, you will collect EMG and goniometer signals for a series of slow arm curls, by bending your elbow, both with and without a set of handweights. You will repeat these measurements for rapid arm curls. The following description of elbow anatomy will help you understand the biomechanics of this action.

Elbow Anatomy

A musculoskeletal model of the prime elbow flexor & extensor muscles with inset views of muscle attachment points at the shoulder and elbow.

 

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Elbow Biomechanics

What forces are acting on the arm?

• gravity pulls downward on the forearm and the ball.
• the biceps muscle pulls upward.
• the lever arms at W and F are greater than that of M therefore the force M must be larger to counterbalance the moment exerted by the forces W and F.
• the biceps muscles support the forearm at 90 degrees of elbow flexion and the lever arm is the perpendicular distance from the tendon to the axis of the elbow joint.

Goniometers

Goniometers and torsiometers are designed for the measurement of limb angular movement. The sendors are attached across the joint employing double-sided medical adhesive tape and connected to instrumentation. The sensors are lightweight an unobtrusive allowing the data of human activity to be displayed or recorded while leaving the subject to move freely in the normal environment.

The goniometers that you will be using in the lab experiment are twin axis which simultaneously measure angles in up to two planes of movement. The measured output is dorsiflexion/plantarflexion, inversion/eversion. The goniometer has two separate output connectors, one measures flexion/extension, the other radial/ulnar deviation. When used to measure a single axis joint such as the knee or elbow, or when measuring a single plane of a twin axis joint, simply connect one channel, the other remains redundant. All twin axis SG series goniometers function the same way, the difference being physical size.

Measurements of the Elbow with Biometrics Ltd. Goniometers

Start with the subject's shoulder in abduction at 90 degrees and the elbow and forearm at neutral. Attach the distal endblock to the forearm with the centre axis of the endblock coincident with the centre axis of the forearm. With the elbow at neutral, move the goniometer to position 2 (maximum length and attach the proximal endblock to the upper arm with the centre of the endblock and the centre axis of the upper arm coincident. The elbow may be fully flexed and extended with the distal endblock freely sliding between positions 1 and 2. Measurement of flexion/extension is obtained from the green plug, the grey plug being redundant. The distal endblock should be mounted in close proximity to the elbow joint. Movements of pronation and supination may be made and will affect the measurement of flexion/extension minimally.

Electromyography is the recording of muscle activation patterns during functional activities such as walking, weight lifting and other motions. The spinal cord accomodates so-called alpha-motor neurons, which are nerve cells having long axons, as part of a nerve, down to the skeletal muscle. Arriving there, each axon branches out and connects to a number of muscle fibers on their individual motor endplates, which are usually located somewhere in the middle of the fiber. In this way, each alpha-motorneuron can simultaneously innervate a group of individual muscle fibers. This structure of motorneuron and its associated muscle fibrs is a basic functional unit in the process of muscle action and is called a motor unit.

When the central nervous sytem activates a motor unit, an electrical impulse travels down the axon and arrives at the motor endplates of the muscle fibers.Upon arrival of the stimulus, neurotransmitters are released here which cause depolarisation waves to propagate along the muscle fibers towards both ends. Because the tissue around the muscle fibers is electrically conductive, this simultaneous depolarization of the fibers in the active motor unit will induce a noticeable electrical effect on an electrode in the vicinity. The signal observed on this electrode is called the motor unit action potential (MUAP). Electrodes placed on or in a muscle will record the algebraic sum of all the MUAPs at a specific point in time.

The Neuromuscular Junction

Nerve impulses (action potentials) traveling down the motor neurons of the sensory-somatic branch of the nervous system cause the skeletal muscle fibers at which they terminate to contract. The junction between the terminal of a motor neuron and a muscle fiber is called the neuromuscular junction. It is simply one kind of synapse. (The neuromuscular junction is also called the myoneural junction.)

The terminals of motor axons contain thousands of vesicles filled with acetylcholine (ACh).

When an action potential reaches the axon terminal, hundreds of these vesicles discharge their ACh onto a specialized area of postsynaptic membrane on the fiber. This area contains a cluster of transmembrane channels that are opened by ACh and let sodium ions (Na+) diffuse in.

The interior of a resting muscle fiber has a resting potential of about −95 mV. The influx of sodium ions reduces the charge, creating an end plate potential. If the end plate potential reaches the threshold voltage (approximately −50 mV), sodium ions flow in with a rush and an action potential is created in the fiber. The action potential sweeps down the length of the fiber just as it does in an axon.

No visible change occurs in the muscle fiber during (and immediately following) the action potential. This period, called the latent period, lasts from 3–10 msec.

Before the latent period is over,

The brief (1–2 msec) period needed to restore the resting potential is called the refractory period

Dr. Gordon Robertson of the University of Ottawa has an excellent website that illustrates the Biomechanical Analysis of Physical Activity . Scroll down to "Course Material" for links to presentations on EMG.

Useful information on the variables of the EMG signal and their applications can be found here