Topics: Muscle, Muscle contraction, Skeletal muscle Pages: 15 (3548 words) Published: August 25, 2013

In this experiment, you will explore the electrical activity of skeletal muscle by recording an electromyogram (EMG) from a volunteer. You will examine the EMG of both voluntary and evoked muscle action and attempt to measure nerve conduction velocity.

Written by staff of ADInstruments.


A skeletal muscle fiber is innervated by a branch of a motor axon. Under normal circumstances, a neuronal action potential activates all of the muscles innervated by the motor neuron. This activation process involves an action potential and a contraction of the muscle fibers. During a contraction, therefore, there is synchronous activity in a number of fibers in the same muscle. The electrical signal recorded from a contracting muscle is called an electromyogram or EMG. Like the electrocardiogram (ECG), this activity can be detected by electrodes placed on the skin. A voluntary muscle contraction is produced by one or more action potentials in many fibers. The EMG activity is not a regular series of waves like the ECG, but a chaotic burst of overlapping spike-like signals.

In this experiment, you will record EMG activity during voluntary contractions of the biceps and triceps muscles of the arm. The raw EMG signal during voluntary contractions may be processed in various ways to indicate the intensity of EMG activity. In the method used here, the negative-going portions of the EMG are inverted, and then the whole signal is integrated in such a way as to smooth out individual spikes, and make the time course of changing activity much clearer. You will also record EMG signals produced by electrical stimulation of a motor nerve supplying a muscle. The abductor pollicis brevis muscle a member of the thenar muscle group on the palmar surface of the hand. The motor nerve to this muscle (the median nerve) is easy to stimulate at the wrist and elbow.

Required Equipment

A computer system
Chart software, version 5.0 or later
PowerLab (with built-in Bio Amp or PowerLab and Bio Amp front-end) Five-lead Shielded Bio Amp Cable & snap-connect Shielded Lead Wires Disposable ECG recording electrodes
Stimulus Bar Electrode
Electrode cream
Abrasive pad
Alcohol swabs
Ballpoint pen
Dry earth strap
Four books of similar weight (about 1 kg each)


Some exercises involve application of electrical shocks to muscle through electrodes placed on the skin. People who have cardiac pacemakers or who suffer from neurological or cardiac disorders should not volunteer for those exercises. If the volunteer feels major discomfort during the exercises, discontinue the exercise and consult your instructor.

Setup of the experiment

A. Subject preparation and equipment setup

1. Select a volunteer for the experiment and have that person remove any watch, jewelry, etc. from their wrists.

2. Plug the five-lead Bio Amp cable into the Bio Amp(Figure 1).

3. Plug the lead of the dry earth strap into the Earth connection of the Bio Amp cable. Since this cable only has one connection pin, plug it into the upper post on the shielded Bio Amp cable (Figure 2).

4. Firmly attach the dry earth strap around the palm or wrist of the volunteer. The fuzzy side of the dry earth strap needs to make full contact with the skin.

5. Swab the skin with alcohol in each area where electrodes will be placed on the volunteer (Figure 1). With a ballpoint pen, lightly mark two small crosses on the skin overlying the biceps muscle, in the position for the biceps recording electrodes shown in Figure 1. The crosses should be 2–5 cm apart and aligned with the long axis of the arm. Lightly abrade the marked skin with an abrasive pad. This decreases the resistance of the outer skin layers and ensures good electrical contact.

6. Prepare the skin over the triceps for attaching the electrodes as outlined in step 5 for the biceps. The position for the triceps recording electrodes is shown in...
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