Electric Fields
Introduction
In the previous activities in this module, we have studied the basic components of an electrical system. In
order for a current to flow, there needs to be a potential difference. This potential can be supplied by one
of a variety of objects such as a battery or a charged capacitor. The current that flows in a circuit will
encounter resistance, and if the circuit is made of ohmic material, the current will be proportional to the
applied voltage.
So far, all of these activities have involved the normal equipment that is used to study electricity:
manufactured circuit elements. We have studied capacitors, resistors, and power supplies. For this
capstone activity, though, we are going to investigate a much different electrical system: the human body.
As we have mentioned before, the cell membrane operates much like a capacitor, as a potential
difference due to ion concentration differences is maintained across the membrane. When a nerve or
muscle cell is activated, the membrane opens in certain protein channels and allows the ions to flow, thus
creating a current. This changes the potential difference across the membrane, which will be picked up
as an electronic signal.
EMG
Electromyography (EMG) is a medical test that records the electrical properties of muscles both at rest
and during activity. It records the voltage that is output by a muscle over time, with a greater output
indicating that more muscle fiber is being activated. In medical settings, it is normally used by doctors to
determine if the reduced function of a muscle is due to damage or whether it is due to pain/soreness. If
the muscle is damaged, then one would expect to see a different size and frequency to the voltage output
by the muscle while it is at rest or contracting. By probing different parts of the muscle, doctors are able
to determine where exactly the damage is, if any, and take
appropriate action. Of course, this localized probing requires close
placement to the area in question, which means the use of needles to
insert the probe to the exact location.1
There are non-invasive versions of EMG’s, though, which are used in
a variety of activities. Athletes use them to study the output by their
muscles as they are training to see if they are getting greater
strength. The stronger the total muscle, the more muscle fiber that
gets activated during exercise, which leads to a greater voltage output
on an EMG chart. They are also used by patients in biofeedback
studies to determine when muscles are tense (contracted) and when
they are not. For example, patients who are suffering from bladder
incontinence can use EMG’s to aid them in building greater strength
in pelvic muscles that prevent the flow of urine. There is also work
being done by high tech firms to implant EMG’s into devices like
gloves that can be used to control robotic devices.2
Fig. 1: Portable EMG (NASA)
