Design Elements of Muscle Stimulators

Due to the wide variety of electrical stimulation units that are available and the numerous combinations of design elements, many practitioners are confused as to what electrical stimulation really is. This is not surprising because researchers and manufacturers have used different terminology to discuss similar systems.

There are several parameters that define the different types of electrical stimulation units. These parameters can be combined in a variety of ways and it is essential that the correct combination be used to obtain a system that is effective and comfortable.

The following information discusses the parameters that are involved in developing electric stimulation systems.

CURRENT TYPE

There are two types of currents used in electrical stimulation

AC or alternating current

• Continuous, two directional flow of current (positive and negative)
• This is the form used in household appliances
• Sometimes incorrectly referred to as Faradic in literature
• A 120 current obtains it’s name because the current reverses direction 120 times per second, completing 60 full cycles
• In physical therapy, this current is generally preferred because of greater patient comfort

DC or direct current

• Continuous, one directional flow of current, the direction is determined by the polarity selected
• Can be positive or negative
• This is the form found in a battery or DC generator
• Sometimes incorrectly referred to as Galvanic in literature
• Since the charge is one-directional, electrolysis at the electrode/tissue contact could occur because there is a non-zero net charge. Therefore, ions accumulate at electrodes, causing excessive accumulation of corrosion at an electrode. This can be more of a problem with denervated
  muscle stimulation and for implanted electrodes.

OTHER DESIGN ELEMENTS

There are several other parameters that define the different types of electrical stimulation units that are available. These parameters can be combined in a variety of ways and it is essential that the correct combination be used to obtain a system that is effective and comfortable.

These different parameters include:

CURRENT FLOW

Current or flow can be used to describe both an alternating current (AC) and a direct current (DC). The term flow is sometimes interchanged with the term “current”. This can be confusing because a “continuous current” is the same as direct current (DC), but a “continuous flow” can describe both a direct (DC) or alternating current (AC).

Continuous flow (current)
No interruption of the current flow

Pulsed flow (current)
Pulsed flow (current) also referred to as Intermittent flow (current)
Periodic interruption of the current flow (interpulse interval)
Allows for an adjustment of frequency and phase

WAVEFORMS

Different waveforms produce different contraction intensities and different levels of fatigue. The “waveform is an important consideration in the choice of an appropriate muscle stimulation regimen” (Laufer et al, 2001)

Three basic criteria are used to evaluate the appropriate stimulation waveform:

1. The mean stimulation current required to achieve the desired muscle contraction tension
2. The subjective comfort of the stimulation
3. The physiological responses to the electrical stimulation (tissue injury)

Waveforms are the change of the current from zero. The value of zero is called the baseline.

Symmetrical

• The same signal is repeated
• The signal can be above the baseline (+) or below the baseline (-)

Asymmetrical

• Different signals are used in one pulse duration

Types of Pulses that Produce a Waveform

Monophasic (Unidirectional)

• Each pulse contains one phase that deviates from the baseline in one direction only
• Can be positive or negative

Biphasic

• Each pulse contains two phases that deviates from the baseline in two directions
• Positive and negative pulse
• Balanced Biphasic (Bidirectional) where both pulse deviations are equal

Polyphasic (each pulse contains three or more phase deviations)

• Also called: Burst AC
• Medium-frequency stimulation
• Carrier-frequency stimulation

Examples of some waveforms include:

Source: Wikepedia

Etc…
Combinations of several different waveforms are also used

PULSE DURATION

Pulse Duration is the length of time the current is flowing.

Nerve tissue responds quickly to current
Sensory nerves respond to the duration of a constant pulse of 100 microseconds or shorter

Muscle tissue responds slowly, therefore longer duration stimuli are used.
Motor nerves respond to the duration of a constant pulse of 500 microseconds or shorter

In electrical stimulation units a single pulse generally produces a short-lived muscle twitch of not more than 250ms. If the pulse duration is longer than this, the muscle does not have time to relax between stimuli and eventually tetanic (continuous) contraction occurs.

FREQUENCIES OF PULSE

The Frequency of the Pulse is the period of time the current flow is active.
Generally the following is used as a guideline, however variations are used to elicit different responses:

• Nerve tissue responds to high frequencies over short durations.
• Sensory nerves respond to 100-150 Hz (cycles per second).

• Muscle tissue responds to a lower frequency, therefore longer duration stimuli are used.
• Motor nerves respond to 25 Hz (cycles per second).

The higher the stimulation frequency, the faster the muscle fatigues

In electrical stimulation units (FES) used for controlling limb movement, a compromise frequency is generally used. This compromise frequency creates a smooth response that does not quickly fatigue the tissue.

EMG values obtained by electrical stimulation look similar to contractions recorded from voluntary movement, however this does not prove true in the clinical setting. If motor neurons are innervated voluntarily in an asynchronous manner, tetany is achieved at much lower rates of 5-25 Hz.

RAMPING OF CURRENT FLOW

The Ramping of the Current Flow is the time the waveform takes to reach maximum amplitude.

Nerve tissue responds quickly to current, but requires a current that rises rapidly to maximum intensity.
Muscle tissue responds with very slowly rising currents

The rate of rise of the pulse is also important for function and comfort.
Too slow of a rise time results in changes in the tissue membrane known as accommodation.
Accommodation gradually elevates the threshold required for the nerve to fire.
The pulse durations used in electrical stimulation do not, in general, allow this effect to occur.

SUMMARY

Nerve and muscle tissue responds to electric stimulation in different ways.

The threshold change necessary for eliciting a muscle fiber action potential is generally much greater than the threshold necessary to activate the neurons of nerves.

Nerve tissue responds quickly to a current that rises rapidly to maximum intensity.

Muscle tissue responds very slowly to a current that rises gradually at a lower frequency, therefore longer duration stimuli are used.

Different electrical stimulation parameters must be used for muscle and nerve stimulation.

Electrical stimulation can be placed on different areas of the body to elicit different responses. The two different types of sites used for stimulation are motor points and sensory points.

Motor points are stimulated to mimic the same signal that the brain sends to the muscle therefore evoking actual muscle contractions.

Sensory points are stimulated to mimic nerve responses.

A comparison of the general differences between the responses of muscle and nerve tissue:

FES uses a compromise of stimulation parameters to activate both muscle and nerve tissue requiring highly sophisticated control of the stimulation parameters.

Over the last several decades, research to find the appropriate combination of design elements has produced an effective and comfortable FES modality.