Muscle Stimulators (EMS)

Or TEMS Transcutaneous Electrical Muscle Stimulation
There are many types of systems on the market that are referred to as muscle stimulators. Some of these systems cannot produce a muscle contraction without causing extreme discomfort. In addition, other modalities can only be applied at lower intensities and therefore can only stimulate the surface of the skin. The electric stimulation modality must be correctly designed to produce the desired beneficial results.

TYPES OF MUSCLE STIMULATORS

Comparisons of the Types of Muscle and Nerve Stimulators are found in the following pages of this website:

Confusion exists when authors mislabel TEMS (Transcutaneous Electrical Muscle Stimulation) systems as TENS (Transcutaneous Electrical Nerve Stimulation) systems. TEMS units and TENS units are two different systems and they are used for two different applications.

In technical terms, electrical stimulation involves depolarizing nerve/muscle fibers through an externally applied electric current. If depolarization reaches threshold, an influx of sodium ions from the extra-cellular space to the intracellular space produces an action potential in these fibers. The action potential will be transmitted across the neuromuscular junction and will cause the muscle fiber to contract in the same manner as occurs in healthy tissue.

At the electrode-tissue interface a conversion occurs between the current of electrons passing through the wires and the current of ions moving within the tissue. Then through this externally applied current, the depolarization of nerve and/or muscle to threshold, is produced by the transport of ions across the tissue membrane.

Stimulation must occur to threshold to obtain a muscle contraction. There is no consistent value that can be given to the intensity of the current necessary to reach threshold. This is because several factors influence the threshold level, which include; the condition of the membrane, the condition of the environment (humidity, temperature, etc), the duration of the current pulse and the resistance of the membrane.

NERVE AND MUSCLE RESPONSES TO ELECTRICAL STIMULATION

Nerve tissue responds quickly to current but requires a current that rises rapidly to maximum intensity. High frequencies over short durations are used. Sensory nerves respond to 100-150 Hz and 100 microseconds or shorter durations. Electrical stimulation on the skin surface always results in activation of sensory receptors on the surface of the skin.

Galvanic skin resistance (GSR) is a measure of the sympathetic nervous system’s response to painful stimuli. Electrodes secured to the skin measure the skin resistance, and the output is amplified using a strain gauge amplifier. (Danilov et al, 1994) The increase in GSR from baseline suggests a diversion of the blood flow due to the vasoconstrictive response of the sympathetic nervous system. (Bennie et al, 2002)

Muscle tissue responds with very slowly rising currents and a lower frequency, and longer duration stimuli are used. Motor nerves respond to 25 cycles and 500 microseconds or shorter durations.

Muscle that is denervated will respond to electrical stimulation in a different manner than will healthy muscle tissue. Denervated muscle produces a slower and more sluggish contraction when compared to the quick and clear response of healthy muscle contractions.

Two different methods of determining motor points on the muscle are:

  1. By using the negative pulse of an asymmetric balanced biphasic waveform, using a repetition rate of 1 pps. Motor points are confirmed with the rate increased to 50 pps (Baker et al, 1988).
  2. By providing stimulation at 200 cycles/sec to the muscle. The points of lowest impedance on the muscle is determined to be the motor points for that muscle (Bennie et al, 2002).