Electrical Current/Waveforms

Three types of currents are commonly used: (1) continuous direct current (DC) (Figure 20-1, A), (2) continuous alternating current (AC) (Figure 20-1, B), and (3) pulsed current (either AC or DC) (Figure 20-1, C). Continuous direct current is a unidirectional electrical current that flows for 1 second or longer; alternating current changes the direction of flow at least once every second. Continuous direct current (also called galvanic current) has been used for wound healing, iontophoresis, and in the treatment of denervated muscle. In current clinical practice it is used only for iontophoresis. Continuous alternating current is not used therapeutically, but is our typical line current (see Figure 20-1, B).

Pulsed current consists of a unidirectional or bidirectional flow of charges (DC or AC) that periodically stops for a finite time period. All NMES devices are pulsed current stimulators. Phase duration (also referred to as phase width) is defined as the time in which the current flows from the baseline in one direction and back to the baseline (Figure 20-2).

Pulse duration (also referred to as pulse width) is defined as the time during which charge flows in both directions (see Figure 20-2). In a monophasic current, the pulse duration and the phase duration are the same. In a biphasic current, two phases make up one pulse. Pulsed current consisting of a bidirectional flow of charge is called biphasic pulsed current.

When the flow in each direction is the same in time and amplitude-dependent features, the biphasic current pulse is considered symmetric (Figure 20-3, A). If the time- and amplitude-dependent features differ, then the biphasic current is termed asymmetric (Figure 20-3, B). Balanced asymmetric biphasic pulsed current occurs when the total charge in one phase equals the total charge of the other phase (Figure 20-4). If the total charge in each phase is unequal, it is termed unbalanced (Figure 20-5). Both pulsed AC and DC current forms are commonly used in portable and clinical model NMES units.

Stimulator Parameters

Current amplitude (also called magnitude or intensity) is defined as the vertical distance from the highest to the lowest peak during one electrical wave and is typically measured in milliamperes (mA) (Figure 20-6). Increasing the amplitude induces a stronger force of muscle contraction by recruiting additional muscle fibers at greater distances from the electrodes.^1,3

Skin produces resistance to current flow by ohmic resistance and capacitive impedance. Capacitive impedance is patient dependent and cannot be modified. Clipping or shaving the hair coat and cleaning the skin with alcohol to wash away skin oils or other substances helps to lower ohmic resistance. Lowering the skin resistance diminishes the driving voltage that is necessary for current penetration of the skin, potentially making the treatment more comfortable.1,3

Increased current amplitude is required to produce a given amount of muscle contraction force if the pulse or phase durations are short (<100 µsec). Portable NMES units are not always capable of producing the current output that clinical models can produce; however, most units sold for electromedical purposes are adequate. Some bargain simulators possess a very low phase charge (<10 microcoulombs) and cannot create adequate contraction of large muscle groups.

Pulse durations of 150 to 400 microseconds produce powerful contractions while minimizing the likelihood of recruiting many pain fibers.1,3 Some stimulators that are made only for NMES do not allow control of pulse duration, but are typically set in this range. As the pulse duration increases, smaller diameter pain fibers are recruited. One study has examined pulse durations in dogs and subsequently recommended pulse durations by individual muscles (Table 20-1). This study also examined the differences in needed pulse durations between people and dogs. Although they are similar, dogs in general do not require as long a pulse duration as human motor nerves to depolarize. This can potentially increase comfort by using the shortest recommended pulse duration.⁵

Pulse rate (also called frequency, rate and pulses per second [pps]) is the number of pulses delivered per second and is measured in hertz (Hz). Tetanic muscle contractions may be produced with frequencies as low as 20 Hz, but only submaximal muscle contractions are typically produced in this range. Maximal force of contraction generally occurs between 60 and 100 Hz. However, as frequency increases the rate of fatigue also increases. Lower tetanic frequencies in the range of 35 to 50 pps reduce muscle fatigue while still providing strong muscle contractions.1,3

Duty cycle is the ratio of on-time to total cycle time, expressed as a percentage (Figure 20-7). On-time is the period of time in which a series of pulses or bursts are delivered to the patient. Off-time is the time between on-times. A single on-time duration plus a single off-time duration constitutes the total cycle time. For example, a stimulator that causes a muscle to contract for 10 seconds followed by a 30-second rest would have a 25% duty cycle, or a 1 : 4 ratio. As on-time increases, muscle fatigue increases.² Optimal duty cycles vary, depending on the patient. A patient with severe atrophy may require a longer off-time to recover between contractions. Many clinicians start with duty cycle ratios between 1 : 2 and 1 : 5 and watch for signs of fatigue, which indicates the need for a longer off-time.

Ramp is a feature of NMES that helps improve patient comfort. It involves a gradual increase or decrease in current amplitude such that the number of recruited motor units gradually increases the force of muscle contraction or gradually decreases the force of contraction (Figure 20-8). The ramp time is the period of time over which the pulse is increasing or decreasing. No data to date have identified the optimal ramp time. However, a ramp up time of 2 to 4 seconds is commonly used to maximize comfort with a 1- to 2-second ramp down.¹ Suggested current parameters for muscle strengthening are listed in Box 20-2.

Electrodes

Many types of surface electrodes are available. The main criteria in choosing electrodes are as follows:

An alternative to traditional electrodes is the use of electrodes with multiple fine wires or prongs that make contact with the skin (see Fig. 20-13). The advantage of these types of electrodes is that the hair does not need to be clipped. Rather the fine wire prongs are worked down among the hair to contact the skin. Wetting the hair before treatment helps with conductivity.

Recruitment

Neuromuscular electrical stimulators recruit Type II (fast twitch) fibers first, then Type I (slow twitch), which is the reverse of the muscle recruitment pattern in a volitional contraction.6–8 Increasing the pulse duration increases the recruitment of smaller diameter motor units at the same depth, but increasing the pulse duration too much (800 µsec) may stimulate undesirable fibers (small-diameter pain fibers). Increasing either the amplitude or the pulse duration affects the strength of contraction because additional muscle fibers are recruited. Increasing the frequency results in the existing motor units firing at a faster rate and will increase the strength of contraction, but it also causes more rapid fatigue. Application of NMES at an optimal frequency results in an optimal physiologic response (the desired contraction) while minimizing fatigue. In a healthy uninjured individual, a maximal voluntary muscle contraction produces a greater torque (more powerful contraction) than occurs in an electrically induced contraction. However, patients with injuries or immediately following surgery may be unable or unwilling to produce a maximum voluntary muscle contraction. In these patients NMES may produce a stronger muscle contraction.

What to Look for When Buying a Neuromuscular Electrical Stimulator Unit

An acceptable NMES unit should allow flexibility in a number of parameters, including frequency (adjustable from at least 1 to 50 Hz), adjustable pulse duration (50 to 200 µsec or more), on/off times, ramp, and multiple channels (at least two to allow co-contraction or alternating contraction of two different muscle groups). Acceptable clinical units may cost $300 or more.

Effect on Muscle Enzymes

In a study regarding the effects of low-frequency electrical stimulation on the metabolic profile of skeletal muscle, a portable NMES unit was used to stimulate the knee extensor muscles of sedentary men and women 6 days a week for 6 weeks.¹⁴ No changes in creatine kinase concentrations were found in vastus lateralis muscle samples before or after NMES. There was a small decrease in phosphofructokinase, the rate-limiting enzyme of glycolysis in human muscle, which was only statistically significant in men. Hexokinase, a regulatory enzyme of skeletal muscle glucose phosphorylation, was only significantly increased in the vastus lateralis of women. Krebs’ cycle and electron transfer chain marker enzyme activity of fatty acid oxidation increased significantly in both sexes, with the greatest change occurring in women. The authors concluded that NMES can significantly increase the skeletal muscle aerobic-oxidative potential of sedentary subjects. In a study by Arvidsson et al.¹⁵ there was no difference between a postoperative ACL reconstruction–NMES-treated group and an isometric exercise group in the activity of the oxidative enzyme citrate synthase or phosphofructokinase. Research by Wigerstad-Lossing et al.¹⁶ showed no significant reduction in the activity of the oxidative and glycolytic enzymes citrate synthase and triphosphate dehydrogenase in NMES patients following knee ligament surgery compared with untreated control knee ligament surgery patients, which had decreased enzyme activity.