the number of motor units activated, and the larger the evoked muscle contraction output of force.
The higher the current amplitude, the higher the effectiveness of NMES for muscle strengthening in
both healthy individuals and patients.
Current output CC
Stimulators used for NMES are constant current type (CC-type) stimulators. This means that a set
currrent amplitude (e.g. 80 mA) remains constant (hence, the term constant current during treatment,
regardless of changes in tissue impedance over time).
The main advantage of CC-type stimulators is that they deliver predictable levels of electrical
stimulation, making therapy more predictable and comfortable for the patient.
Pulse duration
In NMES, the duration of pulses often ranges from 100-600 s. To produce a contraction of a
designated intensity, it should be remembered that the shorter the pulse duration, the greater the
pulse amplitude needed. Recent evidence would suggest that pulse duration is possibly a less
important control than the intensity or the frequency and the most effective setting in the clinical
environment is probably around 200 s. Stronger muscle contractions occur with 300-400 s pulses,
but these will also produce significant stimulation of sensory fibers.
Pulse frequency
The frequency is very important in NMES because the physiological correlate of frequency is temporal
MU activation. In other words: setting the pulse/burst/beat frequency is like setting the frequency at
which all the activated MUs will fire during the evoked muscle contraction. Pulsed biphasic current
may be delivered as pulses (pulse frequency) or bursts of pulses (burst frequency). Setting the
frequency between 30 and 60 Hz is necessary to obtain optimal temporal MU activation dduring the
fused tetanic muscle contraction, because research has established that the mean fusion frequency
for skeletal muscles is approximately 50 Hz (or pps/bups). lnitially, low frequencies (20 Hz) and short
contraction/ long relaxation times can be used to minimize muscle fatigue. It is worth noting that the
rate of muscle fatigue during NMES is greater than that seen during voluntary contraction.
The stimulation frequency also affects force generation. Maximum forces are produced with tetanic
contractions at 60 – 100 Hz. However, these frequencies not only produce higher fatigue, but also
more discomfort and potential for muscle damage, especially with patients (tetanic stimulation is
widely researched with athletes/ fit individuals rather than patients suffering from muscle
dysfunction). Lower forces (about 65% force) are achieved with stimulation at lower frequencies of
20 Hz that produce much less fatigue. However, comparison of strength gains produced at 20 Hz, 45
Hz and 80 Hz in normal quadriceps femoris showed no significant difference.
Duty cycle (ON:OFF ratio)
The ON-time is the time, rneasured in seconds, during which electrical current is delivered to the
target muscle. Its duration corresponds to the duration of the volitional muscle contraction.
Conversely, the OFF-time is the time, also measured in seconds, during which there is no current
flowing in the target muscle. Its duration corresponds to the duration of the rest period between two
successive contractions. The relationship between ON-time and OFF-time is commonly expressed as
a ratio.
For example, setting a 10 second ON-time and a 30-second OFF-time would produce an ON:OFF
ratio of 10s:30s. Such a ratio means that the resting time between two successive evoked contractions
is three times longer than that of contraction time (1:3).
The ON:OFF ratio should be modified to match the fatigue characteristics of the muscle being
stimulated. A moderate ramp of 2-3 seconds should be used except in cases of high current