How do symmetrical and asymmetrical biphasic pulses differ in NMES?

• A. Symmetrical pulses have equal charge in both phases
• B. Asymmetrical pulses have unequal amplitudes or durations to minimize net charge
• C. Symmetrical pulses produce higher contraction for all muscles
• D. Asymmetrical pulses cannot be used for NMES

Answer: (A)

In NMES, a biphasic pulse consists of a positive phase and a negative phase. If the two phases carry equal charge, the waveform is symmetrical because charge per phase is the product of current and time, so equal current-time products give equal charges for both phases. This balance yields zero net charge per pulse, which helps prevent unwanted tissue polarization and electrode wear, and keeps stimulation predictable and comfortable.

Asymmetrical pulses are designed with unequal phase characteristics to shape recruitment or comfort, but their purpose isn’t simply to minimize net charge. They can still be used in NMES, and symmetry does not guarantee higher contraction for all muscles.