Maximal versus submaximal intensity stimulation with variable patterns.

It is essential to determine optimal parameters of stimulation to maintain muscle force during neuromuscular electrical stimulation (NMES). Protocols that increase in frequency and include doublets can prolong force output over time. However, stimulation intensity level could differentially affect muscle force output during variable-frequency NMES. We compared three intermittent stimulation patterns at maximal and submaximal intensities of stimulation of the median nerve: (1) a constant 20-HZ pattern; (2) 90 s at 20 HZ followed by a 90-s increase from 20 to 40 HZ; and (3) 90 s at 20 HZ followed by 90 s of doublets at 20 HZ. At submaximal intensities, the doublet pattern produced the highest overall force-time integral (FTI). At maximal intensities, the doublet pattern produced the lowest FTI and the increasing frequency pattern produced the least amount of fatigue. Thus, double-pulse stimulation was more effective during submaximal than maximal intensity NMES. These data demonstrate that intensity level must be taken into consideration when programming frequency patterns for NMES devices.