Copyright © 2015 Journal of Orthopaedic & Sports Physical Therapy ® . STUDY DESIGN: Laboratory-based experimental study using a repeated-measures design. OBJECTIVES: To determine the effect of varying stimulation parameters of burst-modulated alternating current (BMAC) and pulsed current (PC) on quadriceps femoris muscle force output and fatigue. BACKGROUND: The impact of altering stimulation parameters on muscle force and fatigue using PC has been well described; however, less is known regarding BMAC. METHODS: Quadriceps femoris muscle force was measured during a series of neuromuscular electrical stimulation-induced muscle contractions, with varying combinations of pulse duration and frequency, using PC or varying duty cycles and burst frequencies using BMAC. Additionally, muscle fatigue tests were conducted bilaterally with different stimulation waveforms and parameters. RESULTS: For PC, the product of pulse duration and frequency was strongly predictive of muscle force output (R2 = 0.85, P<.05). When using BMAC, the duty cycle was a strong predictor of force output (R2 = 0.91, P<.05). Altering the frequency during BMAC had no effect on muscle force production, as opposed to the classic forcefrequency relationship consistently observed with PC. Waveform type significantly impacts muscle fatigue, the BMAC resulted in a more rapid rate of fatigue irrespective of stimulation frequency, and it was confirmed again that lower frequencies of PC result in less fatigue during repeated muscle contractions. CONCLUSION: In this study, altering the burst frequency of BMAC did not influence muscle force or fatigue, whereas the duty cycle significantly impacted muscle force production. Frequency of PC impacted both force and fatigue as expected, demonstrating increased muscle force and fatigue with increased frequency.