This study investigated the modulation of prelanding muscle activity with normal or perturbed vision under conditions where jump height was known or unknown. Lower leg electromyography (EMG) was monitored while subjects jumped from a constant (0.45 m) or variable (0.35-0.55 m) heights when vision was unaltered or perturbed by a strobe light, blackened goggles or a visual conflict dome. Results indicated that visual perturbations had a global effect on muscle onsets only when height was randomized. Randomization rather than jump height also affected the onset patterns observed for normal vision jumps. It is suggested that strategies for triggering prelanding EMG are mutable and may depend on availability of sensory modalities and impact force estimation as well as on vision.