When assessing passive joint range of motion in neurological conditions, concomitant involuntary muscle activity is generally regarded small enough to ignore. This assumption is untested. If false, many clinical and laboratory studies that rely on these assessments may be in error. We determined to what extent small amounts of involuntary muscle activity limit passive range of motion in 30 able-bodied adults. Subjects were seated with the knee flexed 90 degrees and the ankle in neutral, and predicted maximal plantarflexion torque was determined using twitch interpolation. Next, with the knee flexed 90 degrees or fully extended, the soleus muscle was continuously electrically stimulated to generate 1, 2.5, 5, 7.5, and 10% of predicted maximal torque, in random order, while the ankle was passively dorsiflexed to a torque of 9 N.m by a blinded investigator. A trial without stimulation was also performed. Ankle dorsiflexion torque-angle curves were obtained at each percent of predicted maximal torque. On average (mean, 95% confidence interval), each 1% increase in plantarflexion torque decreases ankle range of motion by 2.4 degrees (2.0 to 2.7 degrees; knee flexed 90 degrees) and 2.3 degrees (2.0 to 2.5 degrees; knee fully extended). Thus 5% of involuntary plantarflexion torque, the amount usually considered small enough to ignore, decreases dorsiflexion range of motion by similar to 12 degrees. Our results indicate that even small amounts of involuntary muscle activity will bias measures of passive range and hinder the differential diagnosis and treatment of neural and nonneural mechanisms of contracture. NEW amp; NOTEWORTHY The soleus muscle in able-bodied adults was tetanically stimulated while the ankle was passively dorsiflexed. Each 1% increase in involuntary plantarflexion torque at the ankle decreases the range of passive movement into dorsiflexion by amp;gt;2 degrees. Thus the range of ankle dorsiflexion decreases by similar to 12 degrees when involuntary plantarflexion torque is 5% of maximum, a torque that is usually ignored. Thus very small amounts of involuntary muscle activity substantially limit passive joint range of motion.