We have observed the force resulting from sudden stretches (3-10 mm in 10-20 ms) of human adductor pollicis muscle during voluntary contractions maintaining different proportions of the maximum voluntary force from 20% to 100%. The ratio of the peak force during stretch (S) to the voluntary force (I) just before the start of the stretch was calculated. The S/ I ratio was dependent both on the level of activation and the speed of the stretch. At all levels of activation S/ I increased exponentially with velocity towards a plateau value with an exponential constant of about 150 mm/s. The plateau values reached were dependent on activation level (P<0.02, linear regression, n=24). At 96% activation the plateau value of the S/ I ratio was 1.82+/-0.13 (mean+/-95% confidence limits); at 32% activation the value was 2.72+/-0.23.