To evaluate the joint angle dependence of brachialis muscle architecture at rest and changes in brachialis muscle architecture during isometric voluntary contractions in people after stroke. The pennation angle and fascicle length of the brachialis muscle were measured in the affected and unaffected sides of people after stroke at 9 different elbow angles ranging from 10 degrees to 90 degrees at the rest condition. Measurements were also carried out at a fixed joint angle of 90 degrees while the subjects were performing isometric muscle contractions at 5 incremental levels of maximal voluntary contraction (MVC) from 20% to 100% of MVC. The data obtained from the affected and unaffected sides of the subjects were compared. A research laboratory in a rehabilitation center. Seven hemiplegic adults after stroke with passive range of motion in the elbow from 10 degrees to 90 degrees and Modified Ashworth Scale score larger than 1 were recruited. Not applicable. Ultrasonography was used to measure brachialis muscle pennation angle and fascicle length at the rest condition and during isometric contractions. The measured pennation angles and fascicle lengths were found to be joint-angle-dependent in both the affected and unaffected groups at the rest condition. Further comparisons found that the pennation angles of the affected brachialis muscle were significantly larger (P<.05) than the unaffected muscle in the most extended positions (<50 degrees ), whereas the affected fascicle lengths were significantly shorter (P<.05) than the unaffected muscle in most flexed positions (>20 degrees ). As the level of isometric voluntary contraction was increased incrementally from 20% to 100% of MVC, the results showed that pennation angle increased significantly (P<.05) while fascicle length decreased significantly (P<.01) in the unaffected muscle. However, the contraction level has a significant effect only on pennation angle (P<.05) but not on fascicle length in the affected side. In addition, the measured fascicle lengths in the unaffected group were significantly shorter than those in the affected group for isometric contractions above 40% MVC. Our findings suggest that the architectural parameters of the brachialis muscle in people after stroke are elbow-joint-angle-dependent at the rest condition. In the affected side, pennation angle changed the most when the muscle was in an extension position and fascicle length changed the most in a flexed position when compared with the unaffected side. Immobilization and contracture might cause a shortening of the fascicle and an increase in pennation angle in the affected side. Smaller pennation angle and fascicle length changes in the affected side during isometric contraction might be due to weakness in the muscle after the onset of stroke.