The effects of posture on trunk muscle strength under isometric and isokinetic conditions in 21 healthy male volunteers and the associated electromyographic activity for eight trunk muscles were investigated. The men performed 12 trunk extension exertions in standing and kneeling postures. Isometric tests were performed at 22.5, 45, and 67.5 degrees of trunk flexion. Isokinetic tests were performed at three velocities: 30, 60, and 90 degrees per second. Electromyographic data were collected from eight trunk muscles to assess muscle recruitment under each condition. A priori orthogonal contrasts were specified for analysis of both torque and electromyographic data. The findings suggest that the kneeling posture was associated with a 15% decrease in peak torque output when contrasted with standing. However, no concomitant change was noted in trunk muscle activity. Trunk hyperflexion and increasing rotational velocity were associated with reduced torque in both postures. Trunk muscle activity was primarily affected by changes in trunk angle and velocity of contraction. The authors conclude that a reduced extensor capability existed in the kneeling posture in spite of equivalent trunk muscle activity. The similar activation patterns in both postures suggests that the strength deficit did not result from alterations in trunk muscle function. They suggest that it may be due to reduced capability to rotate the pelvis in the kneeling position, due to a disruption of the biomechanical linkage of the leg structures.