A new apparatus has been designed for standardized measurements of the strength of human trunk muscles utilizing the isokinetic (constant movement velocity) technique (Cybex). It is possible to measure the produced torque during maximal voluntary isometric and isokinetic contractions in the whole range of motion during flexion, extension and lateral flexion of the trunk. Effects of gravity are eliminated since the movements are performed in the horizontal plane. Torque can be measured around different centres of rotation of the body. With this experimental set-up the strength of the trunk muscles has been characterized in a group of 14 normal male subjects (18-31 yrs). The torque produced by the trunk muscles varied with movement velocity and trunk position in the arc of motion. Peak torque occurred in a position where the muscles involved were stretched. The strength of the trunk extensors exceeded that of the flexors, but the degree (ratio) varied with trunk position. The relative contribution of the hip muscles to the total torque produced with the centre of rotation at the hip joint was larger for flexors than for extensors and varied with velocity and position. It is concluded that the present technique is useful to characterize the human trunk muscles.