The aim was to evaluate the tone and electric activity of the quadriceps muscle at rest and different torque levels. The second aim was to study whether thickness of soft tissues and change in the joint position would affect muscle tone. Eighteen healthy subjects participated. Computerized muscle tonometer (CMT) and surface electromyography (sEMG) measurements were performed: seated, first at rest with leg straight and then with the knee at 60 degrees . Thereafter measurements were obtained at levels of 80, 60, 40 and 20% of the maximum isometric torque at the same knee angle. Thickness of skin, subcutis and muscle was measured by ultrasound. The CMT values taken were the depth the indenter travelled and the work it did while compressing the right rectus femoris and vastus intermedius muscles. Expressed as mean (SD) depth the change in muscle tone changed from 29.2 (3.6) mm in the relaxed position to 16.9 (5.2) mm at 80% of maximal torque, and expressed as work the values were from 1589 (150) mJ to 739 (149) mJ respectively. The correlation between CMT, sEMG and torque measurements varied from r = -0.52 to -0.71 (p < 0.01). CMT was able to detect a change of 20% in torque production and 4% in tone. Tone values, at each torque level, were significantly separate from the values at the other force levels (p < 0.001-0.04). Soft tissue thickness explained most of the tone results at rest (57%). The repeatability of the CMT measures was good (ICCs 0.75-0.99). Both depth and work correlated with electric activity and muscle torque, but the correlation with work was higher. In conclusion, muscle activity, length and thickness have to be taken into account when evaluating muscle tone.