The purpose of this study was to investigate the effects of 20 days bed-rest on the elastic properties of tendon structures of the human knee extensor muscles in vivo. Six healthy men carried out a 6 degrees head-down bed-rest for 20 days. Muscle volume and maximal voluntary contraction (MVC) torque of the quadriceps femoris muscle significantly decreased by an average of 7.8 (SD 2.7)% and 14.9 (SD 6.9)%, respectively. Before and after bed-rest, the elongation (l) of the tendon and aponeurosis of vastus lateralis muscle was measured directly by ultrasonography, while the subjects performed ramp isometric knee extension up to MVC. The extent of l tended to be greater after bed-rest. The l above 110 N was significantly greater after bed-rest. Furthermore, the mean stiffness after bed-rest [35.5 (SD 7.8) N x mm(-1)] was significantly lower than that before bed-rest [52.6 (SD 19.2) N x mm(-1)]. The rate of torque development significantly reduced after bed-rest by an average of 47%, and the bed-rest induced a lengthening in the electromechanical delay (mean 21%). These results suggest that bed-rest results in a decrease in the stiffness of tendon structures with a reduction of muscle strength and volume. These adaptations of the tendon structures to bed-rest would bring about the changes in electromechanical delay and rate of torque development.