The purpose of this study was to investigate the changes in the elastic properties of tendons in humans in relation to fatigue of knee extensor muscles. The muscle fatigue test (MFT) consisted of maximal isometric contractions performed 50 times. The decline in peak moment was 43.6 (SD 19.5)%. After MFT, the muscle thickness and pennation angle of the vastus lateralis muscle (VL) significantly increased 1.5 (SD 0.7) mm (5%) and 1.7 (SD 1.8) degrees (11%), respectively. Before and after MFT, the elongation (l) of the tendon and aponeurosis of VL was directly measured by ultrasonography, while the subjects performed ramp isometric knee extensions up to maximal voluntary contraction. The l tended to be greater after MFT than before MFT. This difference in the l was statistically significant (P < 0.05) at force developments beyond 220 N. Furthermore, the compliance increased significantly from 2.0 (SD 0.6).10(-2) mm.N-1 before MFT to 2.6 (SD 0.7).10(-2) mm.N-1 after MFT (22.7%). In addition, the electromechanical delay was significantly increased from 60.6 (SD 5.9) ms before to 70.0 (SD 4.4) ms after MFT. These results suggested that the repeated muscle contractions made the tendon structures more compliant.